SOME ASPECTS OP THE WJffiER RELATIONS OP W O MAHOGANY SPECIES A thesis submitted by Gladys Dodoo, B.Sc, (Hons) in part fulfilment of the requirements for the M.Sc. Degree of the University of Ghana. Deco?We^ 5^71 From; Botany Department University of Ghana . University of Ghana http://ugspace.ug.edu.gh G183168 University of Ghana http://ugspace.ug.edu.gh I hereby declare that the work presented in this thesis •Some Aspects of the Water Relations of Two Mahogany Species* was done entirely by me in the Department of Botany, University of Ghana, Legon, from September 1970 to November 1971. No part or parts of this thesis has been submitted for a degree elsewhere. o o (GLADYS D0D00) University of Ghana http://ugspace.ug.edu.gh ABSTRACT The general distribution, growth form and economic importance of Kha%a senega!sir (Bear) A. Juss. and Khaya ivorensis A. Chev. are described. Some aspects of the water relations of seedlings of these two species, the former a savanna species and the latter a forest species were studied with the view of ascertaining whether moisture plays an important role in determining the pattern of their distribution. &rowth of seedlings under four soil watering regimes namely - -0.3 (A), -0,4 (B), -0.8 (C), and -4.5 (D) bars; and under culture solution and culture solution to which polyethylene glycol was added to give the following osmotic potentials (bars): -0.3 (A), -2.8 (B), -5.3 (C), aid -10.3 (D) was studied. The experiment was done in the g r e e n h o u s e . G-rowth of K. S e n e g a lensis was more sensitive to moderate moisture stress and less sensitive to high moisture stress. K, ivorensis on the other hand showed less sensitivity to moderate stress but high sen­ sitivity to severe stress. However when soil moisture stress was -0.3 bars, growth of K. ivorensis was very poor. This was attributed to a reduction in root permeability due to poor aeration as a result of more permanent near saturation of the soil. Studies of diurnal patterns of plant water status were carried out by examining leaf relative water oontent, (R.W.C.) leaf water potential (L.W.P.) and stem diameter variation, in relation to soil moisture stress. R.W.C. was overall higher in seedlings of K. senegalensis than in those of K. ivorensis. L.W.P. on the other hand was lower for IC. senegalensis than for K. ivorensis seedlings. Stem shrinkage decreased with decrease of soil moisture content from 100 to about 50%, field capacity. The decrease in K. senegalensis was greater than in K. ivorensis-. However at 27% field capacity, shrinkage in K. senegalensis was consistently reduced more than in K. ivorensis. This may indicate better water conservation by the former species. Transpiration was also studied in relation to the soil moisture treatments, both in the greenhouse aid in the research room, the latter being a semi-controlled environment where temperature, relative humidity and light intensity were precisely known. The transpiration of seedlings growing in osmotic solution was also studied in the greenhouse, employing stresses of -0.3 and -10.3 bars. Transpiration generally decreased with moisture stress in the root medium. In the research room transpiration of K. senegalensis was higher than that of IC. ivorensis under all soil treatments. In the greenhouse, however, similar higher transpiration rates were recorded for K. senegalensis seedlings than for K. ivorensis seedlings when stress was from -0.3 to -0.8 bars, but at severer stresses (-4.5 snd -10.3 bars) transpiration of K. senegalensis was reduced more than that of K. ivorensis. University of Ghana http://ugspace.ug.edu.gh Infiltration of leaves of K. senegalensis and K. ivorensis with mixtures of liquid paraffin and commercial Kerosene showed that stomatal conductivity of K. senesalensis leaves was greater then that of K. ivorensis leaves at low moisture stress. At more severe stress (-4.5 bars) conductivity of leaves of both species was low. The desorption curve for K. senegalensis seedlings was above that for K. ivorensis seedlings. Curves of leaves for adult trees of both species were however similar. K. senegalensis leaves oould tolerate desiccation better than K. ivorensis leaves. Stomata of K. senega!ensis leaves close at higher relative water content values than those of K. ivorensis leaves. The results are discussed in relation to the distribution of these species and to the general ecological problem of the control of plant dis­ tribution between forest and savanna in West Africa. University of Ghana http://ugspace.ug.edu.gh CONTENTS Page ABSTRACT •*■ INTRODUCTION ... ... ... 1 CHAPTER I j Experimental species, materials, and general methods ... ... 11 CHAPTER II s Growth in relation to moisture stress in the root medium ... ... 25 CHAPTER IH: The diurnal pattern of plant water status 74 CHAPTER IV : Transpiration in relation to moisture stress in the root medium ... 104 CHAPTER V : Tissue water relations ... 139 CHAPTER VI j General Discussion ..« ... 165 SUMMARY ' ... ... ... 174 APPENDIX ... ... ... 181 ACKNOWLEDGEMENT ... ... 217 REFERENCES ... ... ... 218 University of Ghana http://ugspace.ug.edu.gh INTRODUCTION General In general, the natural pattern of distribution of plants in any area is largely controlled by environmental factors. The main physical factors of the environment - temperature, light, moisture, wind and soil, frequently set limits to the occurrence of species, both on a geographical and on an ecological scale (Raunkiaer, 1934; Daubenmire, 1947; Hopkins, 1965)* The importance of any of these factors in a given area is detemined both by its general mean level and by the range between its minimum and maximum levels. Temperature is the primary factor responsible for the main vegetational belts of the world. In many geographical regions it is also the overriding factor controlling the pattern of plant m distribution on an ecological scale. In West Africa, however, where daily and seasonal variations iai temperature are usually small (see Lawson, 1966), this factor is less important in controlling plant distribution. There is rather a general belief that the main division of West African vegetation into forest and savanna is causally related to environmental moisture conditions (see for example Taylor, 1952; Keay, 1959). Hopkins (1965) and Lawson (1966) in their general accounts of vTest African ecology point out that in this region rainfall University of Ghana http://ugspace.ug.edu.gh 2varies greatly between areas, in amount and seasonal distribution* Over 80 ins. (200 cm) of rain falls annually in most parts of the forest areas. This anount decreases progressively inland from the coast until within the savanna areas there is usually less then 45 in. (114 cm) of rain per annum. Rainfall in the forest regions is distributed in such a way that there are not more than two months with less than one inch (2.5 cm) of rain. On the other hand the savanna regions may have up to five or six months with less than 1 inch (2,5 cm) of rain. The balance between precipitation and potential evaporation is perhaps a more meaningful criterion than amount er distribution of rain for assessing water availability for plant growth, From a simple survey of this balance for Africa, Davies and Robinson (1969) reached the tentative conclusion that the forest-savanna boundary, at least in West Africa, coincides reasonably well with the -200 mm isopleth, where although there is no excess of precipita­ tion over potential evaporation for iiie year as a whole, there is some amount of water stored in the soil which could support growth of forest species. Thus it is clear that savanna vegetation type is associated with a dzy climate, hence the general belief that savanna plants are more drought-adapted than plants of the forest regions. Comparative experimental work to test this belief is however largely lacking so that there is little knowledge of the University of Ghana http://ugspace.ug.edu.gh 3mechanisms by which plants from these two regions are adapted to environmental moisture conditions. Mechanisms by which plants adapt themselves to drought have been much studied and reviewed (see for example Parker, 1956, 1968; Stocker, 1956, I960; Iljin, 1957; Oppenheimer, i960, 1968; and Kozlowski, 1968). Oppenheimer (i960) points out that these mechanisms generally fall under three main heads; plants may be drought escapers, drought evaders or drought endurers. Drought escapers apparently have no adaptation to withstand drought. They normally complete their life cycle before the unfavourable period. Unlike the escapers, drought evaders survive under drought by preventing the development of internal water stress under a given degree of external stress. This they do by efficient control of transpiration and efficient water absorption. As has been pointed out by several workers (for example Sullivan and Levitt, 1959; Oppenheimer, I960; and Parker, I968), numerous adaptive morphological characters are associated with the phenomenon of drought evasion. In contrast to drought escapers or evaders, drought endurers are able to cany on their normal life activities under drought even when there is internal stress. Most studies of plant adaptation to drought have been concerned with identifying which of these mechanisms is paramount in any given situation, and with understanding details of plant attributes which make drought resistance possible. Such studies University of Ghana http://ugspace.ug.edu.gh gain in value when they contribute towards understanding the causal factors of the pattern of plant distribution in any area. To achieve such an understanding is the general objective of the present study. The Problem The general paucity of experimental investigations on the water relations of West African plants has already been mentioned. The few studies conducted in this field (for example Lawson and Jenik, 1967; Okali, 1971a), were mostly concerned mth mechanisms of adaptation to environmental stress by seme species on the Accra Plains of Ghana. Although vegetation on the Accra Plains is dissimilar to Guinea savanna in many respects (Wills, 1962; Hall and Jenik, 1968), nevertheless the Accra Plains Lawson (I966)^indicates that ^ ishow'. many interesting ecological factors at work which may throw light on processes in other types of savanna. The vegetation here consists of short grass and tree- thickets which often occur in scattered clumps (Wills, 1962; Aubreville, 1959} Boughey/"1957)j described the Accra Plains as a kind of steppe. As in true savanna regions, the climate over the plains is relatively dry. Less than 30 in. (76 cm) of rain falls annually, (Lawson, 1966; Lawson and ‘•enik, 1967). Thus studies of mechanisms of plant adaptation to Uiis habitat may be useful in understanding the general nature of the control of plant life by University of Ghana http://ugspace.ug.edu.gh 5environmental moisture in West Africa. For example, Lawson and Janik (1967) in their study of the interrelations of microclimate and vegetation on the Accra Plains, compared transpiration rates (by the rapid weighing technique) and other features of species on the wind-ward and leeward sides of a thicket. In general they found that species on the windward side transpired less and had more xeromorphic features than those on the leeward side, thus demonstrating tiiat microclimate (and here particularly desiccating winds) may exert considerable influence on vegetation. Okali. (1971a) further studied the water relations of seme of the woody species on the plains. Using mainly detached leaves or leaf discs taken from plants growing in the field, he compared transpiration rates and the relationships between leaf water content, leaf water potential, stomatal closure and tissue damage. His results showed that the species studied, exhibited several mechanians of adaptation to drought. Neither of the above studies examined the response of whole plants (e.g. growth) to drought, and although they demonstrate the possession of adaptive features by species in a diy area, they do not provide an adequate basis for testing the role of moisture in deteimining the pattern of plant distribution between forest and savanna, because they examined species adapted to one "type of climate alone. A more satisfactory approach for such a test would be to compare the water relations of forest and savanna species. University of Ghana http://ugspace.ug.edu.gh 6There is little information in the literature on the water relations of West African forest species. The only suitable data which permit such a comparison to be made were recently given by Hopkins (l970a,b). Prom a study of species in forest and savanna sites of the Olokemeji Forest Reserve in Nigeria, Hopkins was able to show that for forest species leaf water may be 'severely limiting at the severest part of the dry season*; he could not demonstrate similar limita­ tion for the savanna species although he attributed this to probable effects of leaf age in masking iiie relation between environmental moisture and leaf water status. Hopkins' study examined, only one aspect of the water relations of forest and savanna jpecies and this limits its usefulness as a basis for generalization on the probable role of moisture in controlling plant distribution in this part of the world. Richards (1952) draws attention to the existence in West Africa of certain tree genera whose component species are restricted to sharply contrasting habitats. He cites the genera Khava and Lophira as examples, noting that the foimer genus is represented by K. ivorensis and K. anthotheca in the Wet Evergreen forest, by randif o 1 iola in the Dry Forest and by K. Senegalensis in the savanna. Lophira is similarly represented by L. alata in the forest and L. lanceolata in savanna. To these examples may be added such genera as Afronaosia. Paniellia and Cussonia which have both forest and savanna species. If environmental moisture is indeed the University of Ghana http://ugspace.ug.edu.gh overriding factor causing the restriction of these closely related species to contrasting habitats, it should be possible to demonstrate this by comparing the water relations of any two such species of the same genus - the one from forest and the other from savanna. The object of the study described in this thesis was to examine experimentally the water relations of two such species, Khava senegalensis (Des*) A. Juss, and K. ivorensis A. Chev, commonly called Dry Zone Mahogany and African Mahogany respectively, to find possible differences 'between them which might help in understanding the factors responsible for their restriction to contrasting habitats. The two species are not only taxonomically closely related, they are also as is more fully described at a later stage, similar in growth habit, phenology and reproductive biology. Differences between them could therefore be expected to have arisen more as an adaptation to their respective habitats than would be the case if the two species were dissimilar in the above four respects. If environmental moisture plays a large part in determining the restriction of these two species to different habitats, K. senegalensis would be.expected to show greater adaptation to drought than &. ivorensis. Approach to the problem Several investigators of plant water relations have approached such studies with different objectives. Some investigators have University of Ghana http://ugspace.ug.edu.gh 8studied single species (for example Weatherley, 1950, 1951, 1965; Gates, 1955a,b; Hutter and Sands, 1958; Aspinall, 1965; Klepper, 1969); others have compared the responses of a group of species to similar environmental conditions (for example Slatyer, 1955, 1957’Dj I960; McKell, Perrier and Stebbins, I960; M.S. Jarvis, 1963; Jarvis and Jarvis, 1963a,b,c and e, 1965; Connor and Tunstall, 1968). The objective when single species have been studied has usually been to understand the processes through which environmental moisture affects plant growth. When several species have been studied together, the implied aim has been to understand the comparative effects of environmental moisture with the view 60 explaining ecological situations. The latter is the case in the present study. The value of comparative water relations studies in contributing to an understanding of the causal factors of plant distribution is illustrated by the work of many authors. Thus McKell, Perrier and sub . ^lomerata Stebbins (i960) compared two^species of Daqtvlis and concluded that the sub species the restriction of Iasi f.ani r.a to mesic habitats and i>. Judaic a to xeric habitats in Israel might be due to differences in the water demand of the two species. Similarly, investigations by M.S. Jarvis (1963) on factors which limit the distribution of Saxifrage liypnoifles and Prunus padus to upland areas of North western'parts of Great Britain and Filipendula vulgaris and Thelycrania sanguinea to more low land parts to the University of Ghana http://ugspace.ug.edu.gh 9South east showed that this could be attributed to differences in soil moisture in these areas. Sullivan and Levitt (1959) found from their work that a possible explanation for the restriction of Querous palustris to moist areas and Q. rubra to upland areas in Missouri might be the difference in drought resistance of the two species. Kychnovska and Kvet (1963) accounted for the distribution of Festuoa domini. Corynephorus canesoens and Helichrysum arenarium in Czechoslovakia on the basis of their water relations. Bannister (1964) also related differences in distribution of Calluna vulgaris. Erica cinerea and E. tetralix to the water relations of these species. The distribution of various species of Artemisia in arid and ssmi arid communities in Kazakhstan has also been related to differences in the internal water balance of these species (Sveshnikova, 1965). Irrespective of the objective investigators have sought to understand plant water relations by observing growth and transpiration responses of the species to environmental moisture stress (for example Gates, 1955&; Rutter and Sands, 1958; M.S. Jarvis, 1963; Jarvis and Jarvis, 1963a,b,c), by studying the water relations of tissues through, for example, measurements of the relation between water content and water potential of tissue (Slatyer, i960, 1962c; Connor and Tunstall, 1968), the relation between water content and stomatal closure (Bannister, 1964, Jarvis and Jarvis, 1963d,e), and the relation between tissue water content and tissue damage University of Ghana http://ugspace.ug.edu.gh (desiccation tolerance) (Sullivan and Levitt, 1959; Noy-JSeir and Ginzburg, 1969b; Okali, 1971a). Some investigators have followed changes in stem diameter as a measure of variation in plant water status (Kozlowski, 1967; Ogigirigi, ICozlowski and Sasaki, 1970). The present study represents an attempt to combine several of these approaches. Following an analysis of the growth responses of the experimental species to varying moisture status in the root medium, internal water balance over most of the day for plants growing in wet or dry soil was compared. In order to understand the growth responses and diurnal patterns of plant water status more clearly, transpiration and seme aspects of tissue water relations of the experimental species were then studied. These experiments were carried out in the laboratory and mostly with potted plants as has been previously done by several workers such as Slatyer (1957b, 196l), Rutter and Sands (1958), McKell, Perrier and Stebbins (i960), Brix (I962), Jarvis and Jarvis (1963a,b,c,d,e, 1965)» and Lawlor (1969). There are clear difficulties in attempting to use laboratory studies in unc.erstending natural distribution of plants, but evidence in the literature, suggests that when these studies are comparative, individual differences revealed could be of ecological significance (cf. McKell, Perrier and Stebbins i960, M.S. Jarvis, 1963). 10University of Ghana http://ugspace.ug.edu.gh 11 CHAPTER I EXPERIMENTAL SPECIES, MATERIALS AND GENERAL METHODS ✓ 1.1. The Experimental Species The species chosen for this study - Khaya Senegalensis (Desr) A. Juss. and Khava ivorensis A. Chev. are both trees belonging to the family Meliaceae. They are both widely distributed throughout West Africa. Irvine (l96l) notes that K. senegalensis extends from Senegambia to Cameroons, Sudan and Uganda while K. ivorensis extends from the Ivory Coast to Gabon. Hutchinson, Dalziel and Keay (1954) however, indicate that K. ivorensis could be found as far south as o -O Cabinda, that is between latitudes 4 and 6 south of the equator and o o longitudes 10 and 13 east of the Greenwich Meridian. Throughout their geographical range, the two species occur in distinctly different habitats. Richards' (1952) observation on this has already been mentioned. Hutchinson, Dalziel and Keay (1954) also point out tiiat K. senegalensis is a savanna tree but grows especially by streams. Taylor (i960) describes K. ivorensis as occurring throughout the high forest zone of West Africa. In his book 'Woody Plants of Ghana1 Irvine (1961) indicates that K. sene^alensis is found in savanna and fringing forest, particularly in low lying places besides streams aid K. ivorensis in deciduous and evergreen forests. The contrast in the habitats of the experimental species appears thus to be well docunented. University of Ghana http://ugspace.ug.edu.gh 12 The two species appear to be dissimilar in their soil preferences. K. seneaalensis prefers good alluvial soil but not swampy soils, while K. ivorensis is known to favour moist valley soil and apparently can also survive considerable flooding (Kinloch, 1945). Data to be presented later in the present study suggest that tolerance to flooding is limited in K. ivorensis seedlings. Taylor (i960) further points out that K. ivorensis occurs on heavy or rich alluvial soil with good drainage near water courses and damp areas. The growth form and general morphology of the two species have been adequately described by Lely (1925), Dalziel (1957) and Irvine (1961). K. S e n e g a lensis grows up to 30 m in height and three meters in girth. It has no buttresses. The baric is grey and scaly and the slash is red. Unlike K. senega!ensis. K. ivorensis grows up to 60 m in height and 4.5 m in girth. It is strongly buttressed and has a tall cylindrical bole up to 27 ra above the buttress. The bark is ashy white to brownish black. The slash is crimson. The crowns of adult trees of both species are dense with branches bearing alternate pariplnnate leaves about 25 cm long with 4 - 8 opposite or subopposite, exsiipulate, oblong or oblong elliptic leaflets. Examination of K. senefealensis indicates that each leaflet is about 10 cm long and * shortly acuminate at the apex. Each leaflet has a petiolule which is about one cm long. Each leaflet of K. ivorensis on the other hand is about 14 cm long and 7 cm wide with well University of Ghana http://ugspace.ug.edu.gh defined drip-tips, (see appendix 1, Plate i), smooth, acuminate and usually rounded at the base with a short petiolule about 1.5 cm long. Observations on seedlings in the greenhouse, on herbarium specimens from trees of different ages as well as on trees in the Botanical Garden, Legon, show ,that the well defined drip-tips of K. ivorensis leaflets decrease in length as the tree grows older. In general mature leaves of K. senegalensis are comparatively much smaller, with a thick net of veins, than those of K. ivorensis. This is probably a xeromorphic feature (cf. Lawson and Jenik, 1967)• Detailed examination, of the leaves, made on epidermal prints and on microtome sections revealed that stomata occur mostly on the lower surfaces of leaves in both species; those of K. senegalensis being more numerous (Table l). There are very few stomata (less than l/mm ) on the upper surfaces of the leaves of both species. Metcalfe end Chalk's (1950) statement that stomata occur only on the lower surface in leaves of Khava species probably therefore does not necessarily apply to seedlings. The stomatal frequencies obtained for the two species are comparable to figures quoted by Yanney-Wi 1 son (19.63) for three species on the Accra Plains: Baphia pubescens. Yemonia senegalensis and Pluggea virosa. Yanney-V/ilson considers frequencies more than 502/rm to be an indication of xeromorpby in leaves. On this basis the leaves of both species could be assumed to have some xeromorphic features. Metcalfe and Chalk (1950) note that the palisade mesophyll in Khaya species is more than one layer. University of Ghana http://ugspace.ug.edu.gh Table 1 s e e d l i n g Anatomical features of^leaves of Khava senegalensis and K. ivorensis 14 Leaf anatomical character K, senegalensis K. ivorensis Mean thickness of leaf ( ji) 822 508 Mean thickness of cuticle ( ju) 14 13 Length of stomata ( ju.) 14-20 11-18 Stomatal frequency (mm ) 660 580 No. of palisade mesophyll layers 1 1 Mean height of palisade cell ( p) 272 104 Mean width of palisade cell ( ji) 46 38 Thickness of spongy mesophyll (p) 347 324 Mean diameter of spongy mesophyll cell ( p) 56.5 61.0 Species University of Ghana http://ugspace.ug.edu.gh 15 This has been found to be true of leaves from adult trees of K. senega!easis and K. Ivorensis; they have two or three layers of palisade. However mature leaves of seedlings were found to have only a single layer of palisade (Pig. l) with those of K. senegalensis being much deeper (about 270 microns) than those of K. ivorensis (about 104 microns). In their natural habitats, K. senegalensis and K. ivorensis are never deciduous (Kinloch, 1945; Taylor, i960), Kinlock (1945) however indicates that in the drier areas of the high forest zone where K. ivorensis is seldom found, the tree is deciduous for a short period at the beginning of the dry season. The similarity between the two species extends to tiieir reproductive biology. The flowering season for both ^ecies is July to January. Flowers are arranged in axillary panicles. Fruits of both species are capsules. They ripen and open to release their seeds between February to May. The fruit of K. senegalensis is about 6 cm in diameter and generally four-valved, while that for K. ivorensis is about 9 can in diameter and opens by five valves. Seeds of the two species are brown and winged. Measurements show that the Seeds of K. senegalensis are about 1.8 cm long and 2.4 cm broad and are oblong-elliptic in shape, while that of K. ivorensis are about 2.3 cm long and 3.7 cm broad; they are oblong to triangular in shape. The seed of K. senegalensis is comparatively much thicker with an average dry weight (without testa) of about 16 mg while that of K. ivorensis weighs about 11 mg. Seeds University of Ghana http://ugspace.ug.edu.gh FIG-. 1 CAMERA. LUCIM DRAWINGS OF EXAMPLES OF TRANSVERSE SECTIONS OF LEAVES OF KBAYA SMEGALENSIS (KS) iND K. IVORMSIS (Kl) Seedlings * LEFT (THM L E A V E S 5 7 ^ ® ' (THICK LEAVES)! University of Ghana http://ugspace.ug.edu.gh CUTICLE UPPER EPIDERMIS PALISADE MESOPHVLL SPONGY MESOPHYLL LOWER EPIDERMIS University of Ghana http://ugspace.ug.edu.gh 17 of both species are short lived. Observations in the present study show that gemination, in both species, generally starts eleven days after sowing* by the 21st day almost all the viable seeds geminate. Often K. senegalensis seeds start germinating before K. ivorensis. especially when fresh. Germination is hypogeal for both species. The shoot of K. senegalensis is green and that of K. ivorensis is red and slender. The first two leaves of both species are simple and opposite and are borne about 8 - 10 cm above the soil. The areas of these leaves when about two months old are respectively about 14.0 and 11.5 sq. cm for K. senegalensis and K. ivorensis. The next few leaves are also simple but alternate. These are followed by a few unifoliolate leaves. Compound leaves develop later, normally after two months of gemination. The first few sets are trifaliolate,, later anes are imp a rip inna t e with about 4 - 9 leaflets which are opposite. Leaflets of K. ivorensis are much larger than those of K. senegalensis (see Appendix l,Pla£e ). -One year old seedlings of both species have both paripinnate and impari- pinnate leaves. At emergence seedlings of K. senegalensis are much larger than those of K. ivorensis and by the age of two months, the height of the seedlings is about 13.0 cm with 6 - 9 leaves while that of K. ivorensis is about 10.0 cm with about 4 - 6 leaves. The larger initial size is presumably due to the larger seed size of K. senegalensis (see p. 15). This might be advantageous for quicker establishment. University of Ghana http://ugspace.ug.edu.gh 18 The two species are not only of ecological interest; they are also economically important species. The first exploitation of Mahogany species for timber for export begaa in about 1891 (Taylor 1952). It has since remained one of the most important species in the timber economy in this country. Economic reports of the Forestry Department indicate that in 1969 (JAnon) the log volume output of K. ivorensis. whose trade name is Dubini, from the forest reserves was about one million cubic feet (28000 cu.m). Over 19 million logs from 34 timber species valued at about 24. million new cedis were exported from January to December, 19^9, and out of this about 1.5 million cu. ft. (£2000 cu.m) valued at about 2.2 million new cedis was K, ivorensis. Further reports in 1970 indicate that Mahogany yield from forest reserves in the high forest zone, was about 1.3 million cu. £t.(36000 cu.m) out of which one million was K. ivorensis. K. senegalensis being of smaller size is of less importance. The first export of the species as timber was made from tiie Gambia over a century ago. Although an excellent timber, it is seldom now exported from Ghana because of its small size and weight. Local uses of these two Mahogany species include their conversion to charcoal for fuel. Annual reports of the Forest Froducfss Research Institute here (1968 and 1969) emphasise that because of the general increase in industrial consumption of wood, it is necessary to establish plantations of the more important timber species of which the mahoganies are certainly leading University of Ghana http://ugspace.ug.edu.gh examples. It is clear that successful establishment of such plantations can be enhanced by the availability of adequate information on the ecology of the timber species that are considered as important. Some of the results from the present study might contribute towards providing such information. In particular, the findings from this study might assist in the choice of combination of management practices if it is desired to extend the habitat ranges of these two species with respect to environmental moisture conditions. 1.2, Experimental Plants Seedlings of K. senegalensis and K. ivorensis were used for -this study except in one instance when certain determinations were made on adult trees growing in the Botanical Garden. The main advantage of using seedlings is that they are more convenient to handle in laboratory work. Although the seedling phase represents only & small fraction of the entire life of a plant, distributional patterns of plants in nature are often governed by factors affecting seedlings. (Moore, 1926; Daubenmire, 1943; Jarvis and Jarvis, 1963a). Seedlings of K. senegalensis for experiments between October 1970 and April 1971 were raised from seed supplied by the Forest Products Research Institute, Kumasi, Ghana. The seeds had been collected from Koforidua in May 1970. Seedlings for later studies 19University of Ghana http://ugspace.ug.edu.gh 20 were raised from seeds collected in March 1971> from tiie grounds of the University Campus, Legon where this species lines "the main University avenue. Seedlings of K. ivorensis were raised from seed also obtained from the Forest Product Research Institute, Kumasi. The seeds had been collected from Asenanyo > Forest reserve, in Ghana, also in May 1970. For seme experiments larger seedlings were used to supply sufficient leaf material for investigations of tissue water relations or to provide stems large enough to enable observations on diameter changes to be made. Some of these seedlings were less than one year old while others were just over one year old. At the time of experiment, these seedlings were between 50 and 80 can tall in both species. 1.3. Growth media Plants were grown on soil or in culture solution to which polyethylene glycol was added when it was desired to vary the water potential of the root medium. Both media have advantages and disadvan­ tages for the study of plant water relations. Several investigators (Sands and Rutter, 1959; Jarvis and Jarvis, 19&3&,% Assinall,. 1965), who have used soil as a rooting medium point out that the main disadvantage is that it is not possible to know the exact moisture stress to which roots are subjected, since water potential at the root-soil interface changes continuously with transpiration by the plants and as roots grow into new regions of soil. Exact knowledge University of Ghana http://ugspace.ug.edu.gh of soil moisture stress at the root—soil interface is however important since this factor appears to he the main soil characteristic controlling soil water availability for plant growth (Slatyer, 1967). The use of well stirred solutions containing osmotic substrates such as sodium chloride (Slatyer, 1961; Nieman, 1962), msnnitol (Gingrich and Russell, 1956; Slatyer, 1961, Jarvis and Jarvis, 1963c,d) and polyethylene glycols (Largerwerff, Ogata and Eagle, 1961; Jarvis and Jarvis, 1963d, 1965; Kaul, 1966; Kauftaann, 1968; Lawlor, 1969, 1970), attempts to overcome this limitation with soil. But there are also disadvantages in using aqueous solutions of osmotic substrates. The first is that the water potential will comprise only an osmotic component whereas soil water potential is made up of both osmotic and matric components, although there is evidence (Wadleigh and Ayers, 1945) that this difference may have little significance for plant growth response. Another disadvantage is that some osmotic substances tend to be absorbed by the roots, thus decreasing plant water potential. Jor example, Slatyer (1961) found that sucrose, sodium chloride and potassium nitrate were readily diffusible into the roots of tomato. It has however been found by Lawlor (1970) that undamaged roots have low permeability to polyethylene glycols of molecular weights 1000, 4000 and 20,000. Some of these substances, when absorbed, are also toxic to the plant (Largerwerff et al, I96I; Jarvis end Jarvis, 1963d; Macklon and Weatherley, 1965; Leshem, 1966; Lawlor, 1970). In.spite of these disadvantages, both soil and osmotica have been 21University of Ghana http://ugspace.ug.edu.gh 22 successfully employed, by most of the authors cited a ;ove, to study several aspects of plant response to moisture stress in the root medium, hence their use in the present study, 1,4, Culturing technique Seeds for soil experiments were sown in wooden boxes (51 x 45 x 10 cm) containing John Innes Potting Compost II which was obtained from the Botanical Garden, Legon. Seeds for culture solution experiments were sown in plastic bowls (about 35 cm in diameter and 14 cm deep) containing vermiculite, The containers were placed on a platform in the greenhouse. Gemination normally took place 11 to 21 days after sowing. Germinated seedlings in soil were allowed to grow for one month before they were transplanted. Those in vermiculite were allowed to grow only for three weeks before transplanting. The shorter time here was found necessary as it became more difficult to free the roots from the vermiculite Mien seedlings were allowed to grow for a longer period. At the time of transplanting into the respective media seedlings were mostly at the three to five leaf stage (K. Senegalensis^ or the two to three leaf stage (K. ivorensis). John Innes Potting Compost II was also used for subsequent growth of seedlings in soil eiiperiments. This medium allows good aeration while holding sufficient moisture for plant growth. As prepared by the Botanical Garden the mineral nutrient status of the soil was found to be adequate for growth of University of Ghana http://ugspace.ug.edu.gh the experimental seedlings and there were no harmful organisms since preparation included heat sterilization. A bulk quantity of this soil was obtained at the beginning of the experiments. Soil for each experiment was taken from this bulk to maintain uniformity of growing medium. This also meant that characteristics determined on samples from this bulk were applicable to soil used in each experiment. For culture solution experiments modified half strength Amon and Hoagland (1940) (see Hewitt 1952) nutrient solution was used. This culture solution was recommended by Dr N.I.C. Nwachuku of this department who had found it suitable for growth of tree seedlings. Seedlings were grown in polyethylene containers. These containers were preferred to porous clay pots because of their lightness and also because they prevented evaporation from their sides. Containers for the soil experiments -were 12 cm in diameter with a capacity of about one litre. Five drainage holes were bored at the base of each bucket and over these was placed a snail nylon mesh to prevent loss of soil particles. The containers were filled with soil to about 2.0 cm below the rim so that each held 1.4 - 1.5 kilogram soil at field capacity. There was one seedling in each container. Containers for culture solution experiments were 13.0 an in diameter with a capacity of about 1.1 litres. They were covered with black paper to prevent light reaching ihe solution in order to limit algal growth. Bach container held about 700 ml of culture solution, and was covered by a lid through which seven holes had been made for insertion of seedlings University of Ghana http://ugspace.ug.edu.gh and an aerator. Larger contsiners were used for culture solution experiments in which transpiration was measured. These were about 15 cm in diameter and about 2.5 litres capacity. 1.5. Experimental growth room Most of the experiments described in this study were carried out in a greenhouse in the Botany Department, Legon. Thermohygrographs were installed in the greenhouse to record temperature and relative humidity. Midday temperatures in the greenhouse before the actual experimental work were found to be too high (about 40°C); it was therefore found necessary to shade the house with palm leaves. / On certain occasions, Piche evaporimeters were installed in the greenhouse to record evaporation. Throughout the experimental period temperatures in the greenhouse never exceeded 36°Cs these normally occurred between 11.00 and 14.00 hours depending on the sunniness of the day. Minimum values were obtained at night and these never fell below 18°C. Minimum relative humidities were around 45 to 50$: these also were obtained between 11.00 and 14.00. Some transpiration experiments were carried out in the Research room of tiie Botany Department, Legon, where, through air conditioning, temperature and humidity were controlled slightly more precisely than in the greenhouse. Because of the near constant temperature maintained in this room, determinations requiring more precise temperature control were also carried out here. 24University of Ghana http://ugspace.ug.edu.gh 25 CHAPTER H v GROWTH IN RELATION TO MOISTUEE STRESS IN THE ROOT MEDIUM 2.1. Introduction. Normal plant growth is sensitive to both too much or too little water. Hunt (l95l), Walker (1962), Hosner and Leaf (1962), Gaertner (1963) aid Mueller — Dombois (19&4), observed that excess water is detrimental to most species presumably through poor aeration and the consequent reduction in root permeability (Kramer, 1956; Mees and WeatSserley, 1957). Several workers, however, indicate that water deficiency is even more important than excess water in limiting plant growth (Slavik, 1965; Kozlowski, 1968), Thus Marsden (1950, cited by Kozlowski, 1958) observed that drought in New England had been responsible for great reduction in growth of trees, poor fruit development, early leaf fall, dieback, thin foliage, transplanting failures, sunscorch and premature death of trees. In temperate climates, although duration of the growing season is controlled mainly by photoperiod and temperature, water deficiency during the growing season also has influence on growth of forest trees (Zahner, 1968). In tropical regions* the dry season is usually the least favourable period for plant growth, although the relative importance of moisture as against other environmental factors (for example, photoperiod and low night temperatures, Njoku, 1959J 1964; Longman, 1969) is little understood. University of Ghana http://ugspace.ug.edu.gh 26 Reduction of growth results because of the effects water deficit existing in the root medium has on internal water balance of plants. Plant growth is related directly to the internal water balance of plant (Kramer, IS63; Slatyer, 1967, 1970). In nature plant water status is controlled by the degree of soil moisture stress and the diurnal lag of absorption behind transpiration (Kramer, 1962; Slatyer, 1970). These are in turn controlled by environmental factors such as temperature, solar radiation, humidity and wind, the effects of which are modified by features of the plant which tend to reduce transpiration (e.g. leaf characters) or enhance absorption, (e.g. efficient root system). During the day transpiration usually exceeds absorption, internal water deficits develop, leading to a steep gradient in water potential between plant and soil. At night when transpiration is low or negligible this gradient promotes the absorption of water into the plant so that the internal water deficit is reduced or even eliminated (Slatyer 1967). The extent to which deficits are eliminated each night depends greatly on soil moisture status. When this is high deficits in plant are rapidly eliminated; but when the soil is dxy the overnight improvement in plant water status is limited. Thus the moisture status of the root-medium effectively sets the upper limit of plant water status at the beginning of each day, and this controls plant growth. It is probably for this reason that the effect on plant growth of water University of Ghana http://ugspace.ug.edu.gh stress in the root medium has been studied by a large number of investigators, such as Sates (l955$» Slatyer (1957b), Sands and Sutter (1959), Jarvis aid Jarvis (1963a, 1965), M.S. Jarvis (1963), Aspinall (19^5) and Lawlor (1969). Plant growth is the ultimate expression of the various processes that occur in the plant. It reflects the interaction of environment and plant genotype (cf Lirxmoore and Millington, 1971) and is known to be directly affected by water stress (Wardlaw, 1966). If environmental moisture has different effects on the two species in the present study, because of any adaptive features they possess, this should be reflected in their growth response to soil moisture stress. The experiments described in this chapter were carried out to compare the growth responses of the two species, K. senegalensis and X. ivorensis. to increases in moisture stress in the root mediun, both when grown on soil and in culture solution. 2.2. Growth response to moisture stress in soil. Seedlings which had been transplanted singly onto soil in 12 cm diameter containers (see p. 23 Chapter One) were allowed to grow until they were well established and their roots thoroughly permeated the soil mass. During this establishment period, which lasted about four weeks from transplanting, the soil was kept moist to reduce the development of stress in the plants. 27University of Ghana http://ugspace.ug.edu.gh 28 Seedlings of each species were divided into five matched groups (I, II, III, IV and V) of eight seedlings each, matching being based on leaf number and partly on height. This was done to reduce possible errors which might arise from differences in initial size of plants. Pour of the matched groups of seedlings (II - v) were assigned, each to one of four soil moisture treatments, designated A, B, C and D. Soil moisture level was controlled by pot weighing. Several workers (e.g. Sands and Rutter, 1959) have pointed out that in an experiment such as this soil moisture status is best expressed in terns of moisture tension since this indicates the force with which the soil holds water, and hence the ease with which the plant can obtain water from the soil. Therefore, in order to relate pot weights to soil moisture tension the moisture characteristic of the experimental soil was deteimined. The pressure plate apparatus was used to establish the relation between soil moisture content (as percent dzy weight) and soil water potential (bars) for the range 0 to -1.0 bars. A pressure membrane apparatus was used for the range -1.0 to -11.0 bars. It was not possible because of technical failure in the apparatus to extend the determination to -15.0 bars, which is generally accepted as the permanent wilting point for most soils (Black, 1957; Russel, 1961). The extension of the curve (Pig. 2) to this range was done by extrapolation. The pressure apparatus, as used here measures only the metric potential of the soil University of Ghana http://ugspace.ug.edu.gh 29 whereas total water potential of the soil includes an osmotic component (cf. Jarvis ana Jarvis, 1963a). No attempt was made here to correct for this osmotic component as was done by Jarvis and Jarvis (loc. cit.) since there was no good reason to expect this to be high (cf. Slatyer, 1967). Because the apparatus used was loaned frcm the Soil Science Department, Faculty of Agriculture, Legon, it was not possible to complete determination of the soil moisture characteristic before the experiment was begun. Therefore soil moisture levels in the various treatments were controlled simply by pot weighing throughout the experiment. Conversion of soil moisture contents to the equivalent potentials was done at the end of the experiment, A heavy duty top-pan Mettler balance (Mettler P3) was used for pot weighing so that weights could be obtained to an accuracy of +1.0 gram. The moisture content of the soil at field capacity, that is after standing in water to become thoroughly saturated and allowing the pots to drain for 24 hours was found to be 23.0+1.6$! of the soil dry weight. The pot weight at this point was taken to be 100 percent moisture status. Treatment A consisted of soil kept at near this point by daily addition of water. In the other treatments, the soil, initially at field capacity, was allowed to dry out respectively to 75, 50 and 25% of the weight at field capacity before rewatering. Soil water potentials (bars) corresponding to these weights were subsequently read from the curve in Pig. 2 and were as followsi University of Ghana http://ugspace.ug.edu.gh PI&. 2 THE RELATIONSHIP BSTWE3J SOIL WATER POTENTIAL (BARS) AND 3DIL MOISTURE OONTENT (% DRY ’3EEGHT). EOS THE EXPERIMENTAL SOIL, JOHN INNES POTTING- COMPOST H. (see also Appendix 2, table, i for statistical treatment op the DATA). University of Ghana http://ugspace.ug.edu.gh SO IL W AT ER PO T E N T IA L (B A R S ) 30 F I G . 2 University of Ghana http://ugspace.ug.edu.gh Treatment A (-0.3 bars), Treatment B (-0.4 bars), Treatment C (-0.6 bars) and Treatment D (-4.5 bars). This method of soil moisture control has been successfully used by previous workers (see for example Slatyer, 1957b). The experiment was carried out in the greenhouse. Over the experimental period (13 November 1970 to 12 January 1971) average temperatures were around 28°C at 09.00 hrs and 33°C at 15.00 hrs; relative humidity was around 60jFa (09.00 hrs) or (15.00 hrs). The general light intensity within the greenhouse was compared from time to time with light intensity outside. Two matched selenium barrier layer photometers (BEL Lightmaster Model 18) were used for this purpose. Internal lighting was about kOfo of that outside. The roof of the greenhouse was fairly heavily shaded to reduce mid-day temperatures (see>. p. 24 Chapter i). The experiment was carried out on a centre bench of the greenhouse; a randomized-block design was adopted. Randomization was based on a table of random numbers (Fisher and Yates, 1957). It was found necessary to re-randomize the groups and plant pots weekly because environmental conditions within the greenhouse varied appreciably from one end to the other. In order to examine as fully as possible how soil moisture stress might operate to control growth in the experimental plants, the growth analysis technique (Gregory, 1917) was used. The remaining group of seedlings, group I, was harvested at the beginning of the experiment. Length and breadth 31 University of Ghana http://ugspace.ug.edu.gh 32 of the leaves of each seedling were measured. The leaves were then blue printed on 'Ozalid’ paper and then planimetered to obtain their areas. From the regression of planimetered area on length x breadth (l x b) of these leaves (see .Appendix 2Sgraph I), leaf area of the unharvested plants could subsequently be derived by simple measure­ ments of length and breadth. Dry weights of leaves, stem and roots of the harvested plants were determined separately by oven-diying at 90 - 100°C for 24 hrs. The unharvested seedlings were allowed to grow on for approximately nine weeks. Periodic measurements of seedling height and leaf area were made over this interval. During this period K. senegalensis and K. ivorensis seedlings in Treatment JR received* overall, approximately 26 and 24- watering cycles respectively, while those of treatments C and D received 12 and 6 cycles respectively for both species. Pots in treatment D did not often reach the predetermined weights when wilting took place, therefore the criterion of watering in this group was the apparent inability of vdlted leaves to recover at sunrise. At the end of the nine weeks the seedlings were harvested; leaf areas and dry weights were determined as for the initial harvest. The initial weights of these seedlings were obtained from their initial leaf areas by using the relation between leaf areas and plant dry weight obtained from plants in the critical harvest (see Appendix 2, graph II)* University of Ghana http://ugspace.ug.edu.gh 33 (b) .Analysis of growth From the total diy weights (W-^ and and leaf areas (^^ and, A2) at times ^ and tg, relative growth rates (R.G.R.), net assimilation rates (N.A.H.) end mean leaf area ratios (M.L.A.R.) were calculated as follows: R.C-.R. (g/a/wk) = loge - loge t2 " *1 N.A.R. (g/m2/wk) = (W2 - W^) (loge A2 - loge A ^ A2 “ ^ ^ 2 “ V or * (»2 - V (ii) 2 M»L.A.R. (cm /g) = 2 + A 2 W1 W 2 Fisher (1920) showed that the above equation for R.G.R. is valid irrespective of the relationship between dry weight and time. The equation for IT.A.R and M.L.A.R. are however subject to certain limitations. Thus Williams (1946) emphasized that the use of the above equations for N.A.R. and M.L.A.R. is valid only when the total plant weight (W) is linearly related to the total leaf area (A) over the experimental period. In this experiment there were harvests on only University of Ghana http://ugspace.ug.edu.gh 34 two occasions (initial and final harvests) it was not possible, therefore to know the exact relation between W and A over -the period. Cocmbe (i960) has suggested that *en the relation between A ^ ^ is small (less than 2.0) the difference between the two methods of calculating 1T.A.R. is small. In the above experiment it m s found that for most of the treatments A^/A^ was less than two; the exceptions are for K. ivorensis in treatments B and D. N.A.R. was generally calculated by equation (i) except when A^ - A^ was zero when equation was used. The above equation for the M.L.A.R. was also used to sinplify the arithmetic. Distribution of dry matter between the main organs - root, stem and leaf - was compared between treatments, at the final harvest by calculating the ratios of dry weights of these organs to total plant dry weight, thus obtaining root-weight ratio (R.W.R.), stem weight ratio (S.W.R.) and leaf weight ratio (L.YiT.R.). These were expressed as percentages of total dry weight. In addition, root- shoot ratios (ly’s) and specific leaf areas (leaf area/leaf diy weight) (S.L.&.), were calculated. To bring out the comparison between the species more clearly all these parameters were further expressed as percentages of the values in treatment A. (c) Results During the experimental period it was observed that some of the leaves of K. ivorensis in Treatment A became chlorotic. Towards iiie end of the ej;perimental period, one of these seedlings University of Ghana http://ugspace.ug.edu.gh shed most of its leaves* "This seedling was not included in any of the final calculations. It was also observed that the length of the watering to drying-out cycles differed from pot to pot within the same treatment; the bigger seedlings with larger leaf areas took a shorter time to get to the predetermined weights. The tensions stated for each treatment are therefore averages of the tensions experienced by all the seedlings in that treatment before rewatering took place. Changes in height and leaf area of the seedlings over the experimental period are shown in Pigs. 3 and 4 respectively. Initial and final heights, dry weights and leaf areas, per plant together with the least significant differences (L.S.D., P = 0.05) between these values are presented in Table 2. A breakdown of the dry weights into leaf, stem and root fractions is given in Appendix 2, Table ii. In Table 3, growth functions and mean leaf area ratios derived from the primary data are similarly presented, together with leaf area ratios and ratios indicating the distribution of dry matter within and between the main organs at final harvest. To aid comparison, the derived data are presented again in Fig.4 and Table 4, as percentages of the values in Treatment A. K. Senegalensis seedlings were by far larger than K. ivorensis seedlings, both at the beginning and at the end of the experimental period. The larger initial size is presumably partly due to the larger seed size of K. senegalensis (see p. 15). This difference is reflected in the initial and final dry weights of the seedlings. The growth 35 University of Ghana http://ugspace.ug.edu.gh 36 Table 2 unary data for the analysis of growth of Khaya senegalensis and ivorensis seedlings under varying soil moisture treatment: A (-0.3); (-0.4); C, (-0.8); D, (4.5) bars; t and t^ indicate initial and lal harvest occasions respectively. K. senegalensis ait indices A Treatments B C D least sign diff. (P =0.05) m height per plant (cm) t 14.0 13.9 14.0 11.7 3.2 *2 16.5 16.7 14.6 14.6 3.7 sn total diy weight per ait (g) *1 0.751 0.829 0.876 0.813 0.097 *2 4.100 4.500 3.200 3.000 1.940 sn leaf area per ant (cm^) 114 125 103 114 52 *2 225 246 179 174 73 (continued next page) University of Ghana http://ugspace.ug.edu.gh 37 (Table 2 continued) K. ivorensis lant indices A Treatments B C D least sign. diff. (P=0.05) ean height per plant (cm) t^ 9.7 10.5 10.6 9.6 2.2 *2 12.6 13.3 13.1 12.7 2.3 ean total dry weight per lant (g) *1 0.460 0.410 0.430 0.370 0.130 *2 1.710 2.150 1.980 1.710 0.600 [eaj^leaf area per plant t 1 100 87 93 78 31 t2 169 189 182 153 31 University of Ghana http://ugspace.ug.edu.gh rates obtained for the two species are in general comparable to values obtained by Jarvis and Jarvis (1963a) for tree seedlings (aspen, pine and spruce) in a similar experiment using soil moisture tensions ranging from 0 to 4 atmospheres, and to values found by Ampofo (1969) for Afronaosia elata. a West African rain forest tree; but they are lower than most of the values found by Okali (1971b) for seme West African forest-tree seedlings in full day light, presumably partly because of the relatively heavy shade {kO per cent of full day light) employed in the present study. These comparisons suggest that, apart fran the shade, conditions of growth were relatively good in the present experiment. (i) Height growth At the beginning of the experiment mean heights of K. senegalensis ■ seedlings assigned to Treatments A, B and C were similar but significantly different from those of seedlings for Treatment D (Pig. 3)• This initial difference -was maintained and possibly increased by treatment, over the experimental period. K. ivorensis seedlings assigned to Treatments B and C were similar in height, but significantly taller ihan seedlings for Treatments A and D, which were themselves similar -when the experiment was started, Again, these differences appear to have been maintained, though slightly reduced, over the experimental period. For K. senegalensis. height of seedlings in Treatments A, B and C increased rapidly at the beginning of the experiment but slowed down by the sixteenth day. Seedlings in University of Ghana http://ugspace.ug.edu.gh M E M HEIGHT OF KHAYA SENEGALMSIS (KS) JND K. rVORENSIS (Kl) SEEDLINGS UNDER VARIOUS SOU MOISTURE TREATMENTS (BARS). Tlffi&mENTS: A (-0.3), B (-0.4), C (-0.8) M D D (-4.5). FIG. 3 University of Ghana http://ugspace.ug.edu.gh M EA N HE IG HT (C M .) 39University of Ghana http://ugspace.ug.edu.gh k0 Treatment D showed a slow rate of height increase throughout the whole period. TJnlike seedlings of K. senegalensis those of K. ivorensis showed a more uniform rate of change in height throughout the experimental period. Height growth was more or less linear in this case and there was no striking difference in rate between treatments. However, in both species, there was a tendency for seedlings in Treatment S to be taller than seedlings in all other treatments. This and the slow growth of K. senegalensis in Treatment D were the only observable effects of the treatments applied on height growth. (ii) Changes in leaf area. As with height, total l.eaf area per seedling of K. senegalensis was generally greater than that per seedling of K. ivorensis. both at the beginning and at the end of the experimental period (Fig. For both species initial leaf areas of seedlings assigned to the various treatments were more similar than were heights, and, by the end of the experiment, differences between seedlings in the various treatments were still not significant. This presumably was partly due to the large variability in leaf area of plants within any one treatment. In spite of this, a tendency for more rapid rate of leaf area increase, particularly up to Day 32, is indicated for K. senegalensis seedlings in Treatments A and B. A more uniform rate of leaf area increase is also shown by K. ivorensis seedlings. As vath height growth, seedlings in Treatment B, for both species, tended to have higher leaf areas. University of Ghana http://ugspace.ug.edu.gh PIG-, 4 LEAP AREA OP KHAYA SMEGALBNSIS (KS) JND K. IVORMSIS (Kl) SEEDLINGS UNDER VARIOUS SOIL MOISTURE TREATMENTS (BAHS). TREATIMTS: A (-0.3) 5 B (-0.4) j C (-0.8), D (-4.5). THE BROKEN LINES INDICATE ACCIDENTAL LOSS OP LEAVES WHEN THE GREENHOUSE WAS SPRAYED TO CONTROL INSECTS. University of Ghana http://ugspace.ug.edu.gh LE A F AR EA ( C M 2 ) a FIG.4 University of Ghana http://ugspace.ug.edu.gh 42 (iii) Relative growth rate. In general the relative growth rates of the two species (Table 3) appeared not to be markedly different. They ranged from 0.14 to 0.18 s/g/wk in It. senegalensis and from 0.15 to approximately 0.19 g/g/wk in K. ivorensis. The data presented in Pig.5 suggest that K. senegalensis was more sensitive to the diy treatment than K. ivorensis: its relative growth rate decreased steadily with soil diyness so that in the driest treatment (D) the rate was significantly reduced to 11% of that in treatment A. The differences between seedlings of this species in Treatments A, B and C were however not significant. In / terms of growth rates, the response of K. ivorensis seedlings can be considered as that to a wet treatment (Treatment A) and a dry treatment (Treatments C and D) (cf. Jarvis and Jarvis, 1963a). Relative growth rate was highest in Treatment B and tended to be reduced, though not significantly in either wetter or drier soil. The lcyest growth rate occurred in Treatment A, and although the rate in Treatment D was slightly higher than in C the difference is not significant. (iv) Het assimilation rate. Net assimilation rates obtained for K. senegalensis were much higher than those for K. ivorensis (Table 3). Like the relative growth rates, maximum net assimilation rate for K. senegalensis (22.6 g/m /wk) occurred in Treatment A and the minimum (15.0 g/m2/wk) in Treatment D, The difference between these two values is significant. University of Ghana http://ugspace.ug.edu.gh 43 Table 3 Derived growth data and ratios indicating distribution of dry matter between main organs for Khaya senegalepsis and K. ivorensis seedlings under varying soil moisture treatments; A (-0.3); B (-0.4); C (-0.8); D (-4.5) bars. K. senegalensis Plant indices A Treatments B C D least sign* dSttfifi. (P = 0.05) Relative growth rate (g/g/wk) 0.183 0.179 0.170 0.140 0.033 Net assimilation rate (g/m2/wk) 22.6 19.4 21.9 15.0 6.9 Mean leaf area ratio (cm2/g) 103.6 108.5 106.1 107.6 17.7 p Leaf area ratio (cm /g) 62.1 64.8 62.5 60.5 23.9 Specific leaf area (cm/g) 201.5 219.0 220.2 200.8 59.0 Leaf weight ratio {%) 31.7 29.4 28.6 29.2 2.8 Stem weight ratio {%) 25.0 25.5 24.7 24.2 3.2 Soot weight ratio (j?) 44* 1 45.3 46.3 46.6 6.2 Root shoot ratio (%) 80.0 81.1 87.7 87.5 13.9 $ continued next .'.page) University of Ghana http://ugspace.ug.edu.gh Table 3 (continued) 44 K. ivorensis Plant indices A Treatments B C D least sign, tiff*. (P = 0.05) Relative growth rate ; (g/g/wk) 0.150 0.189 0.165 0.171 0.060 Net assimilation rate (g/m^/wk) 11.3 14.9 13.3 13.5 3.0 Mean leaf* area ratio (cm2/g) 154.2 149.8 151.7 148.4 17.7 Leaf area ratio (cm /g) 93.0 88.0 91.3 88.4 17.7 Specific leaf area (cm2/g) 240.7 217.0 231.6 226.9 34.2 Leaf weight ratio (%) 59.5 40.6 37.7 58.3 5.9 Stem weight ratio {%) 26.7 24.1 24.8 25.1 3.5 Root weight ratio (%) 37.0 35.3 37.2 36.6 9.9 Root shoot ratio 55.2 55.1 59.7 58.6 11.4 University of Ghana http://ugspace.ug.edu.gh THE RELATION BEOTE® RELATIVE GROWTH RATE (R.&.R.), NET ASSIMILATION BATE (N.A.R.) OR MEAN LEAF A R M RATIO (M.L.A.R.) AND SOIL MOISTURE POTENTIAL (BARS), FOR KHAYA SME&ALENSIS ( & ) AND K. IVORMSIS ( 0 ) SEEDL3N&S. THE EATA ARE EXPRESSED AS PERCENTAGES OP THE VALDES IN TREATMENT A. 2TG-. 5 University of Ghana http://ugspace.ug.edu.gh M E A N L E A F N ET A S S I M I L A T l O N A R E A R ATI O R ATE (o/o) (o/o) (NAR) University of Ghana http://ugspace.ug.edu.gh RELATIVE GROWTH RATE <%) University of Ghana http://ugspace.ug.edu.gh 46 There was, however, no significant difference between either of these values and those obtained for seedlings in Treatments B and C. In K. ivorensis. the maximum net assimilation rate (14.9 g/m /wk) was obtained for seedlings in treatment B and the minimum (11.3 s/m /wk) for plants in treatment A, Unlike the situation with K. senesalensis no significant treatment effect was found between K. ivorensis seedlings. (v) Mean leaf area ratio. In all the treatments, mean leaf area ratios of K. ivorensis seedlings were greater than those of K. seneftalensis seedlings. The differences between treatments within the species were however not significant (Table 3 and Fig. 5). (vi) Leaf area ratio, specific leaf area and leaf weight ratio at final harvest. K. ivorensis had higher leaf weight ratio and specific leaf area ratio than K. senegalensis so that leaf area ratios at the second harvest were also higher in the former species (Table 3). Within the species, the treatments appeared not to have had any significant effect on any of these parameters, (vii) R&gfc weight ratio, root-shoot - ratio and stem-weiaht ratio. For both species the treatments apparently had no significant effect on any of these ratios although root weight ratio tended to increase with soil dryness. K, senefcalensis had proportionately more dry matter present in the root than had K. ivorensis. while the reverse University of Ghana http://ugspace.ug.edu.gh 47 Ratios indicating distribution of diy matter between main organs at final harvest, for Khaya senegalensis and K. ivorensis seedlings grown under various soil moisture treatments — A , (0.3)} B (-0.4) C (-0.8); D (-4.5) bars. The data is expressed as percentages of values in Treatment A. Table 4 K. senegalensis Plant indices A Treatments B C D Leaf area ratio 100 104 100 97 Specific leaf area 100 109 109 100 Leaf weight ratio 100 93 90 92 Stem weight ratio 100 102 99 97 Root weight ratio 100 103 105 106 Root shoot ratio 100 101 110 109 K. ivorensis Plant indices A Treatments B c D Leaf area ratio 100 95 98 95 Specific leaf area 100 90 96 94 Leaf weight ratio 100 103 95 97 Stem weight ratio 100 90 93 94 Root weight ratio 100 95 101 99 Root shoot ratio 100 100 108 106 University of Ghana http://ugspace.ug.edu.gh is true for the proportion of diy matter in the leaf (Table 3)» Similarly higher V s was obtained for K. senegalensis than for K. ivorensis seedlings. The ratio of stem weight to total dry weight was similar in both species. The greater proportion of root in K. senegalensis may be of adaptive significance, although, in the present experiment its advantage was not apparent. (a) Discussion The view of VeihSeyer and Hendrickson (1950) that soil moisture stress is equally available for plant grov/th within the available range (0 - 15 atmospheres) has been actively contested (Richards and Wadleigh, 1952; Slatyer 1957a; Stanhill, 1957)* According to Slatyer (19&7), this view was based primarily on field experiments with deep-rooted tree crops. It is now more generally accepted that growth may vary with soil moisture' status within the available range., (Richards and Wadleigh, 1952). In the present study, where potted seedlings were used, same reduction in growth of the two experimental species was expected, but the results presented show that any such reduction was small. Relative growth rate of K. senegalensis generally decreased with increasing soil moisture stress, so that there was a significant reduction for plants in Treatment D (-4.5 bars) as compared with those in Treatment A (-0.3 bars). In K. ivorensis however the rate vt&8 apparently not significantly affected by the treatments although there was a tendency for growth to be depressed in the wettest treatment (A). 48University of Ghana http://ugspace.ug.edu.gh In general net assimilation rate was lower in K. ivorensis than in K. senegalensis but this was compensated by higher mean leaf area ratios in the former species so that relative growth rates were very similar for both species. Mean leaf area ratios for both species showed very little response to the treatments so that reduction in relative growth rate of K. senegalensis iB response to dry soil was due to reduction in net assimilation rate; similarly the tendency for reduced relative growth rate in response to wet soil in K. ivorensis can be accounted for by reduction in net assimilation rate. Jarvis and Jarvis (1963a) obtained a similar relationship between relative growth rate and net assimilation rate in their response to soil moisture status with the species they studied. The apparent low growth rate for K. ivorensis in Treatment A, the wettest treatment, could be attributed to poor aeration. Kramer (l9§f») and Meesand Weatherley (1957) have pointed out that poor aeration in the rooting medium reduces root permeability sufficiently to cause large reduction in water uptake which may limit growth. Jarvis and Jarvis (1963a) observed a reduction in growth of aspen, birch and spruce in the wettest of the series of water regimes they practised in their study. They also suggested that poor aeration was responsible for low growth rates in response to wet soil. In the present study growth of one of the species, K. senegalensis. was however not reduced by Treatment A. The generally larger size of the seedlings and the amount of root they had may have caused rapid transpiration per plant and hence rapid soil moisture depletion, so that any excess water, which might have caused prolonged water logging after each watering operation, was rapidly removed. Transpiration data 49University of Ghana http://ugspace.ug.edu.gh 50 to be presented later support this s suggestion. The growth of K. senegalensis was significantly reduced by soil moisture stress of about 4*5 bars while that of K. ivorensis was not significantly reduced over this isnge. Sands and Rutter (1959) reviewed examples of tensions which have been found to reduce growth significantly in plants and came to the co&clusion that for most species considerable restriction of growth was caused by moisture tensions of about 0.5 atmospheres. In their own work wilii PiBus svlStestris. they found diy weight production by first year seedlings to be sensitive to this tension. Jarvis and Jarvis (1963a) however found that maximum growth of birch, aspen, spruce and pine occurred in soils at this tension but that diy weights of the conifers were reduced by about 335^ in soils dried to 1 .7 atmospheres and those of the engiospeims by about 10 - 20^ in soils at 2 atmospheres. M.S. Jarvis (1963) also observed that measurable growth (as assessed by leaf development) of Saxifraga hvpnoides and ffilipendula vulgaris ceased at tensions of 0.5 and 2.Q atmospheres respectively. Although she could not reach a definite conclusion it appeared that for the woody species, Prunus padus and Thelyorania sanauinea. growth ceased at soil moisture tension above 2 - 3 atmospheres. In contrast to the above results indicating low soil moisture tensions for the cessation of growth, Wadleigh and G-auch (1948) studying the effect of increasing total soil moisture stress on the daily elongation of cotton leaves, found that elongation ceased at a narrow range of stress values, close to 15 atmospheres. Slatyer (1957^) also observed that for privet and cotton increases in dry matter ceased at University of Ghana http://ugspace.ug.edu.gh 51 dawn values of leaf water status corresponding to the permanent wilting point for these ^ecies. It thus appears that a variable range (low or high) of soil moisture stress has been found by different workers to reduce plant growth. Some plants may be sensitive to low moisture tensions while others are sensitive to high tensions. The growth response described here indicate that K. senegalensis. a savanna species was more sensitive to moderate soil moisture stress than K. ivorensis which is a forest species. This result was not expected. M.S. Jarvis (1963) has however observed that high desiccation resistance and ability to grow well at moderate moisture stresses may be inversely correlated in seme species. Thus a plant with high sensitivity to small reduction in water potential may still possess strong drought resistance, because other features which promote drought resistance e.g. rapid / stomatal closure at relatively low moisture stress or slow rate of water loss per unit decrease in water potential of tissues, may be disadvantageous for growth. Stomatal closure increases carbon dioxide diffusion resistance and this could limit growth. Data to be presented later show that K. senegalensis possesses some drought resistant features. Interpretation of differences which arise in plant growth response to soil moisture stress is complicated by the difficulty of knowing the precise moisture stress experienced by plants, particularly at the root-soil interface, throughout an experimental period. 3h the present case it is possible that the correct stress which existed at the root—soil interface has not been adequately described. Since the same type of soil was used University of Ghana http://ugspace.ug.edu.gh and the two species transpire at different rates (see later), the stress experienced at the root surface may have been different (cf. Jarvis and Jarvis, 1963a), because of different rates of depletion of soil moisture in the root zone. The more rapidly transpiring plant could be expected to have experienced greater stress than the plant with a lower transpiration rate. Evidence to be given later indicate that K. senegalensis generally transpires more rapidly than K. ivorensis. It is possible therefore that the significant reduction in growth shown above for senegalensis was in response to more severe water stress than occurred in K. ivorensis or than was detected by weighing of the whole soil mass. The differences in growth of the two species might also have arisen because of variation in the length of time seedlings took to reach the predetermined weights before watering took place, so that the method of taking the mean soil moisture tensions of the individual seedlings in a treatment to represent the tension experienced by the group (see p. 35 ) was probably inadequate. M.S. Jarvis (1963), in order to obtain estimates of the amounts of effective soil moisture tensions experienced by the seedlings she worked with, summed up the areas beneath the drying out curves she obtained for seedlings in each treatment and expressed the results in percentage atmosphere-days. In this way the length of the drying out cycle was accounted for in each treatment. Jarvis (1963) also adopted a similar method for estimating effective soil moisture tensions in his experiments. In the present experiment however no attempt was made to account for the length of the 52University of Ghana http://ugspace.ug.edu.gh drying out cycle,* therefore, differences in the growth results which are due to this factor are not known. In addition to the above complications, Owen aid Watson (1956) indicate that experimental comparison of plant growth response to soil moisture stress is further confounded by the effect of re-watering on plants which had been subjected to drought. They observe that re­ watering could cause temporarily greater increases in dry matter production in plants which have been subjected to prolonged drought as compared with irrigated plants that had never been subjected to severe water stress. For the reasons outlined above it became necessary to examine further the growth responses of the two species to moisture stress in the root medium, this time using a root medium in which the water potential could be more precisely controlled; hence the water culture experiment which is described in the section that follows. By using a culture solution technique it was also possible to extend the severity of stress applied^ to see whether the pattern of response is similar under both high and low stress conditions. As a further improvement upon the soil experiment it was thought desirable to grow the two species in the same container so that there would be no doubt about the similarity of the water potential experienced by the two species at the root surface. This condition however introduces inter-species competition, but there was no good reason to expect large interference effects. 53 University of Ghana http://ugspace.ug.edu.gh 54 2.3. Srowtl re as onse 'to-.moistaitrsu, atre a a in culture solution. a« Method Seedlings for this experiment were raised from seed sown in vermiculite (see p. 22). They were transplanted into half-strength Amon and Hoagland culture solution (see below) held in two large (38.0 cm diameter) polyethylene containers. The solutions were not sterilized as had been done by previous workers (for example Jarvis and Jarvis, 1963d, 1965). The solutions were however aerated continuously, and the seedlings were allowed to grow in this condition until they were required for experiment. This took about four T®eks from transplanting. The seedlings were then divided into matched lots, corresponding to the number of pots to be used in the experiment. Matching was based on lsgf number alone; seedling height was not used for this purpose because at this stage seedling stems (particularly in K. ivorensis) were too short. After matching, the seedlings were carefully inserted through holes made in the lids of the experimental pots. They were supported with cotton v so o l. In order to ensure that the growth medium was the same for the two species in each treatment, each experimental pot carried three seedlings of each species so that there were six seedlings per pot. The experimental pots each held 700 ml half-strength Amon and Hoagland solution aerated continuously. The seedlings were allowed to grow for a further period of two weeks before the experiiaent was started. It was necessary after each transplanting operation, to University of Ghana http://ugspace.ug.edu.gh allow the seedlings to grow for some t ime so that any root damage caused by the transplanting operation could heal, Lawlor (1970) has shown that uptake of polyethylene glycols by plant roots is reduced if roots are undamaged. The total length of time allowed for recovery from any root damage in this experiment was six weeks - four weeks after the initial transfer from veimiculite and two weeks after transfer from large pre-treatment containers to experimental pots. Preliminary experiments, by growing the seedlings in different strengths of Amon and Hoagland (1943), (see Hewitt 1952) solution, showed that the half-strength concentration was optimum for growth of the seedlings. The basic culture medium used consisted ofj 55 Ca(N03)24H20 0.354 g HgSO^HgG 0.247 g KNO^ 0.404 g * V ° 4 0.136 g N\ H2P04 0.231 g "^Microelements 0 .5 ml Distilled water 1000 ml •i(The composition of tine microelements solution was: MnClg.W^O, 1.810 g/lj CuSO^.SHgO, 0.080 g/L; ZnSO^HgO, 0.220 g/L; H^BO^, 2.860 g/L; and (NH^MOyO^.i^O, 0.124 g/L). The seedlings were subjected to four levels of osmotic potential in the root medium: A (control, -0.3 bars), B (-2.8 bars), C (-5.3 bars) and D (-10.3 bars). Treatment A consisted of plants grown in the culture solution alone; the quoted osmotic potential for this medium is derived University of Ghana http://ugspace.ug.edu.gh 56 from the literature (Hewitt, 1952) for half-strength of solutions of identical composition. The solutions in Treatments B, C and D were made respectively by dissolving 100, 150 and 256 g of polyethylene glycol (molecular weight i»JOOO) in a litre of the half-strength culture solution. These weights were derived from the curve of Lawlor (1970, 3?ig. l) relating concentration to osmotic potential for polyethylene glycol of the same molecular weight. The osmotic potentials quoted , I are -therefore only approximate. Because of peculiarities in behaviour of polyethylene glycols (cf. Lagerweft et al, 1961), there was no ready means of measuring osmotic potentials of these solutions directly. Two containers were assigned to each treatment. The volume of solution in each container was maintained at 700 ml by daily addition of distilled water. The solutions were renewed twice over the experimental period of 21 days. The first renewal took place seven days after 12ie experiment had started and the second was ten days after the first renewal. This experiment also was carried out in the greenhouse. Over the experimental period (12 July 1971 to 2 August, 1971) average temperatures were around 26°C at 09.00 hrs and 30°C at 15.00 hrs; relative humidity was around 75/i (09.00 hrs) or 60% (15.00 hrs). The sky was never over­ cast during this period} this made' it impossible for light intensity measurements to be made as was done for the soil experiment. The light intensity might be lower then that encountered in the soil experiment. In all, growth conditions were comparatively less severe for this experiment than during the soil experiment. As in the previous case University of Ghana http://ugspace.ug.edu.gh 57 the experiment m s carried out on the centre bench of the greenhouse and a randomized block-design was adopted. At the beginning of the growth period, ten seedlings of K. senegalensis and nine of K. ivorensis were harvested. At this time alsoj marks were made with white paint a few millimeters above the pot lid, on the stems of the plants allowed to grow on. These marks served as the base line from which height measurements were subsequently made. Height and leaf area measurements were made at intervals during the experimental period. Leaf area and dry weights of leaves, stem and roots were determined for "the harvested plants as was done in the soil experiment. Similar determinations were made at the final harvest. As in the soil experiment, the results were examined by the growth analysis technique, b. Results Wilting was observed frequently in almost all the seedlings of the two species in Treatment D during the experiment. Occasional wilting was also observed in most of K. ivorensis seedlings in Treatment C. The results of the height and leaf area measurements are expressed in graphs (Pigs. 6 and 7 respectively). Table 5 shows the primary growth data together with the least significant differences (L.S.D, P = 0.05), while Table 6 presents the derived growth data and ratios indicating distribution of dry matter between the main organs for both K. senegalensis and K. ivorensis. also with the L.S.D. (P = 0.05). University of Ghana http://ugspace.ug.edu.gh 58 Table 5 Primary data for the analysis of growth of Khaya senegalensis and Kg. ivorensis seedlings in culture solutions of different osmotic potentials (bars)s A (-0.3);® (“2.8); C (-5*3); D (-10.3); and t„ indicate initial and final harvest occasions respectively. Treatment A consisted of culture solution alone, B, C and D consisted of culture solution plus polyethylene glycol. K. senegalensis Plant indices A Treatments B C D least sign diff. (P, 0.05) Mean height per plant (cm) 15.4 16.3 15.8 14.3 2.3 *2 16.6 17.3 16.2 14.4 2.2 Mean total dry weight per plant (g) . T. 0*359 0.406 0.459 0.418 0.117 *2 0.921 0.781 0.743 0.547 Q.223 Mean leaf area per plant (cm2) t 42.1 47.8 53.1 48.5 12.9 t 2 90.2 64.6 62.6 61.4 17.9 K. ivorensis Plant indices A Treatment B C D least sign diff. (P = 0.05) Mean height per plant (cm) t 10.1 12.4 10.4 9.7 2.3 t 2 11.3 13.3 11.0 9.9 2.0 Mean total dry weight per plant (g) t 0.205 0.205 0.209 0.201 0.104 t 2 0.370 0.312 0.202 0.236 0.176 Mean leaf area per plant (cm2) t]_ *2 32.1 61.0 31.9 45.7 32.5 35.1 31.1 34.1 16.4 25.2 University of Ghana http://ugspace.ug.edu.gh 59 In appendix 2, Table iii, is shown the breakdown of the total dry weights at the initial and final harvests into the weights for the main plant organs. Table 7 and Pig. 8 show the derived growth data and the other ratios presented as percentages of the values in Treatment A, Total dry weights of both species decreased with decreasing solution osmotic potential with those of K. senegalensis decreasing less than those of K. ivorensis. The relative growth rates and net assimila­ tion rates of K. senegalensis in this experiment were comparatively higher (maximum values are 0*31 s/gjweek and 29.2 g/m /wk respectively) than in the soil experiment, K. ivorensis on the other hand showed lower values here (maximum values being about 0,18 g/g/wk and 11.3 g/m /wk respectively) than in the soil experiment. In spite of these lower values for K. ivorensis. the rates on the Tiiole suggest that growth conditions were again relatively good, (i) Height growth J)’or both species changes in seedling heights over the experimental period were small (Pig,6), The intial heights of seedlings assigned to the various treatments were variable with those of Treatment D being the lowest (Table 5)• Differences between treatments were not significant for K. senegalensis seedlings while for K. ivorensis mean seedling height in Treatment B vsas observed to be significantly different from that of seedlings in i reatxaent D. idl’ferences between these and the other treatmen were however not significant. Changes in seedling heights over the experimental period both within and between the species appeared University of Ghana http://ugspace.ug.edu.gh MEM HEIGHT OF KHAYA SESEGALENSIS (KS) M D K. IVOHMSIS (El) SEEDLINGS GR0V3N IN CULTURE SOLUTEJNS OP DIFFERENT OSMOTIC POTE'TTIALS (BARS). TREATMENTS: A (-0.3); B (-2.8); C (-5.3); D (-10.3). TREAMENT A, CONSISTED OP CULTURE SOLUTION ALONE} B, C 1 D D CONSISTED OP CULTURE SOLUTION PLUS POLYETHYLENE GLYCOL. PIG. 6 University of Ghana http://ugspace.ug.edu.gh |o *i University of Ghana http://ugspace.ug.edu.gh '0 *1 University of Ghana http://ugspace.ug.edu.gh 61 similar. There was however a significant difference between heights of seedlings in Treatments B end D for K. senegalensis at the end of the experiment. The difference observed for K. ivorensis seedlings in Treatments B and D could be attributed to the initial difference between these seedlings. There was no marked difference in rate of height increase between seedlings under these treatments. (ii) Changes in leaf area Initial leaf area of seedlings assigned to the various treatments were similar within the species (Table 5). Leaf area of K. senegalensis seedlings were generally larger than those of K. ivorensis. Leaf development in both species was stimulated by Treatment A, so that -there was a rapid increase in leaf area throughout the experimental period (Fig. 6). This effect was comparatively more pronounced in it. senegalensis than in K. ivorensis. Leaf area increase was however affected when stress was imposed so that at final harvest (see Table 5) leaf area in both species, for plants in Treatment A was greater than that for plants in the other treatments. The difference between Treatments A and B plants of K. ivorensis was however not significant; that between plants in Treatments B, C and D was also not significant for either species. (iii) Relative yxowth rate Relative growth rate decreased significantly with decreasing osmotxc potential of the root medium (Table 6). Negative rates of growth were recorded for some seedlings of K. ivorensis in Treatments C University of Ghana http://ugspace.ug.edu.gh FIG. 7 LEAF AHEA OF KHAYA SMEGALENSIS (ivS) AND K. IVORMSIS (Kt) SEEDLINGS GROWN IN CULTURE SOLUTIONS OP DIFFEBMT OSMOTIC POTH'TTIALS (BARS). TREATMENTS A (-0.3); B (-2.8); C (-5.3); D (-10.3). TREATMENT A CONSISTED OP CULTURE SOLUTION ALONE: B, C AND D, CONSISTED OP CULTURE SOLUTION PLUS POLYETHYLENE GLYCOL. University of Ghana http://ugspace.ug.edu.gh LE AF AR EA (C M *) 62 FIG. 7 University of Ghana http://ugspace.ug.edu.gh 63 Derived growth data and ratios indicating distribution of dxy matter between main organs at final harvest, for Khaya senegalensis and K. ivorensis seedlings grown in culture solutions of different osmotic potentials (bars). A (-0.3); B (-2.8); C (-5.3), D (-10.3). Treatment A consisted of culture solution alone; B, C and D consisted of culture solution plus polyethylene glycol. K. senegalensis Table 6 Plant indices A Treatment B C D least sign diff. (P = 0.05) Relative growth rate (g/g/wk) 0.310 0.212 0.163 0.083 0.009 Net assimilation rate (g/m /wk) 29.2 23.5 16.8 9.1 6.7 lean leaf area ratio (cm /g) 107.8 100.9 100.9 106.6 14.6 Leaf area ratio (cm2/g) 98.0 84.0 86.0 97.0 14.6 Specific leaf area (cm /g) 225.0 221.0 215.0 210.0 28.2 Leaf weight ratio {%) 44.0 38.0 40.0 46.0 4.5 Stem weight ratio (/?) 24.0 28.0 29.0 27.0 4.5 Soot weight ratio {%) 32.0 34.0 31.0 28.0 7.3 Root shoot ratio 02) 51.0 53.0 47.0 40.0 11.3 (continued next page) University of Ghana http://ugspace.ug.edu.gh 64 Table 6 (continued) K. ivorensis Plant indices A Treatments 0 B C D least sign cliff. (P = 0.05) Relative growth rate (a/a/;*) 0.184 0.134 0.025 0.045 0.08 Net assimilation rate (g/m /wk) 11.3 9.3 1.8 3.3 5.4 Mean leaf* area ratio (cm2/g) 157.7 151.0 159.9 157.5 39.6 Leaf area ratio (cm2/g) 171.0 146.0 165.0 160.0 39.6 Specific leaf area (cm2/s) 311.0 318.0 320.0 317.0 71.1 Leaf weight ratio (%) 55.0 46.0 52.0 50.0 4*9 Stem weight ratio {%) 25.0 31.0 29.0 25.0 5.1 Root weight ratio {%) 20.0 23.0 19.0 25.0 5.8 Hoot shoot ratio {%) 26.0 30.0 24.0 33.0 6.8 University of Ghana http://ugspace.ug.edu.gh 65 and D. Fig. 8 suggests that K. senegalensis was more sensitive to moderate moisture stress (up to about -2.8 bars) than K. ivorensis. so that around this point relative growth rate of K. senegalensis was significantly reduced to 68% of -that in Treatment A, vrtiile that of K. ivorensis was still about 73?® of the value in Treatment A. However K. ivorensis showed greater sensitivity to moisture stress beyond -2.8 bars so iiiat at 5 .3 bars the rate was significantly reduced to 12# of that in Treatment A while the rate for II. senegalensis was reduced only to 53%. For K. ivorensis the percentage was however slightly raised to 21& in Treatment D (-10.3 bars) but this was still slightly lower than the percentage reduction for K. senegalensis which was about 27% at this point. The slight increase for K. ivorensis was accounted for by one large value out of six in ttie sample. Without this value which was 0.149 &/g/iak, the mean relative growth rate for K. ivorensis at 10.3 bars would be 0.024 g/g/wk vhich is about 13% of the rate in Treatment A. (iv) Net assimilation rate Net assimilation rates (Table 6) were higher for K. senegalensis than for K. ivorensis. The responses of net assimilation rate to decreasing osmotic potential for both species followed the same tread (Fig. 8) as was encountered with relative growth rate. It is remarkable that there was no significant difference between the net assimilation rates of seedlings of K. ivorensis subjected to Treatments A (-0.3 bars), and B (-2.8 bars). This further indicates the tolerance of this species to low moisture stress (cf. p. 51 ). University of Ghana http://ugspace.ug.edu.gh THE RELATION BETWEEN RELATIVE GROWTH RATE (R.&.R.), NET ASSIMILATION RATE (N.A.R, ) OR MEAN LEAP AREA RflTIQ (M.L.A.R.) AND MEAN WATER POTENTIAL OE THE BOOT MEDIUM FOR KHAYA SME&AMSIS ( & ) AND K. ITORMSIS ( 0 ) SEEDLING-S. THE DATA ARE EXPRESSED AS PERCENTAGES OF THE VALUE M TREATMENT A. FIG-. 8 University of Ghana http://ugspace.ug.edu.gh M E A N LE A F NE T A S S I M I L A T I O N RE LA T IV E G R O W T H AR EA RA TI O RA TE R A T E (o /o ) (° /o ) (° /o ) 66 (RGR) (N AR) (MLAR) WATER POTENTIAL(BARS) University of Ghana http://ugspace.ug.edu.gh 67 (v) Mean leaf area ratio K. ivorensis had higher mean leaf area ratios than K. senegalensis (Table 6). Within the species, there appear to be no significant treatment effects on this ratio. Thus the reductions in relative growth rates could not, again, be attributed to changes in mean leaf area ratio. (vi) Leaf area ratio, specific leaf area and leaf weight ratio at final harvest The results presented in Table 6 shows that leaf area ratio, specific leaf area and leaf weight ratio, at final harvest, for Kg ivorensis were greater than those for K. senegalensis. The osmotic potentials imposed apparently had no significant effects on any of these parameters. However specific leaf area for K. senegalensis decreased slightly with decrease in osmotic potential. (vii) Root weight ratio, root-shoot ratio and stem weight ratio. For both species the treatment effect did not seem to have followed a particular trend (Table 6). However for K. senegalensis the difference between root/shoot ratios of Treatments A and D was significant. K. ivorensis had the greatest root development in Treatment D and poor development in Treatment C, but there appears to be no marked difference in the root weight ratios of Treatment A and B plants. Stem weight ratio was also not greatly affected by the stresses imposed on the seedlings of both species. University of Ghana http://ugspace.ug.edu.gh 68 Ratios indicating distribution of dry matter between main organs at final harvest, for Khaya tables i to v. It was found that presentation of all the results in a University of Ghana http://ugspace.ug.edu.gh 81 graphical form complicated the curves for the individual treatments. For this reason only the results for Treatment A, Hie wettest treatment and Treatment D the driest, have been presented as curves (See Pig. 10). On all the five days maximum relative humidity was over 80$ and this occurred in the early mornings and evenings. Minimum temperatures ranging from 22 to 26°C were recorded also in the early mornings aid evenings. Minimum values of relative humidity ranging from 48 to 58$ were obtained between 10.00 and 16.00 hours G.M.T. Maximum daily temperature was over 30°C for all the five days with the highest value of 36° C occurring on 5 April. The minimum temperature value was obtained between 10.00 and 16.00 hours G.M.T. Although evaporation was not recorded, it could be assumed that the high temperature and low relative humidity values occurring generally between 10.00 and 16.00 hours were conducive to high evaporation around that time. The early morning and evening low temperature and high relative humidity values conversely reflect low evaporation rates. As expected from theory (Slatyer, 1967), Fig. 10 shows that under both moist and dry soil and with variable microclimatic conditions, the experimental seedlings started each day with a high water status in their leaves: over 90% relative water content, except on 5 April when K. ivorensis seedlings in Treatment D started with less than this percentage. As evaporation increased, relative water content decreased so that minimum values were attained usually between 10.00 and 16.00 hours. University of Ghana http://ugspace.ug.edu.gh PIG. ID DIURNAL VARIATION OP LEAP RELATIVE WATER CONTENT 02) ®OB KHAYA SEMBGALENSIS (LEST) iHD K. IVORENSIS (RIGHT) SEEDLINGS IN RELATION TO SOIL MOISTURE STRESS (BARS); (TBE4MMT Ay OPM SYMBOLS) AND (TREA355MT D. CLOSED SYMBOLS) ON SEVERAL DAYS; I (15/12/70), II (6/3/71), H I (16/3/71), IV (3Q/3/71), V (5/V71). (SEE APPMDIX 3, TABLE i TO v POR PURTHER DETAELS) University of Ghana http://ugspace.ug.edu.gh R EL A TI V E W AT ER C O N TE N T |« /o | 82 l O O r OS: A 9Ql F I G . I O 0600 10-00 1400 l»00 22-«B06-00 10-00 1400 1800 2200 T I M E O F DAY I H R S J k /• (continued next page) University of Ghana http://ugspace.ug.edu.gh R E L A T I V E WA TE R C O N T E N T (° /° ) 83 FIG ,IO .(CONT.) University of Ghana http://ugspace.ug.edu.gh 84 The relative water contents rose again in the evening when microclimatic conditions suggest lowered evaporation. Similar patterns are well documented for other plants (Weatherley, 1951; Rutter ena Sands, 1958; and Slatyer, 1962c). Comparison between the two species under similar soil moisture treatments shows that throughout most of the day seedlings of K. senegalensis maintained overall slightly higher relative water / contents in leaves than those of K. ivorensis. Over the experimental period, the maximum values obtained for Treatments A, B, C and D of K. senegalensis were 99.8, 99.4, 97.6 and 96.?!^ respectively, while the corresponding maxima for K. ivorensis were 99.2, 98.6, 97*7 and 95»0JS>. Minimum values for these treatments were, in the same order 84.6, 87.8, 85.0 and for K. seae^alensis and 81.5, 84.2, 81,5 and 72.0/t for K. ivorensis. G-enerally differences between the values for Treatments A, B and C were not great so that comparison between these three treatments appeared to be unprofitable. In contrast, the differences between plants in Treatments A and D were more appreciable. In these latter treatments and for both species, seedlings in Treatment D had lower relative water content values for most parts of the day than did those in Treatment A. Comparison within the species shows that, for K. senegalensis. relative water content curves fluctuated more often in Treatment A than in Treatment D (except in Pig. 10 I H and v): that is, seedlings in the latter treatment maintained more constant relative water content values throughout the day than those in the former. Unlike the situation vdth K. senegalensis. the curves for University of Ghana http://ugspace.ug.edu.gh seedlings of K. ivorensis in Treatment D generally fluctuated as often as those for seedlings in Treatment A (except in Fig. 10, Ij). Improvement in relative water content in both species generally started when environmental conditions were still severe, between 10.00 and 16.00 hours. As has already been stated, at night, all the seedlings tended to make up for water lost by their tissues during the day. Recovery of plant water status at night tended to be more complete when soil moisture was readily available. Although measurement was not made throughout any night, the trend towards better recovery in Treatment A, rather than in Treatment D plants was already dear each day by 22.00 hrs when measurement ended. Under this treatment also, seedlings of K. senegalensis appeared to recover more fully at night than those of K. ivorensis! the mean relative water contents at 22.00 hrs for the five days of observation were 96 and respectively for K. senegalensis and K. ivorensis. In Treatment D, relative water content values obtained for K. senegalensis seedlings were generally higher than those for K. ivorensis seedlings throughout most of the day and this was so even when K. ivorensis seedlings in Treatment D started the day with slightly higher relative water content values (Fig. 10, curve l) than K. senegalensis seedlings. Unlike the rest of the curves, curves obtained for the two species on 5 April showed comparatively lower relative water content values * for Treatment D. Recovery at night was also very poor, On that day the seedlings in this treatment started the day mLth much lower values 8 5University of Ghana http://ugspace.ug.edu.gh 36 of relative water content, 90.1$ and 87.8% respectively for K. senegalensis and K. ivorensis. This might have accounted for the low relative water content values obtained throughout the day. The dawn values of relative water content, which normally reflect soil moisture regime (Rutter and Sands, 1958), were as follows on the days when observation was made: Relative water content % Date of observation A K. senegalensis B C D A K. ivorensis B C D 15/12/70 95.3 97.0 94.1 94.6 95.3 - 93.3 95.0 6/3/71 98.6 96.3 96.1 95.1 96.9 95.8 94.5 95.0 16/3/71 96.7 99.4 97.6 94.3 95.8 98.6 97.7 92.5 30/3/71 98.1 99.1 93.5 93.4 95.2 97.2 95.2 90.3 5A/71.5 98.4 98.1 9 5.3 90.4 97.3 98.3 96.9 87.8 Means with standard error 97.3 +0.7 98.0 +0.6 95.3 +0.7 93.6 +0.8 96.1 +0.4 97.5 +0.6 95.5 +0. 8 92.1 +1.4 - No record Comparison of mean values for the five days shows that dawn values of relative water content for K. senegalensis seedlings in Treatments A and B were similar but different from those of Treatments C and D, which were themselves dissimilar. For K. ivorennis. values for seedlings in University of Ghana http://ugspace.ug.edu.gh 87 Treatment B was significantly higher than those in Treatments A, C and D. The values for Treatments A and C seedlings were similar but significantly higher than values in Treatment D. With the exception of Treatment A, where the dawn relative water content values for K. senegalensis were significantly higher than those of K. ivorensis. the values ft>r seedlings of both species experiencing tjie same treatment were similar. The minimum relative water content values obtained on -the experimental days were as follows: Relative water content (%) ox observation' K. senegalensis K. ivorensis A B C D A B C D 15/12/70 93.1 91.5 93.9 92.7 91.5 - 92.1 90.0 6/1 /71 89.3 92.6 91.3 91.2 88.4 89.5 90.5 89.9 16/3 /71 88.8 89.4 85.0 82.5 81.5 86.9 81.5 79.5 30/3 /71 84.6 87.8 88.8 89.8 84.1 84.2 89.2 84.7 5/4/71 89.6 89.7 88.2 7 6.3 84.4 87.4 89.6 72.0 Means with standard errors 89.0 +1.4 90.1 +0.8 89.6 +1.6 86.1 +,3*0 86.0 +1.8 87.0 ±1.1 88.6 +1.8 83.2 ±3.4 - No record Minimum relative -water content values for Treatment B of K. senegalensis University of Ghana http://ugspace.ug.edu.gh were significantly higher than those of Treatment D. These values were howevei not different from those obtained for the other treatments. Similarly for K. ivorensis with the exception of values in Treatment C, which irere significantly higher than those of Treatment D, these values were similar to those of the other treatments. (b) Leaf water potential The diurnal variations in leaf water potential are shown in Fig.11 and Appendix 3, tablesvi to x. As has already been indicated in the method (see p. 77) these data were derived from the measured relative water content values and the curves relating leaf water content to leaf water potential (see Fig. 20, p. 150). Although the relative water content values for K. senegalensis were higher than those for K. ivorensis these corresponded to lower leaf water potentials. Thus the leaf water potential values for K. ivorensis were generally higher than those of K. senegalensis. The'dawn values of leaf water potential were as follows: 88 Date of Leaf water potential (Bars) observation K. A senegalensis B C D 1 A K. ivorensis B C D 15/13/70 -6.5 -4.5 -8.0 -7.5 -4.0 - -6.5 -4.5 6/1/71 -2.5 -5.5 -5*5 -6.5 -2.5 -3.5 -5.0 -4.5 I6/3/7I -5*0 -1.5 —4.0 -7.5 -3.5 -1.5 -2.0 -7.0 30/3/71 -3.5 -2.0 -8.5 -8.5 -4.0 -2.5 -4.5 -9.0 5A/71 -3.5 -3.5 -6.5 -12.5 -3.0 -2.0 -2.5 -11.5 Mean-' with standard error -4.1 +0.8 -3.4 +0.9 -6.5 +0.8 -8.5 +1.1 -3.4 +0 .3 -2.4 +0.4 —4.1 ±0.8 -7.3 +1.5 -No record University of Ghana http://ugspace.ug.edu.gh DIURNAL VARIATION OF LEAP WATER POTENTIAL (BARS) FOR KHAYA SBfBfiALHTSTS (LEFT) iKD K. IVORENSIS (RIGHT) SEEDLING-S B i RELATION TO SOIL MOISTURE STRESS (BARS) j -0.5 (THEA3MENT A, OPM SYMBOLS) AND - i .^5 ( m « 5I D, CLOSED SYMBOLS) ON SEVERAL DAYSs ( I ) ISA ^TO , ( I I ) 3/ 6/ 71, (m ) 16/ 3/ 71, (17) 30/ 3/ 71, (V) 5/4/71*fsss AFPMDIX 3, TABLES v i TO x FOR FURTHER DETAILS) PIG-. 11 University of Ghana http://ugspace.ug.edu.gh LE AF WA TE R PO TE N TI A L (B A R S ) 89 TIME OF DAY (HRS. ) FIG.II (continued next page) University of Ghana http://ugspace.ug.edu.gh LE AF WA TE R PO TE N TI A L (B A R S) 90 T I M E O F DAY ( H R S . ) FIG.II CONT. University of Ghana http://ugspace.ug.edu.gh 91 Comparison of the mean using their standard errors shows that for K. Senegalensis. seedlings in Treatments A and B had similar dawn leaf water potentials; these were significantly higher than values in Treatments C and D which were themselves significantly different. For K. ivorensis the highest dawn values of leaf water potential were found for seedlings in Treatment B. Values for seedlings in Treatment D were significantly lower than those for seedlings in A and C. The latter were not significantly different from each other. Minimum values of leaf water .potential obtained on each day were as follows: Leaf water potential (Bars) Date of observation K. senegalensis A B C D A K. ivorensis B C D 15/1^70 -9.0 -11.0 -8.0 -10.0 -8.5 - -7.5 -9.5 6/1/71 -14.5 -10.0 -12.0 -12*0 -11.5 “9*5 *9.0 -9.5 16/3/71 -14.5 -34.0 -19.0 -22.0 -17.5 -12.0 il7.5 -18.5 30/3/71 -20.0 -16.0 -14.5 -16.0 -15.0 -15.0 -10.5 -14.5 5A/71 -14.0 -14.0 -15.5 -28.0 -14.5 -12.0 -10.0 -25.5 Means with standard errors -14.4 -13.0 +1.7 +1.0 -13.8 -17.6 +1.3 ±3.3 -13.4 ±1.6 -12.1 +1.1 -10.9 +1.7 -15.4 +2.0 - No record The mean minimum value of leaf water potential obtained for K. senegalensis seedlings in Treatment B was significantly higher than University of Ghana http://ugspace.ug.edu.gh 92 that of Treatment D. Values obtained for these and the other treatments were however similar. J?or IC. ivorensis the effect of soil moisture treatment on minimum leaf water potential values of seedlings in Treatments A, B and C were the same. The effect on Treatment D was however different from those of B and C in such a way that the minimum leaf water potential reached by Treatment D was significantly lower than that reached by Treatments B and C. (c) Diurnal variation in stem diameter. The diurnal patterns of variation in stem diameter are shown in Fig. 12: Cycles I, Hi and III, for 12 to 17 May, 18 to 21 May and 24 to 3 June respectively. The mean stem diameters recorded during the various cycles are shown in Appendix 3, Tables xi to xiii. The general pattern of s&xdnkage in stem diameter shown by the two species under the different soil treatments was similar to that obtained in the leaf water status study. The diurnal changes were appreciable. Maximum shmkage occurred around mid-day and there was, generally, recovery during the evenings (cf. Kozlowski, 1967). Steal shrinkage in K. senegalensis was generally greater than in K. ivorensis. Comparison between seedlings on soil at different stages of drying out or on soil at the same stages but on different days are perhaps not justified since environmental conditions may have varied (see Table 8). If comparison is allowed, it could be seen that stem shrinkage in K. senegalensis increased gradually with decrease in soil moisture content. Drying out Cycle III was the only case in which seedlings University of Ghana http://ugspace.ug.edu.gh PIG. 12 DIURNAL VARIATION OP STM DIAMETER FOR KHAYA SMEGALENSTS (LEFT) AMD K. IVORMSIS (RIGHT) SEEDLINGS IN RELATION TO SOIL MOISTURE CONTENT (%) DURING THREE SOIL DRYING-OUT CYCLES (CYCLES I, H AND III). STEM DIAMETER AT EACH TIME OP DAY IS EXPRESSED AS A PER* CMTAGE OP THE VALUE AT 06.00 HRS; THE FIGURE ACCOMPANYING EACH CURVE INDICATES MEM SOIL MOISTURE CONTMT (AS A PERCENTAGE OP THE VALUE AT FIELD CAPACITY) ON DAY OP OBSERVATION. * THESE RECORDS WERE MADE OUTSIDE CYCLE III, SEE TEXT P. 96 University of Ghana http://ugspace.ug.edu.gh 9 * 9 4 9 2 IOO 9 6 9 6 9 4 9 2 IOO 9 8 9 6 9 4 93 9 4 / T v v >.9 93 T*£-V64 - I 2 7 n-D- y ,•70 - O 9 4 1 o— a- J7 6 4 so in ' % / # - ■ 2 7 * DO 10 -00 1 4 00 18 0 0 22-00 06-00 10-00 1400 16-00 2 2 0 0 TIME OF DA.Y (HOURS) F I G . 12 University of Ghana http://ugspace.ug.edu.gh subjected to soil at about 50% moisture content generally showed less shrinkage than those on soil at 6l$. There was a slight shower on the day when the measurements at 50% soil moisture content in Cycle III were made, Although shrinkage increased with decreasing soil moisture content y down to about 50%, when soil moisture content fell to about 27'%, stem shrinkage was markedlv reduced in K. senegalensis (Cvcles I and II). Tie pattern of stem diameter change in response to soil moisture treatment in seedlings of IC. ivorensis were less variable than for K. senegalensis seedlings, particularly during Cycles II and III. In Cycle I, the shrinkage of seedlings tended to increase gradually with decreasing soil moisture content, from around 100 to 50%, but at 27% soil moisture content very little shrinkage occurred. This response is similar to that observed with K. senegalensis seedlings under this treatment. However K. ivorensis seedlings at about 27% soil moisture content in Cycle III did not show a similar response. In general it was difficult to repeat observations often enough with seedlings at 27% soil moisture content because stem shrinkage in both sp ecies continued to an extent that the pointers of the dendrometer were off the scale and the polystyrene material was no longer fitting firmly to the stem. Towards the end of Cycle I, there was only one seedling for both species, with the pointer still on the scale, so that the results given for 27% iu this cycle are based on measurements on one seedling of each species. .Although in cycles II and III no seedling of either soecies 94University of Ghana http://ugspace.ug.edu.gh 95 Experimental conditions during observations on stem diameter changes in seedlings of Kha^a senegalensis and K. ivorensis. Relative humidity and air temperature are taken froi? themiohygrograph records. Table 8 Date of Cycles Cycles Soil # Microclimatic data moisture content Relative humidity ($£) Air temperature (°C)OS) Min. Max. Min. Max 12/5/71 93 59.0 88.0 25.0 35.0 to I 64 54.0 91.0 23.5 36.0 51 54.0 90.0 24.0 35.0 17/5/71 27 54.0 90.0 24.0 35.0 18/5/71 94 61.0 94.0 22.5 32.0 to n 70 52.0 90.0 25.0 33.0 21/5/71 51 50.0 90.0 23.0 35.0 24/5 /71 94 52.0 94.0 23.0 34*0 to i n 64 50.0 88.0 23.5 34.0 28/5/71 50 68.0 98.0 24.0 34.0 3/6/71 27 60.0 94.0 23.5 33.0 University of Ghana http://ugspace.ug.edu.gh could be measured at TJ°/o for the reason given above, during routine weighing of pots, six days long after Cycle III was completed, a number of pots (two of K. senegalensis and four of K, ivorensis) were found in which the soil had dried .to around 27% moisture content. The pointers on the dendrometers on these- were also well within the scale hence measurements were made on the seedlings to supplement data already obtained. The curves obtained from these measurements are plotted along with the data for Cycle H I (see footnote in Fig. 12 ). Environmental conditions differed on the two occasions (Cycles I and Hi) when measurements mre made with soil at 27f° moisture content, and this may account for the difference in response of K. ivorensis seedlings between these occasions. Environmental conditions were very variable. During Cycle I, maximum temperature for the day was 35°C while relative humidity reached a minimum of 54$. During Cycle H I the corresponding values were 33°C and 60/2 respectively. Thus K. ivorensis seedlings subjected to 27% soil moisture content in Cycle I may have experienced more severe drying conditions than similar seedlings in Cycle III. It is possible that the more severe environmental stress (aerial and soil) was responsible for the reduced shrinkage of K. ivorensis in Cycle I when compared with similar seedlings in Cycle H I (cf. data for K. senegalensis). Thus K. ivorensis seedlings can probably regulate their water economy, only when external stress is severe. Since stem daiiLnkag.© for K. senegalensis seedlings at 27/* soil moisture content was similar on the two occasions, it may be that the regulatory mechanism operates in this species even University of Ghana http://ugspace.ug.edu.gh before environmental stress becomes very severe. Data to be presented later, on the relation between stomatal closure and leaf relative water content, support this suggestion. 3.4. Discussion. As discussed previously (Chapter II, p. 26 ) the water status of a plant is controlled by the relative rates of absorption and transpiration (Kramer 1962). Thus plant water status would be expected to change at any one time with variations in the evaporative demand of the air or with the water availability in the soil. (cf. Cowan, 19&5)• In the experiments reported above^ the water status of seedlings of K. senegalensis end K. ivorensis as measured by leaf water status or stem diameter changes was found to be high at dawn.. Thus, relative water content measured at dawn was over Styo for both species on most days and stem diameter was also large around this time. As evaporation increased later in the day plant water status decreased so that minimum values were obtained when evaporation was at its peak which was generally between 10.00 and 16.00 hours, &.M.T. Plant water status subsequently improved in the evenings when there was less evaporation and presumably also because of stomatal closure (see p. 131 ). Weatherley (1951) working on G-ossypium species, Rutter and Sands (1958) on Pinus svlvestris. Slatyer (1962c) on Acacia aneura. Kozlowski (I967) on Acer negundo. Fraximus smericana. Picea glauca and Pinus resinosa. and Klepper (1968) on Pyrus communis and Prtmug ameniaca. all obtained 97University of Ghana http://ugspace.ug.edu.gh 28 similar diurnal patterns in plant water status. In the present case comparison between the species under study shows that seedlings of K« senegalensis maintained in general a more favourable water balance in terms of relative water content than those of K. ivorensis. This might have arisen as a result of the greater volume of roots of this species than in the former species. The growth results reported in Chapter II showed that K. senegalensis had a higher mean ratio of root: shoot (about 0.8) than did K. ivorensis (about 0.6). This may have contributed to more rapid rate of water uptake in K. senegalensis then in IC. ivorensis. Slatyer (1955) observed a similar maintenance of higher water balance by grain sorghun, than by peanut or cotton. He attributed this largely to the more extensive xoot system of grain sorghun than in the other plants. The comparatively slightly higher relative water content at dawn for K. senegalensis than for K. ivorensis further suggest that a difference exists between the two species in the rate of mter uptake. Both species showed sensitivity to water loss by improving their water status #ien evaporative conditions wars still high. This could be a reflection of the pattern of stomatal opening and closure. Comparison was made between Treatments A, the wettest treatment, and D, "the driest treatment. The results showed that there wexe fewer fluctuation, in the relative to.ter content curves of Treatment D then in those of Treatment A for K. senegalensis. Unlike K. senegalensis fluctuations in the curves of .Treatments A and D seedlings of K. ivorensis seemed to University of Ghana http://ugspace.ug.edu.gh 9$ be the same. Comparison between the species showed that less fluctuations occurred in Treatment D of K. senegalensis then those of K. ivorensis. Slatyer (l962o) observed that diurnal fluctuations in leaf water content of Acacia aneura were more when the soil was moist than when the soil was dry. He attributed this to effective stomatal control when soil moisture was limiting. Thus it is possible that stomatal control of water loss by K. senegalensis seedlings was more effective than -that by K. ivorensis. Kramer (1963) pointed out that knowledge of relative water content as an indicator of leaf water status is meaningless unless it is interpreted in terms of leaf water potential, because plants with similar relative water contents may still be eiqperiencing different internal stresses. Hence, for effective comparison of the water status between the experimental species and their effect on plant processes, the relative water content values were interpreted in terms of leaf water potential. K. senegalensis leaves showed overall lower water potentials tiian did K. ivorensis seedlings. This may also have contributed to i±te comparatively more favourable water balance observed at dawn for K. senesalensis rather than for K. ivoransis. Low leaf water potential during tiie day may have steepened the water potential gradient between seedlings of K. sene^alensis and the soil, so that water was brought into the plant at night more rapidly than in K. ivorensis. This added to the more extensive root system in K. senefialensis may have been responsible for the slightly better water balance in this species than University of Ghana http://ugspace.ug.edu.gh 100 in K. ivorensis (cf. KLepper, 1968). Examination of the effects of the soil treatment on dawn values of leaf water potential in the seedlings of K» senegalensis (Pig. ll) shows that higher leaf water potential; were obtained in Treatments A and B than in Treatment C, and the lowest value was obtained in Treatment D. The highest dawn values of leaf water potential for K. ivorensis were obtained in seedlings in Treatment B. This was followed by the values for seedlings in both Treatments A and C. Here also, Treatment D had the lowest dawn values. Unlike the study of leaf water status, where discs are punched, saturated and dried before the value at any particular time is known, measurement of stem diameter is made directly on the seedling. There are thus likely to be fewer sources of error in the latter kind of observa­ tion; hence the recorded stem diameter changes are probably a reflection of variation in plant water status than are the measured changes in leaf water status. Diurnal sfamnkagja in stans of K. senegalensis was observed to be greater tin an that for K. ivorensis when soil moisture content was about 100 and 50%. This indicates that greater internal moisture stress developed in K. senegalensis than in It. ivorensis. This observation supports the conclusions reached from comparison of leaf water potentials in the two specie s. The latter were also lower in K. senegalensis than in K. ivorensis. The greater depression in the curves for K. senegalensis then in those for IC. ivorensis stem could possibly be attributed to University of Ghana http://ugspace.ug.edu.gh higher transpiration rates in the fomer species (see p. 129 )• apparently greater depression when soil moisture was high than when soil moisture was low for K. senegalensis may he a reflection of lower transpiration rate under low soil moisture content. The present study also shows that shrinkage in stem, when soil moisture was low (around 27% soil moisture content), was less in K. senegalensis than in K. ivorensis. This could be attributed to better stomatal control in the former species than in the latter species. Experiment to be described later (Chapter V) shows that IC. senegalensis closet its stomata at higher leaf relative water content than does K. ivorensis when leaves are allowed to dry out from saturation. Hence it was possible that water loss which could have * resulted in greater stem ditinkage occurred less often in K. senegalensis than in K. ivorensis as a result of stomatal control. As has already been stated, growth of plants is expected to be affected by internal water deficits. The results of ihe present study suggest that this expectation is realised only in a general way. Thus for example the high sensitivity of growth of K» senegalensis seedlings to low moisture stress in the root medium (see Chapter1 II) could be related to the apparently higher internal deficits experienced by these seedlings when comp-ared with those of K. ivorensis. Similarly the apparently lower growth rate of K. senegalensis seedlings in Treatment D could be attributed to the high deficits which developed in these seedlings. However the higher internal deficits, found here for K. ivorensis in Treatment D Aen compared with other treatments were 101University of Ghana http://ugspace.ug.edu.gh not reflected in the growth rate observed under this treatment. Other workers have also had difficulty in attempting to explain growth results in detail in terms of internal water deficits, ft!.8, Jarvis (1963) could not relate the differences she observed in the response of growth rate of Prunus aadus and Thelvcrania sanguinea to increases in soil moisture tensions in terms of differences in the relation between leaf water deficits and increase in soil moisture tension. Although Lawlor (1969) observed that the growth of ryegrass, cotton and maize in response to decreasing osmotic potential of the root medium were closely related to the leaf water potential which prevailed, he also found that decreased in growth of bean could not be accounted for by the leaf water potential. The investigations reported in this chapter suggest that differences between It. senegalensis and It. ivorensis. with respect to diurnal patterns of internal water balance, are slight. However, these slight differences reveal a trend which may be important in the water relations of the two species in nature. The lower leaf -water potentials and apparently higher tensions in stem for K. senegalensis. when environmental stress is not severe, suggest that under this condition a steeper water potential gradient exists between this plant and the soil, and this may contribute to more rapid water uptake than in K. ivorensis. At the same time, the lower leaf water potentials may mean that growth is more directly limited in K. senegalensis than in K. ivorensis when environmental stress is not severe, hence the lower growth rates already reported (Chapter II, p. 45). On the other hand, when environmental stress 102University of Ghana http://ugspace.ug.edu.gh 103 is severe (for example, when soil moisture content was reduced to about 27% of its value at field capacity), stem shrinkage in K. senegalensis rather than in K. ivorensis is more consistently reduced from its level at higher soil water contents. This suggests a better mechanism in K. senegalensis than in K. ivorensis for conserving water when stress is severe. Thus, although the differences are small, they provide some support for conclusions tentatively drawn from the growth rate studies. University of Ghana http://ugspace.ug.edu.gh 104 CHAPTER IV TRANSPIRATION IN RELATION TO MOISTURE STRESS IN THE ROOT IEDIM 4.1. Introduction Attention has already been drawn to the relationships between rate of water loss (transpiration), rate of water absorption and internal water balance of plants (see Chapter H). Plant water status at any one time reflects the rates of transpiration and absorption (Kramer, 1962). When the rate of transpiration exceeds that of absorption water deficits develop within.the plant, and when the rates are reversed there is an improvement in plant water status. It is well known (see Kozlowski, 1968) that during the day the rate of transpiration frequently exceeds that of absorption for most plants. A plant which is able to control its rate of transpiration effectively may be able to conserve water, and so have a water balance that is more favourable for growth than would a plant in which control of transpiration is poor. In -this chapter, transpiration rates of the study species are compared, particularly as’ these are affected by moisture stress in the root medium. Both the experiments on growth (Chapter H) and the observations on diurnal patterns of plant water status (Chapter Hi) indicate that differences, sometimes only slight, exist between the two species, with respect to their response to moisture stress in the root medium. The primary object of the comparisons described here University of Ghana http://ugspace.ug.edu.gh 105 B&s to examine the extent to which differences found in the above studies are related to differences in transpiration response. Since water loss from plants and C02 uptake by leaves occur mainly through stomata (Meidner and Mansfield, 1968), when other factors are favourable, transpiration rate should reflect the degree of stomatal conductivity and hence also the ease with which 00^ may diffuse into the leaves for photosynthesis. In this sense both transpiration and growth may be expected to respond broadly in a similar way to external moisture stress although it is known that additional factors affect the rate of CO supply to chloroplasts. To examine the correlation between transpiration and stomatal opening, especially as these are affected by water stress in the root medium, the diurnal ps.ttem of stomatal opening of the experimental species was compared. This comparison is also described in this chapter. 4.2. Methods (a) Transpiration Transpiration was studied mostly with plants growing on soil as the rooting medium. For reasons already discussed (see Chapter II, p.Jl) the study was subsequently extended to plants growing in culture solution. The pot-weighing technique was used in all cases. (i) Transpiration of plants rooted in soil. The diumal variation in transpiration of seedlings was measured on several days and under vaiying soil moisture oonditions. The University of Ghana http://ugspace.ug.edu.gh experiments were carried out with seedlings from the same s took as those used for the study of growth in soil. By the time of transpiration measurements the seedlings were about 4 to 5 months old, but were s till growing singly in polyethylene containers. Leaf area was determined, for each seedling prior to the measurements. Leaf areas of seedlings of both species were comparable at this p stage (about 286 and 292 cm*' for K. senegalensis and K. ivorensis respectively). Soil moisture treatments were as for the growth experiments, namely: Treatments A (-0.3 bars), B (-0.4 bars), C (-0.8 bars) and D (-4.5 bars). Two series of experiments were done. The first series m s performed in 'the greenhouse where records of evaporation, temperature and relative humidity were kept throughout each experimental period. Evaporation was measured with Piche evaporimeifcers with green paper disc (3 cm diameter). Lighting was natural daylight. Measurements were made on at least two seedlings of each species in each treatment. On raost days Treatments C and D were not represented because the soil had not dried to the prescribed limits. The second series of experiments was carried out in the research room where environmental conditions were better controlled. In this environment, temperature ranged between 23 and 24°C and relative humidity fluctuated between 56 and 70$, but was mostly around 60%\ light intensity, measured with an 'EEL' lightmaster (Model 18) was 106University of Ghana http://ugspace.ug.edu.gh between 19.0 and 22.0 Klux at plant level. This intensity was obtained by supplementing normal room lighting (fluorescent tubes) with Osram (150 W) reflector spot lamps. The light from this latter source was filtered through a 3 cm thick layer of running water to reduce the heating effect on the plant. Evaporation was again recorded with Piche^evaporimeters. Table fans were used to pass air over the plant at a speed of about 0.3 Vsec. The same seedlings as in the first series of experiments were also used here. Space did not allow seedlings in all treatments to be measured at the same time. Only four pots could be accommodated at any one time under the lighting and fanning arrangements made. Measurements were therefore staggered such that at each time two pots in Treatment A (one of each species) were always included. The remaining two pots, also one of each species were from any of the other treatments. However, on some days, all the four pots belonged to one species, talcing one from each of the four treatments. On experimental days, each pot was enclosed in a sturdy polyethylene bag, to prevent water loss from the soil surface. The bag was securely sealed with 'sellotape' around the stem. Measurements in the greenhouse were made by weighing the pots at hourly intervals starting at 07.00 and ending at 18.00 hours. In the research room the experiments were run from 06.00 to 18.00 hrs G-.M.T. Because of ihe slow rate of change in weight here, weighing was done at two hourly intervals from 06.00 to 10.00 hrs and from 16.00 to 18.00 hrs. 107 tUniversity of Ghana http://ugspace.ug.edu.gh Weighing was done at hourly intervals for the remaining period, that is, ) from 11.00 to 14.00 hrs, when rate of weight change was higher. From measured leaf areas and the recorded weight changes it was possible to calculate rate of transpiration in teims of unit area of 2 leaf (mg/cm /hr). The calculated rates were then plotted against time. (ii) Transpiration of plants growing in culture solution. Transpiration of seedlings growing in culture solution was studied also in the greenhouse. Measurements were made on seedlings that were about five months old. These seedlings had been maintained for about four months in aerated half-strength toon and Hoagland solution which was renewed periodically. The containers for these seedlings were 15 cm in diameter with a capacity of about 2.5 litres. \ These had air-tight lids through which holes had been made to admit the seedlings and an aerator. Each seedling was supported in its hole by two halves of a rubber bung. There were at least two plants in each container, except in one case of K. ivorensis which was too large to be studied with another plant. Between experiments the seedlings were aerated continuously through one hole in the lid. On each experimental day and immediately before measurements started the aerator was removed and the hole was blocked with a rubber bung so that the whole container was airtight. There were seven buckets in all, three of K. senegalensis and four of It. Ivorensis. Measurements were made on 26, 27 and 28 October 1971. Moisture 108University of Ghana http://ugspace.ug.edu.gh stress in the root medium was varied by replacing the culture solution in some containers with nutrient solution to which polyethylene glycol (Kolecular weight 1000) had been added to give an osmotic potential of about -10.3 bars. The concentration (190 g/litre) of polyethylene glycol which gives this potential was again calculated from the data of Lawlor (1970, Pig. l) (see p. 56 ). On each of the three days of measurement at least one pot of e§ch species was allowed to remain in culture solution to which no polyethylene glycol had been added. These served as controls against which the effect of adding polyethylene glycol could be observed. In order to compare transpiration of seedlings in culture solution with that of seedlings growing on soil, and also as a check on earlier measurements, plants growing in soil at field capacity were included in these experiments. There were two such pots for each species, and each pot carried one seedling. As before, temperature, relative humidity and evaporation were recorded throughout each experimental period. Each experiment lasted four hours and was performed in -the morning. V/'eights were recorded at intervals of 30 min. The short duration of each experiment was designed to reduce any effect of lack of aeration aid entry of polyethylene glycol into the seedlings (Slatyer, 196l). At the end of each experiment the polyethylene glycol solution was replaced with culture solution after rinsing the roots and containers with water. Aeration was then re-started in readiness for the next measurement. 109University of Ghana http://ugspace.ug.edu.gh 110 Transpiration was again measured as water loss per unit area of leaf (m^cm /30 min). (b) Stomatal conductivity and soil moisture stress. The method adopted for this study was the infiltration technique. This method has-been used successfully by other workers such as Alvin and Havis (1954) and Vomer end Ochs (1959)* Two sets of seedlings were used in this study. The first set were lferger seedlings that were just over one year old; the second set of seedlings were about eight-months old. The seedlings were subjected to the usual soil moisture treatments, controlled by allowing pots to dry to prescribed limits: A, -0.3 bars; B, -0.4 bars; C, -0.8 bars and D, -4.5 bars. For each species one pot from each set of seedlings was assigned to one of the treatments. The observations were again made in the greenhouse, where microclimatic records were kept. The degree of stomatal opening was estimated by using an infiltration series prepared from commercial kerosene and liquid parsC&in (cf. Halevy, l^^Oa, b; Shillo and Halevy, 1964; Puehring, Masaheri, Bybordi and Khan, 1966), in the proportions shown in Table 9. Preliminary trials with several mixtures including Nujol mixed with n-dodecane, Nujol mixed with xylene, had shown kerosene/paraffin to be the most suitable for these experimental leaves. Infiltration was scored on a scale which ranged from 1 to 14. An infiltration score of 10 for example means that mixtures containing up to 45$> liquid paraffin penetrated the leaf. University of Ghana http://ugspace.ug.edu.gh Ill Table 9 Composition of the infiltration series and the infiltration score they represent Kerosene (ml) Paraffin (ml) Infiltration score 10.0 0.0 1 9.5 0.5 2 9.0 1.0 3 8.5 1.5 4 8.0 2.0 5 7.5 2.5 6 7.0 3.0 7 6.5 3.5 8 6.0 4.0 9 5.5 4.5 10 5s0 5.0 11 4.5 5.5 12 4.0 6.0 13 3.5 6.5 14 University of Ghana http://ugspace.ug.edu.gh 112 Measurements were made on nature leaves that were at comparable positions on the plant axis. Observations were made on 3, 14- and 17 June. On all the days seedlings from all four treatments wre used. On 3 June smaller seedlings from Treatments A and D were measured in a separate experiment. Determinations were made by putting a drop of the mixture on the abaxial surface of the leaf, starting from mixtures with low infiltration scores and increasing the scores till there was no penetration. The score of the last mixture which penetrated was recorded. A high infiltration score means the stomata are widely open. For the larger seedlings 2 to 3 leaflets of a leaf were tested, while 1 to 2 leaves of the smaller seedlings were tested at each determination, A day's deteminations were made on different parts of a selected leaf. 'When all parts of the leaf were used up other leaves were selected for the continuation of the day's experiment. At the end of each experiment the mean infiltration scores at each hour were calculated and plotted against time of day. 4.3. Results (a) Transpiration. (i) Transpiration of plants rooted in soil. Transpiration rates of seedlings of K. senegalensis and K. ivorensis growing in 3oil in the greenhouse, together with some microclimatic data, are shown in Figure 13, for 4 and 12 February, and, in Appendix 4, University of Ghana http://ugspace.ug.edu.gh 113 Tables i to iv. the data for 8, 5> 10 an|i H February are given. 7 Transpiration rates recorded in the research room are similarly summarized in Pigs. 14 and 15 and in Table 10. Microclimatic data collected in the greenhouse over the experimental period show that the rate of evaporation was generally low in the early mornings. Records made up to 08.00 hrs ranged from 0.1 to 0.4 ml/hr. The rate of evaporation increased from 11.00 to 15.00 hrs. Maximum values obtained on the six days ranged from 0.6 to 0.9 ml/hr. The rate of evaporation decreased after 15.00 hrs so that by 18.00 hrs ’shen the experiments ended, the range was from 0.1 to 0.4 ml/hr. The pattern of evaporation adequately reflects changes in humidity and temperature. Considering first the experiment in the greenhouse, the general pattern observed was a low transpiration rate in the morning when evaporation was low. The rate increased to a maximum from about 11.00 to 15.00 when evaporation was also high; there was generally a reduction in transpiration rate in the evening vhen rate of evaporation was low. Comparison between the two species shows that, except on 8 Pebruaiy when rates in Treatment B were observed to be t he highest, the transpiration rates on the other days generally decreased with soil dryness. The rates observed for K. senegalensis seedlings were generally higher than those for K. ivorensis seedlings especially viien soil moisture was resdily available. However the rate when the soil was diy tended to be slightly University of Ghana http://ugspace.ug.edu.gh PIG. 13 TRfflJSPIBmON OF sbsdlin&s o f khaya s m e g a l m s i s (ks) and K. 3TVDBMSIS (KC) IK DELATION TO .90IL MOISTURB STRESS, BARS; V ( - 0 . 3 ) , * (-0.4), C3 (-0.8), AND » (-4.5); TOGETHER HTH M1C10CUMATI0 MSA (K) - AIR TMPERATERE (•— •), RELATIVE HUMIDITY ( O ™ O ) WD EVAPORATION (HISTOGRAM). FOR h/2/71 (1SFT) AND 12/2/71 (EIGHT), GREEN HOUSE University of Ghana http://ugspace.ug.edu.gh TEMPERATURE «C. T R A N SPIR A T IO N RATE ( M G / C M 2 / H R ) H £ o o ° o O ■ M EVAPORATION ( ML ) 6 ° - „ * -4 • « 2 r e l a t i v e h u m i o i t y ( • / • ) .S 5 o O O O O University of Ghana http://ugspace.ug.edu.gh 115 4ii lower in K. senegalensis than in K. ivorensis. Thus taking all experimental days together, the maximum rate for It. senegalensis in Treatments A, B, C and D respectively were 21.8, 21.5, 15*6 and 6.7 o mg/cm /hr. Those for K. ivorensis were 18.1, 16.0, 13.3 and 9.5 2mg/can /hr for corresponding treatments. The degree of fluctuation in the curves for the two species tended to decrease with soil dryness. On 12 February no fluctuation * was observed in the transpiration rate of Treatment D plants of K. senegalensis. The fluctuations in both species generally occurred between 11.00 and 15.00 hrs when the rate of evaporation was high. Figure 13, curve I shows the daily mean rates of transpiration recorded for seedlings of K. senegalensis and K. ivorensis throughout the experimental period in the research room. Curves II and III show the daily means for Treatments A and D respectively. The means are made up of the date, from five and two experiments respectively for Treatments A and D. The transpiration rates recorded in the research room for both species were lower than those obtained in the greenhouse. Maximum. rates for K. senegalensis seedlings in the research room (see Table 10) for Treatments A, B, C and D respectively were 5.0, 4.7, 4.3 and 3.8 2 mg/cm /hr. Corresponding rates for K. ivorensis were 4.8, 4.7, 3.8 and 2 3.1 mg/cm /hr. These values are considerably less than those quoted above for corresponding treatments in the greenhouse, Evaporative conditions in these environments cannot account for all this difference. University of Ghana http://ugspace.ug.edu.gh 116 The recorded maximum rates of evaporation (compare Pigs, 13 snd- 14) are only slightly higher for the greenhouse. The lower temperature in the research room could, conceivably, have had a more direct effect on the plant, perhaps through controlling stomatal opening (see Meidner and Mansfield, 1968) or the rate of water uptake and transport (Slatyer 1967). The general diumal pattern obtained -with seedlings in the green­ house was repeated in the research room. Thus lower rates of transpiration were recorded in the mornings and evenings, and the highest rates were recorded between 11,00 and 15.00 hrs. However unlike the greenhouse experiment, this pattern was distinctly less correlated with the evaporation rate in the research room. Thus transpiration rate was low in the evening while evaporation was still high. Further, a distinct drop in rate of transpiration was observed on most days, and for both species, betvreen 10.00 and 12,00 hrs. This drop was more pronounced when soil moisture status was high (Fig. 14, Curve II) than when it ?jas low, end is suggestive of the so-called 'mid-day closure’ reported for many plants (see Meidner and Mansfield 1968; Slatyer, 1967). He-examination of the transpiration curves for plants in the greenhouse (i1ig, 13) suggests that this depression also occurred to varying degrees in all Hie treatments on 8 February, The reasons for its marked occurrence in the research room are not known, but taken together with the low transpiration rates in the evening when evaporation was still high the phenomenon suggests that University of Ghana http://ugspace.ug.edu.gh PIG. 14 DIUOTAL VARIATION IN TRANSPIRATION (B&/GM /HR) OF SEEDLINGS OP KHAYA SME&AL3MSIS ( & ) AND K. IVORMSIS ( Q ). I MEM TRANSPIRATION RATE FOR SEEDLINGS IN ALL TREATMENTS. H AND III - MEAN TRANSPIRATION RATE FOR SEEDLINGS IN TREATMENTS A AND D RESPECTIVELY. (HISTOGRAM - EVAPORATION (ML)) University of Ghana http://ugspace.ug.edu.gh so 4 0 3 . 0 a-o 6 0 S O 4 0 3 0 2-0 S O 4 0 3 - 0 2-0 117 (II - 0-8 ■ 0-6 ■ 0 4 - 0 2 ■0-0 (II) - O- 6 - 0 - 4 - 0 2 - 0-0 llll) O O 0 8 - 0 0 1 0 - 0 0 1 2 0 0 1 4 - 0 0 1 6 0 0 l « 0 0 T I M E O F DAY ( H O U R S ) F I G- I 4 • 0-8 - O * - 0 - 4 - O- 2 - O O E V A PO B A T IO N (M L ! University of Ghana http://ugspace.ug.edu.gh the patterns of transpiration rate observed in the research room reflect more the characteristics of the plants or their response to soil treatment rather than to the aerial environment. Curve I (Pig. 14) shows that transpiration in K. senegalensis was generally higher than in It. ivorensis. and that the rates obtained during certain periods of the day were significantly higher in the fonaer species. As in the experiment in the greenhouse, in Treatment A, K. - senegalensis seedlings transpired more rapidly than those of K. ivorensis. However the results for Treatment D were in contrast to that obtained in 'the greenhouse experiment. Here the rate for K. senegalensis remained higher than that for K. ivorensis which is the reverse of what was observed in the greenhouse. In Table 10 are the mean transpiration rates for seedlings in all four moisture treatments compared at thfc same time on 19 February and 1 March (for K. senegalensis) and on 21 February and 2 March (for K. ivorensis). These data are presented again in Fig. 15 where each value is expressed as a percentage of the rate in Treatment A at the same time of day. Transpiration of K. senegalensis was generally greater in Treatment B than in Treatment A. Thus most of t he points were observed to be more than lOO/o. Transpiration rate of seedlings of this species in Treatment D was however generally lower than that of those in Treatment A. On both days of measurement transpiration of IC. senegalensis in Treatment A was markedly reduced between 13.00 and 14.00 hrs, such that the percentage rate obtained for all the treatments including D was more than 100?« of the rate in Treatment A during this time of the day. 118University of Ghana http://ugspace.ug.edu.gh 119 Diurnal variation in transpiration of seedlings of Kha.ya senegalensis and K. ivorensis under varying soil moisture treatments (bars). Treatments A (-0.3), B (-0.4), C (-0.8) and D (-4.5) bars. (The experiment was done under semicontrolled environment). Table 10 Time of day (hrs) Transpiration rate K. A senegalensis B C D A K. ivorensis B C D 06.00-06,00 2.2 2.9 2.9 2.0 3.3 3.2 1.9 2.1 08.00-10.00 3.6 4.0 3.9 3.0 4.4 3.8 2.1 2.5 10.00-11.00 3.8 4.6 4.2 2.7 4.4 3.5 2.5 3.1 11.00-12.00 5.0 4.7 4.3 3.8 3.6 3.6 2.9 2.5 12.00-13.00 4.6 3.7 3.6 2.7 4.8 4.7 2.3 2.8 13.00-14.00 2.6 4.1 3.3 2.9 4.7 3.6 2.5 3.1 14.00-16.00 4.0 4.6 3.3 2.7 4.3 3.9 3.8 2.6 16.00-18.00 2.5 3.2 2.1 1.5 3.5 3.0 1.8 1.7 University of Ghana http://ugspace.ug.edu.gh PIG-. 15 DIURNAL VARIATION IN TRANSPIRATION OP SEEDLIN&S OP KHAYA SBJSGALM3IS AND K. ITORMSIS SUBJECT3D TO VABYIN& SOIL MOISTURE TRBATMMTS - B ( 0 , -0.4), C ( O , -0.8) AND D ( ■ , -4.5) BARS. THE BATA ARE EXPRESSED AS PERGENTA&BS OP VALUES IN TREAM2NT A (-0.3 BARS). University of Ghana http://ugspace.ug.edu.gh 120 T I M E O F DAY { H O U R S ) F I G . I S University of Ghana http://ugspace.ug.edu.gh P ig .15 also shows th a t unlike transpiration of K. senegalensis, transpiration of K. ivorensis decreased consistently Yjith increasing soil dryness such th a t Treatment A seedlings transpired more rapidly than those of Treatment B, which in turn transpired faster than seedlings in Treatments C and D. The rates for seedlings in the latter two trea tm en ts followed each o th e r closely so that th e curves obtained for them overlap at several points (Pig. 15). Calculation of mean daily transpiration in the greenhouse for Treatments A and D (from data given in Pig. 13, left and right) and , 2 for both species shows that water loss per cm and day for K. senegalensis in Treatment D was 29.3$ of the rate in Treatment A. The corresponding reduction for K. ivorensis was to 47.3$. However, in the research room, the mean daily transpiration in Treatment D as a percentage of Treatment A on days when measurements were made on seedlings from both treatments together were 84.6 and 81.8$ respectively for K. senegalensis and K. ivorensis. It is clear from the results obtained in the research room that transpiration rate of both species was again markedly reduced by the driest treatment (D). K. ivorensis seedlings however appeared to be comparatively more sensitive than those of K. senegalensis under this slightly controlled environmental condition. (ii) Transpiration of plants growing in culture solution The results for this esq?eriment, together with the data collected at the same t ime for seedlings growing in soil, are shown in Pig. 16. The data for evaporation are also given in the figure. Temperature and relative humidity < c* e_s. ? r-.-vs1 121 University of Ghana http://ugspace.ug.edu.gh PIG-. 16 TRANSPIRATION BATE OF SEEDLINGS OF KHAYA SEWEflALBNSTS (LEFT) AND K. IVORmSIS (EI&HT) IN RELATION TO MOISTURE SDBSSS IN THE BOOT MEDIUM:-0.3 MSS MATHIS POTOTTIAL (OPEN SYMBOLS), -0.3 BASS OSMOTIC POTENTIAL ( T ) AND -10.3 BASS OSMOTIC POTMTIAL ( ■ ) (HISTOGRAM - EVAPORATION (Ml)) (i) 25/l0/?l, (il) 26/10/71, (m ) 27/10/71. University of Ghana http://ugspace.ug.edu.gh T R A N SP IR A TI O N (M G /C M /3 0 M IN .) 122 T I M E OF DAY ( H R ! ) TIME O F DAY [HRS) FI G. 14 E V A P O R A T IO N (M L ) University of Ghana http://ugspace.ug.edu.gh 123 data during the experiments are presented in Appendix 4, Table v. The transpiration rate recorded, for both species, with seedlings growing in soil here, are comparable to data presented above for seedlings under similar soil moisture treatment (-0.3 bars) (cf. ]?ig. 13)* (Note that transpiration values in Fig. 16 are plotted as rates for 30 rain intervals). Comparison of the soil-grown plants here with seedlings in culture solution (-0.3 bars) shows that for K. senegalensis the rates of transpiration of both types of seedlings were closely similar. Thus the effects of both matric and osmotic potentials on transpiration of this species were the same. K. ivorensis seedlings however did not show the same similarity. The curves obtained for seedlings growing in soil were consistently higher than for plants in culture solution. One of the seedlings used in the culture solution experiment had a greater absolute leaf area so that there may have been a considerable degree of mutual shading among its leaves. This may have caused the low transpiration rate recorded for this species under this treatment. However the sensitivity of K. ivorensis to poor aeration has earlier been pointed out (p. 49 )• It is possible that stopping the aeration (see Methods) during transpiration measurements may have contributed to reduce transpiration of K. ivorensis seedlings in culture solution. Considering the effect of osmotic potential on transpiration rate of the two sfjecies,it was observed that transpiration decreased with decrease in solution osmotic potential. The maximum rate of transpiration for K. senegalensis, taking the three days together, were 8.8 and 2.5 University of Ghana http://ugspace.ug.edu.gh o » Dig/cm /30 min respectively for Treatments A and D. The transpiration rate of seedlings in the latter treatment remained fairly steady for the first lj - 2 hr after exposure to the polyethylene glycol solution; thereafter, it declined steadily while that of seedlings in Treatment A continued to increase. The mean rates obtained for the three days at 11.00 hrs (that is, four hours after the start of the experiment) were about 7.0 and 0.6 mg/cm /30 min for Treatments A and D respectively. Maximum transpiration rate of K. ivorensis in Treatment A was 4.6 2 2 mg/cm /30 min, and that for Treatment D was 1.2 mg/cm /30 min. There was an increase in the initial transpiration rate of seedlings in Treatment A during the experimental period. The initial rates in Treatment D however appeared to have been maintained throughout the experimental period. The mean maximum rates reached on ihe three days 2 at 11.00 hrs were 3«0 and 0.6 m^cm /30 min respectively for Treatments A and D. Transpiration rate of Treatment A plants of K. senegalensis was greater than that of the corresponding K. ivorensis seedlings. This could be due to the mutual shading which occurred in K. ivorensis leaves, but the results from the other transpiration experiments reported earlier suggest that it could also be a reflection of real species difference. Under Treatment D, K. ivorensis seedlings seemed to have shorn more immediate sensitivity to the moisture stress than did K. senegalensis seedlings. Decrease in transpiration rate from the initial value occurred 12kUniversity of Ghana http://ugspace.ug.edu.gh frequently earlier in the former species. The rate of transpiration of K. senegalensis under this treatment was also initially greater than that of K. ivorensis. hut by the end of the experimental period, the rate was similar in botyi species. On the whole, these results suggest that reduction in transpiration rate, as a result of decrease in solution osmotic potential to about -10.3 bars, was greater- in K. senegalensis than in X. ivorensis. Thus within four hours of applying the treatment, transpiration rate of treated K. senegalensis was reduced to less than 10^ of that of untreated plants. The corresponding reduction in K. ivorensis was to about 20^. (b) Effect of soil moisture stress on stomatal conductivity. Patterns of stomatal conductivity are illustrated in Pigs. 17 and 18. Pig. 17 shows conductivity on 14 June for smaller^and 17 June for larger seedlings respectively of all the four treatment for the two species; while Fig. 18 shows conductivity of leaves of Treatments A and D of the two experimental species on 3 June. Ta Appendix 4, Table vi, are presented data for the smaller seedlings from all treatments recorded also on 3 June. Higher infiltration scores (maximum about ll) were commonly recorded for K. senegalensis than, for K. ivorensis (maximum about 9)* The period during fiiich stomata were open and the degree of opening decreased with increasing soil moisture stress, so that stomata were widely open when soil moisture m s at -0,3 bars,. 125University of Ghana http://ugspace.ug.edu.gh JIGfc 17 CURVES OP STOMATAL CONDUCTIVITY OF LEAVES OF KHAYA SMjjGk&LMSIS (KS) AND K. IVORMSIS (Kl) MDER VARYING SOU MOISTURE TREAMENTS (BARS). A (-0.3), t ), B (-0,4, 0 ), C (-0.8,0 ) AND D (-4.5, )} IN RELATION TO MICROCLIMATE (M) MICROCLIMATE - AIR TEMPERATURE — •), RELATIVE HUMIDITY ) SMALLER SEEDLINGS (LEFT) AND LARGER SEEDLINGS (RIGHT). University of Ghana http://ugspace.ug.edu.gh 86 4 2 O 10 6 6 4 2 0 30 25 126 I— w. \ \ ^ \■ •— «— »Xo— D \ \■ — f «_«_g— 1»— m 0— 0 -O-. #" (M ) 30 08-00 I l 'OO 13-00 IS'OO 17-00 07-00 09-00 IIOO 13-00 15-00 TIME OF DAY (HRS.) T3X a)> ujce - loo-o - SO-O ■ 60-0 . 40 O tig. 17 University of Ghana http://ugspace.ug.edu.gh PIG-. 18 DIURNAL VARIATION OP STOMATAL CONDUCTIVITY OP LEAVES OP KHAYA SMEGALENSIS ( & ) AND K. IVOKEKSIS ( O ) tMDER m > SOIL MOISTURE THEATMfflTS (BAHS), A (m«m~ ) -0.3 AND D ( - - - - ) -4.5 IN RELATION TO MICROCLIMATE (M). MICROCLIMATE: AIR TEMPERATURE (#*“ #), RELATIVE HUMIDITY 0-^0). University of Ghana http://ugspace.ug.edu.gh T E M P E R A T U R E t ° G ) I N F I LT RA TI ON S C O R E 127 0 o' >- K 5 1 3 r u > UJ a: • IOO ■ 80 ■ 60 ♦O TIME OF DAY t HOURS) FIG. 18 University of Ghana http://ugspace.ug.edu.gh 128 (Treatment A), end only partially so when soil moisture stress was about -4.5 bars (Treatment D). This is well illustrated in Pig. 18. The figure also shows evidence of early stomatal closure in leaves of seedlings subjected to Treatment D. The results, both in Pig. 17 and 18 show that the stomata of K» senegalensis opened widely during the early hours of the day, and generally began to close around 11.00 hrs. In contrast, E. ivorensis ^ kept its stomata widely open longer. In this ^ecies closure was observed to start after 14.00 hrs on most days. The pattern of stomatal opening did not appear to be closely related to environmental conditions (see Pigs. 17 ana 18). The infiltration score obtained for K. ivorensis smaller seedlings in the early hours of the day (Pig. 1?) were surprisingly high and did'not agree with the other data for this treatment (see Pig. 18 and Appendix 4, Table iv). Similar early hour high infiltration scores were obtained for leaves of the larger seedlings of both species under this treatment. The pattern of stomatal opening observed for Treatment A, B, and C on the smaller seedlings (Pig. 17, left) were comparable to the pattern of transpiration shcmnby similar seedlings under similar treatment in the greenhouse (see for example Pig. 13, right). 4.4. Discussion Transpiration rates of K. senegalensis and K. ivorensis seedlings were studied, both in the greenhouse and. in the research room, in University of Ghana http://ugspace.ug.edu.gh relation to moisture stress in the root medium. In the greenhouse transpiration of both species generally increasedfrom sunrise and attained a peak between 11.00 and 15.00 hrs vfoen evaporation vsas high. The rate fell in the evening when low evaporation prevailed. In the research room transpiration showed a similar pattern but this Tjas not correlated with changing evaporative conditions. Both species responded to low moisture stress by high transpiration rate. The trsnspiration rate of K. senegalensis was however consistently greater than that of K. ivorensis. This is in agreement with the difference in stomatal frequencies of these two species (see Table 1, P. 14 ) and possibly also with the higher mean ratio of root to shoot in -the former species. For both species the rate of transpiration generally decreased with increasing moisture stress. Several workers have observed similar reductions in transpiration in response to increasing moisture stress. Most of these findings have been sunmarised by Richards and Wadleigh (1952). In the greenhouse the sensitivity of transpiration of K. senegalensis seedlings subjected to stress of -k-*5 bars (in soil) and -10.3 bars (in culture solution) vr&s found to be greater than that of K. ivorensis under similar treatments. The reverse was however observed in the research room where K. ivorensis appeared to be slightly more sensitive than K. senegalensis to soil moisture stress of -4.5 bars. Transpiration of seedlings in culture solution was not investigated in the research room. Since water loss from plants occurs mainly through the stomata, 129 University of Ghana http://ugspace.ug.edu.gh 130 transpiration rates would be expected to reflect the degree of stomatal conductivity. A diurnal periodicity was observed in stomatal conductivity of the two species. The stomata were vddely open in the morning. For plants subjected to low soil moisture stress (-0 .3 to -0 .8 bars) the degree of opening generally started to decrease around 11.30 hrs in leaves of K. senegalensis and the larger plants of K. ivorensis (Fig.17)• For the smaller plants of K. ivorensis (Fig. 17) stomata remained widely open, to about 14.00 hrs. For both species the stomata of all plants rooted in soil at -0 .3 to -0.8 bars moisture stress generally showed seme conductivity after 15.00 hrs. Under conditions of greater stress (-4.5 bars) there was generally no conductivity of stcmata of the seedlings after 12.00 hrs. Several workers, for example, Dale (1961) and Y/ormer (1965) have also observed a diurnal periodicity in the opening of stomata of the species they worked with. Thetwo species also differ in the degree of stomatal opening - K. senegalensis opening its stomata comparatively wider than K. ivorensis when soil moisture stress was low (-0.3 to -0.8 bars). Under conditions of greater stress (-4.5 bars) stomatal conductivity of both species (Fig. 18) was small. Earlier stomatal closure and low conductivity of stomata when plants are subjected to severe moisture stress have been observed by other irorkers such as Loftfield (1921), Magness and Furr (1930), Yemm and Willis (1954) Rutter and Sands (1958) and Yforraer (19&5)• Parker (1968) points out University of Ghana http://ugspace.ug.edu.gh 131 that earlier stomatal closure helps to maintain a more favourable internal water balance. In the present investigation it is perhaps most meaningful to compare transpiration and stomatal conductivity for the smaller seedlings which were used in both types of investigation in the greenhouse. Although transpiration aid stomatal conductivity were measured at different periods, prevailing environmental conditions during these periods were comparable. Thus temperatures and recorded on these occasions ranged from 23.0 to 34°C and 23.5 to 35.5°C on days of the stomatal and transpiration experiments respectively. In the same order relative humidity ranged between $4 to 58% and 96 to 51% respectively. The general pattern of stomatal conductivity of K. senegalensis under low moisture stress (-0.3 to -0.8 bars) appeared not to be well correlated with the transpiration pattern observed. For instance high stomatal conductivity was observed early in the morning while transpiration rate had just started increasing. The latter attained a peak around 11.00 to 15.00 hrs just at the time when stomata had begun to close. On the other hand there seems to be a better relation between the two patterns for K. ivorensis seedlings when soil moisture stress was around -0.3 to -0.8 bars. This is well illustrated in Figs. 13 (right) and 17 (left), where the patterns obtained for both transpiration and stomatal conductivity appear similar. For both species both transpiration rate and stomatal conductivity were greatly reduced in the evening. University of Ghana http://ugspace.ug.edu.gh Although the periods of high transpiration rate and of wider stomatal opening, observed in K. senegalensis seedlings when soil moisture was readily available (-0.3 to -0 .8 bars) did not coincide, the higher stomatal conductivity of this species may in part account for the higher transpiration rate vahen compared with K. ivorensis. Stomata of both species speared to close by 12.00 hrs -when soil moisture stress was about -4 .5 bars, while transpiration still continued but under a reduced rate. This is not surprising since several workers, for example Pisek and Winkler (1953) (as cited by Parker, 1968) indicate that stomatal closure may lead to slowing down, but not necessarily complete cessation of transpiration. This probably reflects the fact that there are other passages of water loss such as through the cuticle when the stomata are closed (see Meidner and Mansfield, 1968). In the present case it is also possible that stomata were closed only to a degree which prevented entry of the infiltration mixture used. Thus transpiration could have continued through slightly-open stomata. The primary objective of the transpiration studies was to see whether differences observed in growth and diurnal pattain of plant water status could be related to differences in transpiration of the experimental species. Considering first the diurnal pattern of plant water status, it was observed that the overall patterns of this and transpiration were closely similar especially when moisture stress was around -0 .3 to -0.8 bars. Thus the plants started each day with a favourable internal 132University of Ghana http://ugspace.ug.edu.gh 133 water balance. Transpiration sets in and the internal water status is reduced. The greatest reduction occurred around 11.00 and 16.00 hrs. The rate of transpiration was high between 11.00 and 15.00 hrs. In the evening transpiration rate decreased; plant water status was observed to improve around this time, presumably also as a result of increased absorption. At low soil moisture stress transpiration m s observed to be higher in K. senegalensis seedlings than in X. ivorensis seedlings. Higher transpiration means more water loss. The potential gradient which acts as the driving force for water movement into the plant also depends on the amount of water loss, and on the relation between water content and water potential of leaves. Data to be presented later suggest that decrease in leaf water content is accompanied by a greater drop in leaf water potential for K. senegalensis than for K. ivorensis. Thus a steeper potential gradient for water absorption may be expected to exist in K. senegalensis than in K. ivorensis as a result of the differences both in transpiration rate and in the water content; water potential relationship (cf. KLepper, 1968). The steeper gradient would in turn be expected to lead to a more rapid rate of water uptake in K. senegalensis when moisture is freely available in the soil. Such an effect could contribute to a more rapid rate of recovery of plant water status at night following a day of high transpiration. It is probably for this reason that dawn values of leaf relative water content tended to be higher in K. senegalensis than in K. ivorensis. although the differences found were not significant. University of Ghana http://ugspace.ug.edu.gh Despite its higher transpiration rate, relative water content in leaves of K. senegalensis were generally slightly higher, throughout most of the day, than in leaves of K. ivorensis (see p, 84). However, because of the difference in leaf relative water content: leaf water potential relationship between the two species (see later), the higher relative water oontents of K. senegalensis leaves corresponded to lower water potentials. This indicates that severe stress or higher tensions existed and may account for the more marked stem shrinkages in K. senegalensis rather than in K. ivorensis plants, at low soil moisture content* When seedlings were subjected to a stress of -4.5 bars (Treatment D), the transpiration of K. senegalensis was reduced to about 29.2$ of that in Treatment A, while the corresponding reduction was to about 47*3/? for K. ivorensis. This may have contributed to the better relative water content observed in leaves of K. senegalensis in comparison with K. ivorensis (see p. 98) and to the greater reduction in shrinkage of stsn of K. senegalensis rather than of K. ivorensis under this treatment. Within species, low transpiration and earlier stanatal closure in Treatment D plants probably accounts for their less pronounced stem shrinkage, when compared v/ith plants in the other treatments. However this explenation may not apply to K. ivorensis in all cases. It taay be recalled that stem shrinkage under the severest soil moisture condition {27fa) (Pig.l?) was not consistently reduced in comparison with other treatments for this species. 134University of Ghana http://ugspace.ug.edu.gh 135 The other objective of the transpiration studies was to see how far differences in growth response to moisture stress could be explained in teims of differences in transpiration response. , Photo synthetic rate on which growth depends is known to be correlated •with transpiration rate (Briy, 1962). Si the present study, such a correlation m s expected. It has earlier been indicated that transpiration rate of seedlings rooted in soil in the research room probably reflect more the species characteristics and response to soil than do those of seedlings in the greenhouse, because the former were le*s correlated with environmental evaporation conditions. Results from Hie research room may therefore be used in attempting to interprete the growth of the seedlings in relation to soil moisture stress. Transpiration rates of K. senegalensis seedlings when soil moisture stress was -0.3 to -0.8 bars (Treatments A to C) were high. Similarly the growth rates of seedlings under this oondition were high in comparison with seedlings under severer stress (Treatment D). This higher growth rate could therefore be attributed, at least in part, to wider stomatal opening which favours COg uptake. In the research room also, transpiration of K. senegalensis seedlings in Treatment D (-4.5 bars) was reduced to about 8k*&% of that in Treatment A, whereas that of « K. ivorensis seedlings under the corresponding treatment was reduced to 81.8$; yet, growth was significantly reduced by Treatment D in K. senegalensis but not in K. ivorensis. It is reasonable therefore University of Ghana http://ugspace.ug.edu.gh 13 6 to suggest that the significant reduction in growth of K. senegalensis seedlings in Treatment D may not be mainly due to lower rate of COg uptake, but rather to the higher internal water deficits in Treatment D plants. Such a suggestion is supported to some extent by the leaf water potential data. These were lower for K. senegalensis than for K. ivorensis seedlings (see Chapter 3, p. 99 ). For K. ivorensis. transpiration also decreased with increasing soil moisture stress; that is, Treatment A seedlings had the highest transpiration rate and seedlings in Treatment D, the lowest. However Treatment A seedlings also showed poor growth, and this wa3 attributed to reduced root permeability as a result of poor aeration due to water logging. The seedlings used in the transpiration experiment were from the same stock as those of the growth experiment, but by the time of the transpiration experiments they were much older. These seedlings had received regular watering during the growth experiment, as for seedlings in Treatment A; but at the end of the growth esqperiment watering was less regular. Regular watering was started again a week to the transpiration experiment. If water­ logging was responsible for the poor growth of K. ivorensis seedlings in Treatment A of the growth in soil experiment, it is possible that witS less regular watering during the interval between conclusion of growth experiments and starting of transpiration studies, the seedlings had the opportunity to develop their root systems more fully. Hence, during the transpiration experiments, they were able to University of Ghana http://ugspace.ug.edu.gh 137 absorb any excess water rapidly, after each re-watering, thus reducing the duration of the water-logged condition. This; probably accounts for the high transpiration of Treatment A seedlings of K. ivorensis. although . their growth under this treatment was poor. Transpiration was also high in Treatment B, under which the highest growth rate for K. ivorensis seedlings was found. This could be attributed to more rapid 00^ uptake as a result of wider stomatal opening in seedlings under this treatment. As mentioned earlier transpiration of Treatment D plants of K. ivorensis was reduced to about 81.8% of that of seedlings in Treatment A. Growth of seedlings of K. ivorensis in this treatment appeared however to be unaffected. Transpiration of both species was greatly reduced when moisture stress was increased to -10.3 bars by addition of polyethylene glycol to the culture solution. This reduction agrees with the .growth response shown by the two species under this treatment. The transpira­ tion rate of seedlings in the polyethylene glycol solution, as compared to those in the control (culture solution alone) by the aid of the experimental periods, was reduced to about 10^ for It. senegalensis and only to 20^ > for K. ivorensis. Growth was however observed to be reduced more in the latter than in the former qiecies under this treatment. The transpiration data suggest that the greater reduction of growth for K. ivorensis did not arise from a greater reduction in 002 uptake. Rather, it is possible that the higher transpiration for K. ivorensis at -10.3 bars resulted in higher internal deficits which affected growth. University of Ghana http://ugspace.ug.edu.gh The investigations reported in this chapter show that transpiration rate of K. senegalensis and K. ivorensis seedlings is reduced by increasing moisture stress in the root medium. When soil moisture stress was at -4.5 bars, and in the greenhouse, transpiration rate was reduced more for K. senegalensis than for K. ivorensis. However in an environment (research room) where transpiration m s less dependent on evaporative conditions, the reverse situation was found. At a moisture stress of -10.3 bars (in solution) transpiration was again reduced more in K. senegalensis than in K. ivorensis. These results are to some extent accounted for by the response of stomata to moisture stress, and themselves provide some explanations for the patterns of plant water status and of growth response described earlier. 138University of Ghana http://ugspace.ug.edu.gh 139 CHAPTER V TISSUE WATER RELATIONS 5.1. Introduction Plant growth response, transpiration rate and internal water balance in relation to environmental moisture stress are all a reflection of the properties of the tissues of the plant concerned. The results described in the preceding chapters suggest that differences in response to variation in external moisture stress exist between seedlings of K. senegalensis and those of K. ivorensis. In particular growth of K. senegalensis was observed to be more sensitive to low moisture stress than that of K. ivorensis. while at high moisture stress the latter species tended to have the greater sensitivity. As previously indicated (p. 51 ) a plant with a high sensitivity of growth to small reductions in water potential may still possess drought resistance features in its tissues. Therefore examination of some aspects of the water relations of tissues of the study species appeared worth while. In the present study, changes in growth rate in response to moisture stress in the root medium (Chapter II) were found to be related more to changes in net assimilation rate than to changes in leaf development. Net assimilation rate is determined primarily by net photosynthetic rate, viiile net photosynthetic rate itself is affected by rate of COg uptake (which depends greatly on stomatal University of Ghana http://ugspace.ug.edu.gh conductivity to COg), and by biochemical processes. These processes that control photosynthetic rate are know to be affected by water stress in the plant (slatyer, 1967,1970} Trcughton,. I968) (cited by Slatyer,1970)in a way which may be understood in terms of the water relations of plant tissues. ?or example, the relationship between water content and water potential of leaf tissue (Weatherley and Slatyer, 1957) indicates the amount of water held by the leaf at different degrees of stress. Conversely, it indicates what tension, in terms of water potential, may be expected to exist in the leaf for the loss of any given amount of water. In two plants in which this relationship differs, different degrees of stress may exist in their leaves for the loss of the same amount of water. To the extent that stress in the leaves limits biochemical processes of photosynthesis (cf. Slatyer, 1970) the rates of growth of these two plants may differ for the loss of similar amounts of water. Again the extent to which water stress affects photosynthesis through stomatal conductivity depends on the sensitivity of stomata to water stress. If the stomata are very sensitive, CO^ uptake through them is soon reduced resulting in reduction of photosynthesis, net assimilation rate and hence growth. But high stomatal sensitivity also means that water is readily conserved so that the development of high internal water deficits is checked. On the other hand low stomatal sensitivity, though permitting CO^ uptake to occur over a wider range of moisture stress, may result in increase in internal water deficit which 140University of Ghana http://ugspace.ug.edu.gh may directly reduce growth or damage tissues. Thus it is important to know v*ether a difference exists, between the present study species, in the relationship between leaf water status ana stomatal closure. In the following experiments the relationships between (1) relative water content and water potential of leaves and (2) relative water content and stomatal closure are compared for the two species. The tissue water status at which damage occurs is also briefly compared for the two species. Jarvis and Jarvis (l963e) have already stressed that the tissue water balance for pptimum growth of plants depends greatly on these relationships. 5.2. Methods (a) The relation between relative water content aid water potential of leaves. This relationship was investigated by the gravimetric vapour equilibration technique of Slatyer (1958). The technique basically involves comparing relative water content (R.W.C.) of leaf tissues equilibrated to different water potentials. Relative water content is measured as; R.W.C. = Fresh weight - Pry weight x 100 Saturated weight - Dry weight The determination of H.W.C. thus involves saturation of leaf discs by flotation or equilibration in a humid chamber. Saturation is aimed at eliminating, by water uptake, all the deficits existing in the tissue end at providing a standard water content against which the 341University of Ghana http://ugspace.ug.edu.gh 142 amount of water in the leaf tissue aft any time may be compared. ¥eatherley (1950) and Barrs and Weatherley (19&2) have pointed out that the curve of mter uptake during this kind of saturation comprises two distinct phases! an initial phase of rapid but passive uptake (Phase i) followed by a period (Phase II) when there is a slow and steady uptake due to cell growth. Deficits existing /" in the leaf are normally completely eliminated by the end of Phase I, so that the water content at this point is the ideal basis for estimation of leaf water status. For the present study, it was tiius desirable first to determine the duration of the Phase I water uptake period in discs of the experimental species, before determining the relative water content; water potential relationship. (i) Determination of water uptake curves Ten discs (diameter 0.95 ran) of each species x&re punched from mature leaves of about six-month old seedlings and rapidly weighed. They were then placed singly in holes in a humid chamber similar to that used by Okali (1971a) to become saturated. Each disc was reweighed at intervals of one hour until there was negligible increase. Before each re-weighing the discs were quickly dried between four layers of filter paper. The weight of each disc at each re-weighing was expressed as a percentage of the original weight. The mean percentages for each batch of ten discs were then plotted against time to give water uptake curves. The determination was carried out under nonnal room lighting with temperatures fluctuating between 23 end 25°C. There were six University of Ghana http://ugspace.ug.edu.gh determinations in all. The results (Pig. 19) show that in both species the first phase of the water uptake curve was virtually complete by 3 hrs, and phase II had probably begun. Many workers (for example Yemm and Willis, 1954; Catsky, 1959), have suggested that over this length of time the contribution mads by Phase II uptake to the total water uptake is small. In the studies reported in Chapter III on diurnal patterns of leaf water status, this length of time was strictly adopted as the saturation period. In the present’ experiment, where the primary object was not just to eliminate existing deficits in leaves, but rather to obtain standard saturation water contents prior to equilibration at different potentials, it was convenient to use a ten-hour saturation period. Prgeta the slopes of the curves after 3 hrs (Fig. 19), it is clear that the contribution made by Phase II water uptake to the total saturation weights m>ula still be small after 10 hrs (no more than 1.0 and 1,6% for K. senegalensis and K. ivorensis respectively). (ii) Determination of leaf relative water content at various water potentials. Discs 0.95 om in diameter -were punched randomly from mature leaves of seedlings of each species, that were about one-year old. The seedlings were Rowing in the greenhouse, on well-watered soil.. There were about six seedlings of each species. Preliminary comparisons showed that the relative water content;: water potential relationship remained constant between lower and upper leaves of the same plant 143 University of Ghana http://ugspace.ug.edu.gh WATER UPTAKE! CURVE OF LEAF DISCS OF KHAYA SME&ALENSIS ( & ) JiD K. IVORENSIS ( O )• (VERTICAL LINES INDICATE 21, STANDARD error). PIG. 19 University of Ghana http://ugspace.ug.edu.gh TIME IN (HOURS) F R E S H WEIGHT A S P E R C E N T A G E OF O R IG IN A L F R E SH W E IGHT University of Ghana http://ugspace.ug.edu.gh or between different plants of the same species, provided the leaves used were mature but not senescent. Therefore 50 discs were taken randomly from six plants of each species. The punched discs for each species were thoroughly mixed before being allowed to become saturated as described in the preceding section. The saturated discs were surface-dried between filter paper and weighed to obtain saturated weights, before they were transferred into ten micro-desiccators. Each microdesiccator consisted of a 130 ml screw-capped jam jar containing plastic foam material soaked in one of the following NaCl (Sodium chloride) solutions; 0.50 M, 1.00 M, 1.50 M, 2.00 M, 2.25 M, 2.50 M, 2.75 M, 3.00 M, 3.50 M end 4.00 M, to give known water potentials (bars). These water potentials were calculated from the data of Owen (1952) relating osmotic potential to molarity of sodium chloride solutions. A small grid of nylon mesh supported by glass rods, about 0.3 cm above the plastic foam material, carried the discs. Five discs cf each species were arranged in a too* order in each micro-desiccator, so ’that there were ten discs in all in a micro-desiccator. The micro-desiccators were tightly closed and were maintained at constant temperature in a double water-bath of which the outer bath was controlled to 25+0.02°C. Twenty four hours was allowed for equilibration before discs were re-weighed to obtain the equilibration weights (E.W.). Prom similar determinations carried out previously with these species (Dodoo, 196$) 24 hrs was found to be sufficient for equilibration and also to be more convenient. 145 University of Ghana http://ugspace.ug.edu.gh The discs were ovendried for 21+ hours at 90-95°G to obtain the dry weights (D.W.). The water retained by each disc at each water potential was calculated as,, (equilibrated weight-dry weight/saturated weight-dry weight) x 100. The mean percentage for the five discs of eafih species v/as then plotted against the corresponding water potential (bars). The determination was repeated three times, giving four mean values of leaf water content for each species and at each sodium chloride concentration. Since this relationship may vary with age or previous experience of the plant, the determination was further carried out with leaves taken from adult trees. These were about 13 years old and were growing in the Botanical Garden at Legon. There is one tree of K. senegalensis growing about 3 m or 6 m away from two trees of K. ivorensis. The K. senegalensis tree was about 9 m tall while the II. ivorensis trees were 7.6 and 9.0m. The determination with leaves from these trees was done twice and the mean values for the two determinations were plotted, against the corresponding water potentials. (b) Leaf water status and tissue damage. Desiccation tolerance of the two species was briefly examined by adopting the methods of both Jarvis and Jarvis (l963e) and Okali (1971a). The former authors assessed tissue damage by the ability of desiccated discs to regain tneir original saturated weights; while the latter made a visual assessment of tissue damage. In both methods assessment of tissue damage is made on discs which are re-saturated after equilibration over.sodium chloride solutions in -fee usual way. 146 University of Ghana http://ugspace.ug.edu.gh Discs which had "been desiccated over a series of sodium chloride solutions were resaturated over the same period (10 hrs) as for the first saturation. The discs were surface-dried between four layers of filter paper during which visual assessment of damage was made. This was done by observing the potential at vhich 50% or more of the discs were extensively discoloured and there was heavy discharge of coloured matter. After this, the resaturated weights (R.S.W.) were determined. Dry weights (D.W.) were obtained after drying at 90-95°C for 24 hrs. The percentage recovery of original saturated weight after re-saturation was calculated as: R. S.W. - D.W. x 100 S.W. - D.W Percentage recovery was then plotted against equilibration water potential. As a check on the visual assessment of tissue damage epidermal strips and transverse sections of some of the resaturated discs were stained in equal volumes of 0.015j« methylene blue and neutral red solutions by the Ruczicka-Tronchet method (M clean and Cook, 1952, P. 51). The results obtained from staining were in general agreement with the visual assessment made. (c) The relation between leaf relative water content and stomatal closure. The method used was a modification of Hygen1s quick weighing method (1951) for the study of transpiration of detached leaves. Leaves were obtained from the same seedlings as described in the previous section but the determinations were made when these were younger (about six 147 University of Ghana http://ugspace.ug.edu.gh months old). The petioles of the leaves were re-out under water and the leaves were kept standing in water over night and in sealed polyethylene bags. The bags were kept in the dark overnight. On the following morning the leaves, still within the bags, were then exposed for 60 min under light from a 150 W Osram reflector spot Lamp, to allow the stomata to open. The light intensity at the level of the leaves, as measured with a photometer ('EEL' Lightmaster Model 18) was about 2t->000 lux. After this period, the bags were opened; leaflets were then detached from the leaves. The leaflets were quickly dried between four layers of filter paper and weighed within 10-20 sec on a Mettler H£> balance to obtain their saturated weights (S.W.). The leaflets were subsequently placed horizontally with their abaxial surfaces upper most on a nylon mesh platform. A fan about 160 cm from the platform passed air at a speed of about 0.3 m/sec over the leaflets. Fresh weights (P.W.) of the leaflets were recorded at intervals of five minutes for 70-90 min. After each weighing the leaflets were returned to the platfonn. The experiment was carried out in the resea:.ch room. A thermohygrograph was installed near the platform to record temperature and relative o humidity. Room temperature was about 23 C and relative humidity ranged between 66 and 68^ 6. Temperature read from an ordinary mercury-in-bmlb thermometer at the middle of the platform was 2i^l.0°C. Evaporation from Piche*evaporimeter in the same position was about 0.6 ml/hr. University of Ghana http://ugspace.ug.edu.gh 149 Dry weights (D.W.) were detenained at the end of each experiment. The experiment was repeated six times. Relative water content (R.W.C.) at any point during a drying-out cycle was calculated as: R.W.C. = P.W. - D.W. x 100 S.W. - D.W. The change in leaf water status with time was then plotted. A similar determination was made on leaves of the adult trees described earlier. Three determinations were made with these, in all using seven leaves of K. senegalensis and nine of K. ivorensis. The relative water content at point of stomatal closure was also determined for leaves of the two species growing in culture solution (-0.3 bars) and in culture solution with polyethylene glycol added (-10.3 bars). This determination was done only once, using 2 to 3 leaves of each species from each treatment. 5.3. Results (a) The relation between water content and water potential of leaves. This relationship is summarized in Pig.20 for seedlings of both species. Each point on the curves is the mean of four determinations. Variation between the four values for each point was not great so that the same relative position was maintained by.each species on each determination. The least significant difference between any two points on-the curves is 3.2^ relative water content (P = 0.05). Hence the curve for K. senegalensis is significantly different from that of K. ivorensis especially at high water potentials (0 to -66 bars). University of Ghana http://ugspace.ug.edu.gh FIG. 20 RELATIONSHIP BETWEEN RELATIVE l&TER CONTMT ($2) AND LEAF WATER POTENTIAL (-BARS) OF SEEDLINGS OF KHAYA SMEGALMSIS ( A ) AND K. IVORENSIS ( Q ). .." University of Ghana http://ugspace.ug.edu.gh RELA TIVE WATER CONTENT (®/«\ r~ University of Ghana http://ugspace.ug.edu.gh University of Ghana http://ugspace.ug.edu.gh For K. senegalensis the relative water content fell to 88.5$ when the isater potential k s reduced from 0 to -15 ’oars, and a reduction of water potential to about -it-6.5 bars was required before the relative water content fell to 50%, In contrast, for K. ivorensis the corresponding values were R.Y/.C. at.-15 bars or a reduction of water potential to -41 bars before water content fell to 50%. Although the values at lower potentials for K. senegalensis remained higher than those for K. ivorensis. they were however not significantly different from those of K. ivorensis (except at -110 and -154 bars). In general, the curves make it clear that a larger decrease in leaf water potential accompanies a unit drop in relative water content in K. senegalensis than in K. ivorensis; the difference though significant in parts is however strikingly snail. The results for the relative water content: water potential relationships for leaves of the adult trees are shown in Fig.21. On the figure aire also plotted the results for the seedlings just described above. The curves for the adult trees of both species were very similar. Thus there seems to be no difference in the relationship, between the two species, when adult trees are compered. Comparing results for adult trees with those for seedlings within each species shows that for K. senegalensis. above -44 bars both adult tree and seedling seem to have similar relationships. However when the potential was lowered there was a tendency for leaves of the adult tree to have higher relative water contents than leaves from seedlings at the same potentials. In contrast, for II. ivorensis the two results University of Ghana http://ugspace.ug.edu.gh PIG, 21 RELATIONSHIP BETWEEN RELATIVE WATER CONTENT (%) AND WATER POTENTIAL (BARS) OP LEAP TISSUE 50R SEEDLINGS JttD ADULT TREES OP KHAYA SENEGALENSIS AND K, IVORENSIS ADULT SEEDLING K. SENEGALENSIS A A K. IVORENSIS * O * University of Ghana http://ugspace.ug.edu.gh RE LA TI V E W AT ER CO NT EN T (• *. ) 152 F I G. 21 University of Ghana http://ugspace.ug.edu.gh were quite different. All the points for adult tree leaves were far above those of seedling leaves. Thus there is ’an upward shift in the fiurve of the relationship between relative water content said water potential of leaves of K. ivorensis adult trees. (b) Leaf water status and tissue damage. The degrees of recovery of desiccated discs, in terms of water content after a period of resaturation expressed as a percentage of original saturated weight (cf. Jarvis and Jarvis, 1963e) are shown in Pig. 22. The curves show that when the equilibration potential was higher than about -44 bars, K. senegalensis tissue showed greater tolerance than K. ivorensis tissue. At this point the recovery percentage was 83 and 8l/o respectively for K. senegalensis and K. ivorensis. However the response was reversed when the potential was lowered further, (below -44 bars), this time tissues of the latter species appearing more tolerant than those of the former. It wss however observed that beyond this low potential desiccated discs which showed extensive discolouration and exuded a great deal of coloured matter gave a higher percentage of weight, than desiccated discs which looked normal. It was thus possible that the high percentages obtained from the former discs were due to extensive infiltration of damaged tissue (cf. Okali, 1971a). The recovery curve of K. ivorensis at low equilibration potentials was above that of K. senegalensis probably because of this infiltration of extensively damaged tissues. The visual assessment pf tissue damage was accepted as the more reliable estimate of desiccation tolerance. Results obtained by this method 153 University of Ghana http://ugspace.ug.edu.gh FIG* 22 CURVES SHOWING TOE PMC®SAGS REXX)?EKY OF EB9SATUSIATBD DISCS IN BELAOQSl !K) EQUILIBRATION WAT®. POTER TI1L (BARS) KHAYA SHSEGALfflSIS ( & ) AND K. IVORBSSIS ( O ). University of Ghana http://ugspace.ug.edu.gh FIG .22 R E S A T U R A T E D WATER C O N T E N T C *fcO F O R IG IN A L SATURA TED WATER University of Ghana http://ugspace.ug.edu.gh 155 agreed well with observations made on vitally stained tissues. The overall results therefore show that the critical levels of relative water content and leaf water potential at which tissue damage occurred were 23.0% and -81 bars for K. senegalensis seedlings and 34.0/? and -59 bars for K» ivorgnsis. These values show that the difference between 'the critical relative water content for tissue damage of the two species was snail (a difference of 5.0/3). On the other hand the leaf water potential values differed considerably (a difference of -22 bars). Hence it could be said that K. senegalensis tissues become damaged at a lower potential than those of K. ivorensis. (c) The relation between leaf relative water content and stomatal closure. Table 11 summarizes the results obtained for this comparison. In allj seven determinations were made during which 22 leaves of it. senegalensis and 16 of K. ivorensis were observed (see .Appendix 5, Table i). Examples of the curves from ’Aiich relative water content at stomatal closure was estimated are shown in Pig.23. Extrapolation of the straight line portions of such curves (cf. Jarvis and Jarvis 1963d) suggests that the mean relative water content for stomatal closure in seedlings of K. senegalensis was 87.6j£ and that for K. ivorensis was 81.2$. The standard error of the difference between these means is 3. 94$?, giving t value of 4.69 ( P 0.001). Thus the relative water content at which stomata closed in the two species were significantly different. The stomata of K. senegalensis having the higher relative water content for closure could therefore be said to be more sensitive than those of K. ivorensis. University of Ghana http://ugspace.ug.edu.gh 156 Table 11 Mean relative water contents (R.’ilf.C.) of leaves of Khaya sanapja.I ensis and K. ivorensis at points of stomata,1 closure. Species K. senegalensis K. ivorensis Expt. No. of observations R.W.C. 00 Expt. No. of observations R.W.C. 00 1 3 88.0 1 3 81.0 2 3 89.0 2 1 87.0 3 1 93.0 3 2 77.0 4 4 85.0 4 3 82.0 5 5 86.0 5 1 80.0 6 3 81.0 s 3 83.0 7 3 91.0 7 3 80.0 Totals 22 613 16 570 Mean va!Lue '................... 87.6 81.4 Standard error of difference = 3.94 t = 4.69 P < 0.001 University of Ghana http://ugspace.ug.edu.gh FIS. 23 KXMPLE3 OF CUEVES OF 3HE RELATION BETWEEN LEAF WATER CONTENT (LOGARITHMIC SCALE) M D STOMATAL CLOSURE, SHOWING- EXTRAPOLATIONS OF THE STRAIGHT LINE PARTS INTERSECTING AT CLOSURE. KHAYA SB'IBGALMSIS ( A ) K. IVORMSIS ( O ) University of Ghana http://ugspace.ug.edu.gh Lo g tO RE LA TI VE W AT ER C O N TE N T 157 F I G . 2 3 University of Ghana http://ugspace.ug.edu.gh 158 The mean relative water content values at stomatal closure recorded for leaves of the adult trees in the Botanical Garden were 90.7+1.5 and 86.2+1.6$ for K. senegalensis and K. ivorensis respectively. This also shows that the relative water content at stomatal closure of ihe two species differs significantly. Similarly seedlings of K. senegalensis and K. ivorensis growing in culture solution were observed to close their stomata at about 93 and 83/a relative water content respectively, again showing that K. senegalensis stomata are more sensitive than those of K. ivorensis to water loss. However the trend was reversed when leaves from the -10.3 bars polyethylene glycol solution were comp.ared. This time It. senegalensis had 82/2 relative water content at stomatal closure and K. ivorensis 86jS. These last two experiments (using culture solution and culture solution with polyethylene glycol added) were done only once, so that thejr may be less than the determinations vhich were more adequately replicated. 5.4: Discussion. The curves relating relative water content and water potential of leaves for both species in this study, may be compared with similar curves reported by Okali (1971a) for some woody plants in the Accra Plains of Ghana. Relative rater content at -15 bars water potential, or the water potential at 50%. relative water content may be used for this comparison (cf. Jarvis and Jarvis, 1963©; Okali, 1971a). The data presented indicate that these values were 88 and 82$ or -47 and -41 bars respectively for K. senegalensis University of Ghana http://ugspace.ug.edu.gh 159 and. Iv. ivorensis. Thus K. senegalensis oould be placed at the intermediate position in Okali's ranking of drought resistance (see Okali, 1971} Table 5) i*1 terms of this parameter^ that is, between Clausena anisata which had $Ofe relative water content at -15 bars leaf water potential and Dichapetalum guineense with 87% relative water content at the same leaf water potential. K. ivorensis. on iiie other hand, would be placed nearer the bottom of the scale below the last woody species, Fagara. 2an.th0xvl0id.es. which had 85fi relative water content at -15 bars leaf water potential. The evidence from this comparison would then suggest that K. senegalensis like the Accra Plains species is more drought resistant than K. ivorensis in terms of the relative water content: water potential relationship of leaf tissue. The curves of relative water content and water potential of leaves of adult trees growing in the Botanical Garden, were very similar for both species. At higher potentials (above -44 bars) the curve for K. senegalensis adult trees was similar to that for seedlings. However there was a slight upward shift below this potential. On the other hand K. ivorensis showed a clear upv/ard shift in the whole curve for the adult trees. Sle.tyer (i960) compared the desorption curves of Acacia aneura of different ages and in different situations, and observed no shift in this relationship which could be attributed to tissue age or to prior treatment. However results have been obtained for other species which suggest that age of plants 01- their growth localities may lead to a shift in the relation (see for example, 'tfhiteman and Wilson, University of Ghana http://ugspace.ug.edu.gh 160 1963; Gavande and Taylor, 1967)* Jarvis ana Jarvis (1963d,e), and Shepherd (1964) also observed that prolonged prior desiccation may result in a shift in the position of the relative water content: leaf water potential curve. The grounds of the Botanical. Garden where the adult trees are growing form part of the Accra Plains, hence environmental conditions here are more or less comparable to those in the natural habitat of K. senegalensis (see p. 4 ). The slight upward shift in the curve for IC. senegalensis adult tree at lower potential may be attributed to age alone. For K. ivorensis. however, the Accra Plains represent a more severe environment than its natural habitat. It would be expected therefore that K. ivorensis growing on the Accra Plains would snow some modifications towards this environment. It is possible -that such a modification has led to the change in position in the relative water contentjleaf water potential curve in these plants which had grown in the Botanical Garden for 13 years. 0® course, the shift could also be an age effect (cf. Whiteman and TTilson, 1963); but it is remarkable that it is more pronounced for this species than for K. senegalensis of thffi same age. The upward shift in the curve suggests that a small drop in relative water content urould lead to a large reduction in leaf water potential which, as has been pointed out, favours survival under severe moisture conditions, while limiting growth. In this connection, it is interesting to point out that the two trees of K. ivorensis in -the University of Ghana http://ugspace.ug.edu.gh 1 6 1 Botanical G-arden are much smaller in size (girths about 0.6 and 0.3m) than the K. senegalensis (girth about 1.2 m) growing beside them. They are also smaller (7 .6 and 9.1 m tall) than would be expected of trees of similar age growing in forest conditions. Relative water contentsleaf water potential relationships were not detemined for seedlings under the various soil moisture treatments. However the stability of the relationship for K. senegalensis ' • along certain parts of the curve, when seedlings and mature trees are compared suggests that the difference between this species and E. ivorensis may be a real interspecific difference. It is possible then to use this difference to attempt an explanation of the growth responses described in Chapter II. There^it was suggested that the high sensitivity of growth of K. senegalensis seedlings to low soil moisture stress and 'that of K. igorensis to severe moisture stress could be related to the type of relative water contentsleaf water potential curves possessed by the two species. The present data are in agreement with -those suggestions (cf. Jarvis and Jarvis, 1963e). The critical levels of leaf water potential at which tissue damage occurred, was observed to be lower in It. saiegalensis ("81 bars) than in K. ivorensis (-59 bars). This suggests that K. senegalensis is more tolerant to desiccation than K. ivorensis. Comparison of leaf water contents at point of stomatal closure shows that I t . senegalensis closed its stomata at a significantly higher relative water content (about 88.0?S) than did I t . ivorensis (about 8l.0jS). University of Ghana http://ugspace.ug.edu.gh 162 The number of stomata per unit leaf area of K. senegalensis is greater than that for K. ivorensis (see p. lit- ), transpiration was also noticed to be generally higher in the fonaer species than in ’ the latter when subjected to low moisture stress in the root medium. It is therefore remarkable to observe a higher relative water content at stomatal closure in K. senegalensis ' than in K. ivorensis. Stomatal closure results in reduction in gaseous exchange, la general,a species which possesses a high tolerance to desiccation and a low sensitivity of stomata to desiccation could carry on photosynthesis over a wider range of low water potentials, then can a species which does not possess these features. K. senegalensis showed a high degree of tolerance to desiccation, but its stomatal sensitivity to water stress was equally high. This means that while the leaves of this species may survive severe internal moisture stress, photosynthesis in them may be readily limited by reduced CO^ uptake as a result of stcmatal closure when environmental moisture stress is low. The latter effect may have contributed to the low growth rate of K. senegalensis in response to low moisture stress (Chapter II). This suggestion is supported by the data on transpiration in relation to moisture stress. Transpiration (which depends, partly on stomatal conductivity to water vapour) was found to be lowered when soil moisture stress was at -4.5 bars. Stomatal studies also showed earlier diurnal stomatal closure under this treatment for K. senegalensis. K. ivorensis on the other hand m s comparatively less tolerant to University of Ghana http://ugspace.ug.edu.gh desiccation, but it also showed a lower relative water content for stomatal closure. Its lack of sensitivity to low moisture stress in terms of growth rate, may reflect this lower water content at which stomata close. On the other hand, its poor growth rate when moisture stress was high may be a reflection of its poorer tolerance to desiccation, and of the fact that a larger amount of water is lost for a given decrease in water potential, when compared with K. senegalensis. It is significant to note that although the absolute values of leaf relative water content at which stomata close differed between the one year-old seedlings growing in soil near field capacity, on which this relationship was mainly investigated, and adult trees or seedlings grown in culture solution, the trend of difference between the two species was always the same. Stomata in K. senegalensis always closed at a leaf relative water content that was 4.5 - 10% higher than that at which they closed in K. ivorensis. However in the single unreplicated experiment in which plants that had grown in polyethylene glycol solution (water potential about -10.3 bars) were used, the trend was reversed. The stomata of K. ivorensis closed at a leaf relative water content of 86% as against Q2fo for K. senegalensis. Whether this reversal is a real effect is not known; but it may reflect differences in absorption of polyethelene glycol by the two species (cf. Jarvis and Jarvis, 1963d), although all necessary precuasions were taken to reduce this. Although the aspects of tissue water relations examined in this chapter contribute to understanding growth and transpirational responses of the experimental species, it is much more difficult to apply them to 163University of Ghana http://ugspace.ug.edu.gh explaining the diurnal patterns of plant water status described earlier. However comparing the relative water content at stomatal closure with the diurnal relative water content values obtained in Chapter III, shows that relative water content low enough to cause stomatal closure, occurred during the day for both species especially when soil moisture stress was high. The frequency of occurrence of such low leaf water content was observed to be comparatively higher in K. senegalensis than in K. ivorensis. This may have contributed to the better water status observed in Treatment D of K. senegalensis than in K. ivorensis seedlings. The studies described in this chapter perhaps, provide the clearest evidence of a difference in adaptation to drought by the species under comparison. It is clear that the leaves of It. senegalensis possess adaptive features - a slower rate of water loss for a given drop in potential and a greater tolerance to desiccation - which may enable this species to survive severe drought better than K. ivorensis. Its higher stomatal sensitivity to water loss however can only allow it to grow at a slower rate than IC. ivorensis when moisture stress is not severe. l6k-University of Ghana http://ugspace.ug.edu.gh CHAPTER 1/1 GENERAL DISCUSSION Taylor (1952) suggests that moisture conditions are the most important factors controlling the division of the natural vegetation of West Africa into forest and savanna regions. Forest regions are known to receive, on the whole, higher rainfall and so are moister than savanna regions (Lawson, 1966). Distribution, growth— form and physiological behaviour of plants in this geographical area may therefore be expected to reflect control by environmental moisture. However, very few experimental studies have been carried out to test ihis -expectation. The few studies that have been made were concerned mostly with mechanisms of drought-adaptation, in certain species on the Accra Plains of G-hana, a type of dry environment (cf. Lawson aid Jenik, I967, Okali, 1971a). It was only recently that Hopkins (1970,a,b) attempted to make a direct comparison of leaf water status of forest and savanna species in the Olokemeji Forest Reserve in Nigeria. He observed that for the forest species lesf water may be 'severely limiting at the severest part of the dry season'. He was however unable to demonstrate such a limitation for the savanna species. The investigations cited above do not provide an adequate basis for testing the role of moisture in determining the pattemof plant distribution between forest and savanna, because the studies were either, not comparative between species taken from the two contrasting habitats, or they were concerned with only one aspect of plant water relations. 165University of Ghana http://ugspace.ug.edu.gh 166 The present study was therefore undertaken to compare experimentally and more fully the water relations of two species, one from each of these two contrasting habitats. It was hoped that if environmental moisture as against other factors such as fire, exerts the most control, species from these two habitats would show differences in their water relations. In order to observe differences which result mainly from adaptation to habitat, comparison was made between two species that are taxonomically closely related, the Mahogany trees - Khava senegalensis and K. ivorensis. These two species are restricted to contrasting habitats, the former to savanna and the latter to forest (Richards, 1952), but they are similar in their growth habit, phenology and reproductive biology; hence, differences observed from comparing them may be expected to be largely due to adaptation to habitat. The experiments were essentially carried out in the laboratory. Their relevance to the field situation derives therefore :mainly from their comparative nature. Again although seedlings were mostly used for these investigations, there are indications that seedlings respond sensitively to environmental factors such as moisture, and that differences in seedling response may affect plant establishment and hence the pattern of distribution of adult plants. Comparison of leaf morphology of the two species suggested a priori that the species differ in their structural adaptation to drought. The leaves of K. senegalensis. for example, have thicker veins than those of K. ivorensis. Mature leaves of the former species are also thicker University of Ghana http://ugspace.ug.edu.gh (about 0.8 mm) than those of the latter (about 0.5 mm), and stomata per unit leaf area are more in K. senegalensis than in It. ivorensis leaves. All these features are generally known (Oppenheimer i960) to be associated with drought-adapted plants, and have been noted, in particular, for West African plants of dry habitats (Yanney-7/ilson, 1963; Lawson end Yenik, 1967). Since plant growth summarizes all plant processes, it was argued that any difference in adaptation of two specie's to environmental moisture conditions should be shown in the growth responses of such species to varying environmental moisture. In the present study, comparison of growth in relation to moisture stress in the root medium (Chapter II) showed that IC. senegalensis the savanna species, is perhaps adapted to more severe drought than K. ivorensis. the forest species. The latter species was however less affected by moderate stress in the root medium. Thus the experiment on the effect of varying soil moisture stress (-0.3 to -4.5 bars) on growth showed that the growth rate of K. senegalensis was significantly reduced at -4.5 bars as compared with -0.3 bars. The growth rate at -4.5 bars was about 77/5 of that at -0.3 bars. The growth rate of K. ivorensis was however not significantly affected by this degree of reduction in soil water potential although it tended to be reduced by the wettest treatment, presumably as a result of sensitivity to poor aeration. When moistire stress was varied in solution by addition of polyethylene glycol, the higher sensitivity to low moisture stress in K. senegalensis as against K. ivorensis was again observed (Pig.8). But with severer 167University of Ghana http://ugspace.ug.edu.gh stress (for example below a water potential of -5 .3 bars in the root medium) growth rate of It. senegalensis was comparatively less reduced than that of K.. ivorensis. K. ivorensis seedlings did not show poor growth in aerated culture solution (-0.3 bars) as they did under a similar potential in soil. The growth analyses carried out suggest that reduction in growth of the two species under increased moisture stress was probably caused by changes in net assimilation rate rather than changes in mean leaf area ratio. The analyses also showed that K. senegalensis had proportionately more root than K. ivorensis seedlings. The ways by which water stress may have caused the observed difference in growth response of the two species, through its effect on net assimilation rate are suggested by ihe results of the investigations on diurnal pattern of plant water status, transpira­ tion and especially tissue water relations. The comparative studies on diurnal pattern of plant water status showed that a difference existed between the two species in internal water status. In terns of relative water content of leaves K. senegalensis maintained overall a more favourable internal water balance than did K. ivorensis. But in terns of leaf water potential and also in tenas of tension in the stem (as indicated by degree of stem shrinkage) stress was greater in K. senegalensis seedling^fteix external moisture stress was moderate. The greater reduction in growth rate of K. senegalensis when external stress was moderate probably reflects a direct effect of this higher internal tension on photosynthesis aid hence on net assimilation rate and growth rate, rather than an indirect effect through limitation 168University of Ghana http://ugspace.ug.edu.gh of C02 uptake. That COg uptake was not the main limiting factor is suggested by the results of the transpiration studies particularly in the. research room. Here, transpiration rate, mfaich partly reflects the degree of stomatal conductivity, was reduced less in K. senegalensis than in K. ivorensis iuhen soil water potential was about -k-,5 bars, yet the growth rate of K. senegalensis alone was significantly reduced by this soil tension. When external moisture stress was low (-0.3 to -0.8 bars) transpira­ tion rate was higher for K. senegalensis than for K. ivorensis (Pigs. 13 tpltlo). It is possible that this higher transpiration rate, combined with the type of relative water contents water potential relationship exhibited by K. senegalensis leaf tissue, contributed to cause tensions high enough to limit growth in this species when external stress was low. Fig. 20 shows that water loss from leaves is associated with greater reduction in tissue water potential for K. senegalensis than for K. ivorensis. Growth rate was reduced more for K. ivorensis than for K. senegalensis when external stress was increased by reducing water potential in the root medium (solution) to -10,3 bars. Transpiration rate under this potential was however reduced more (to about 10/6 of that in the control) for senegalensis than for I t . ivorensis (20% of control). Thus it is unlikely that the greater reduction in growth rate of K. ivorensis at «10.3 bars derived from a greater limitation to C02 uptake. The higher sensitivity of growth of I t . ivorensis to severe stress was probably then 169University of Ghana http://ugspace.ug.edu.gh due to large deficits which existed in its tissues as a result of higher transpiration rate. v* The high sensitivity of K. ivorensis to severe stress may also be due to -the fact that the tissues of this species are more readily damaged by desiccation than those of K. senegalensis (see Chapter V). Thus tissues of the fomer species were damaged when exposed to a stress of -59 bars as compared to -81 bars for those of K. senegalensis. It is worth while drawing attention here to the agreement between the results of the growth studies and theoretical expectation from considering the desorption curves of the study species. Jarvis and Jarvis (l963e) had suggested that a species which loses comparatively little water, for a given decrease in water potential, could withstand more severe moisture stress than a so ecies which loses a greater amount of water for e similar drop in potential. Conversely, the latter type of species is at an advantage from the point of view of growth when moisture stress is moderate. The evidence in this study suggest that IC. senegalensis represents the first type of species while IC. ivoreasis represents the latter. These considerations taken together with the results from comparison of stomatal sensitivity to water loss (Sig. 23) are perhaps the strongest evidence that K. senegalensis is more drought-adapted than K. ivorensis. Stomata of jC. senegalensis consistently closed at higher relative water * content (around. 88ji) than those of K. ivorensis (around 8l$?>). The relevance of this to the maintenance of a favourable internal water balance has already been pointed out. Perhaps the greater reduction in 170 University of Ghana http://ugspace.ug.edu.gh transpiration rate at -10.3 bars or the less pronounced stem shrinkage when soil moisture status m s 27% of field capacity, for K. senegalensis rather than for K. ivorensis. reflect this higher sensitivity of stomata in K. senegalensis. As has already been mentioned moisture is believed to play an important role in the control of the main pattern of plant distribution in "\7est Africa, K. senegalensis. one of the studied species, is restricted to the savanna region, a dry habitat, while K. ivorensis the other species, is restricted to tiie forest region which is moister. The results of the present experimental study confirm that one possible basis of this difference in distribution of the two species is a difference in drought adaptation, Kr senegalensis possesses features which adapt it to severer drought conditions than does K. ivorensis. An ability to reduce transpiration rate more effectively through high stomatal sensitivity to water loss perhaps helps K. senegalensis to conserve water, when environmental moisture is severely limiting. A more gradual rate of water loss from its leaves with increasing aivironmental dryness, as is suggested by its desorption curve, coupled vdth a greater tissue tolerance to desiccation assist M s species rather than K. ivorensis to survive and thus grow better under severe drought. It is possible also that the better development of the root system in K. senegalensis combined with the steeper water potential gradient which may develop between its tissues and the soil (see Chapter III ) contributes to enhance water uptake from a wider area of soil when moisture is freely available. Enhanced water uptake may support higher transpiration 171University of Ghana http://ugspace.ug.edu.gh 172 rate in K. senegalensis than in K. ivorensis. Higher transpiration rate was generally observed in K. senegalensis when moisture was freely available (see p* 129 )• The higher transpiration rate of K. senegalensis under such conditions may help to lower the temperature of the leaves and so prevent than from overheating in the more exposed conditions of its natural habitat (cf. G-ates 1964). Though not a drought adaptive feature, the rapid growth rate of seedlings of K. senegalensis is another factor which may help this species to compete actively with fast growing annuals which are abundant in the savanna, but not in the forest habitat. Conversely ivorensis is less adapted to drought and does not have most of the features mentioned above. Its natural habitat is comparatively always moist so that drought adaptive features are not essential to its survival. However its tolerance to moderate moisture stress as shown by its growth response may be of ecological importance. The forest regions exp erience one or two months of dry season during the year. This feature may therefore help K. ivorensis to withstand this period of reduced water availability. The results of the present study clearly cannot be extaided to apply to ik all 1 -forest and savanna species, but if the findings can be shown to apply to a wider variety of plants, especially to pairs of g?ecies from these contrasting habitats, they could provide a more valid basis for the assumption that moisture plays an important role in detemining the pattern of plant distribution between forest and savanna regions in West Africa, University of Ghana http://ugspace.ug.edu.gh In the introduction to this thesis it was pointed out that the species studied, particularly Khaya ivorensis. are of economic importance. Establishment of plantations and nurseries of these species are at the moment being given attention by the forestry department of this country. The results presented here on the water relations of these two species may perhaps be of value in providing information on suitable sites of establishment of such nurseries or plantations. For example, since K. ivorensis is sensitive to water-logging, it may not do well particularly as seedlings in sites which are often water-logged. Similarly, while It. ivorensis clearly cannot be considered for planting in veiy dry habitats such as extreme savanna regions, the evidence presented suggest that it may do as well as K. senegalensis in moderately dry environments such as the margins of forest areas. A complete investigation of the role of moisture in the distribution of West African plants should probably be extended to the field situation. This has not been done in the present study. However comparisons such as have been attempted here will always be of value at some stage of such a complete investigation. 173 University of Ghana http://ugspace.ug.edu.gh SUMMARY 1. The water relations of two Mahogany species Khava senegalensis and K. ivorensis. the former a savanna species and the latter a forest species, were studied in an attempt to understand the role moisture plays in determining their distribution. 2. Leaves of K. senegalensis were generally thicker (about 822)!.) with thicker veins than those of K, ivorensis (about 508 )i thick) aid 2 thin veins. Stomata in the former ^ecies are denser (about 660 per mm ) 2.than in the latter species (about 580 per mm ). These mostly occurred on the lower epidermis in both species. Stomata on the upper epidermis 2 are fewer than 1 per mm in both species. 3. Seedlings of K. senegalensis grow faster than those of K. ivorensis: at the age of two months the mean height of seedlings of K. senegalensis is about 13.0 cm with 6 to 9 leaves while that of K. ivorensis is about 10.0 cm with about 4 to 6 leaves. 4* Growth of the two species m s examined when both soil and osmotic solution were used as rooting medium. The soil treatments used were - A (-0.3), B (-O.4), C (-0.8) and D (-4.5) bars. The osmotic solutions (bars) were - A (-0.3), B (-2.8), C (-5»3) and D (-10.3)* 5. Considering first the growth in relation to soil moisture stress experiment. Height growth and leaf area of seedlings of both species in Treatment B tended to be greater than in all the other treatments. 6, Relative growth rate of K. senegalensis ranged from 0.14 to about 174University of Ghana http://ugspace.ug.edu.gh 0.18 g/g/wk} while that of K. ivorensis ranged from 0.15 to aijout 0.19 g/g/vik. 7. Relative growth rate of K. senegalensis decreased with soil dryness, so that value in Treatment D was significantly reduced to 77$ of that in Treatment A» However differences between Treatments A, B and C, and B, C and D were not significant. For K. ivorensis relative growth rate was highest in Treatment B (about 0.19 g/g/wk). The rate tended to be reduced though not significantly in both Treatment A, the wettest treatment and Treatment C & B which were comparatively dryA soils. Treatment A of K. ivorensis gave the lowest growth rate (0.15 s/s/wk) for this species. Most leaves of this treatment were observed to be chlorotic during the experimental period. 8. Wet assimilation rate of K. senegalensis seedlings was higher than that of K. ivorensis seedlings. The rate of the fomer species 2 ranged from 15.0 to 22.6 g/m /wk. Here also the rate for Treatment A was significantly higher than that for Treatment D. The net assimilation rate for the latter species ranged from 11.3 to 14.9 g/m /wk. Treatment B of this species gave the highest net assimilation rate while Treatment A gave the lowest rate. 9. Mean leaf area ratio, leaf area ratio, specific leaf area and leaf weight ratio of K. ivorensis were greater than those of K. senegalensis. For both species the treatments did not have any significant effect on any of these parameters when both soil and culture solution were used as rooting medium. 175University of Ghana http://ugspace.ug.edu.gh 10. The ratio of root to shoot obtained for K. senegalensis seedlings in all treatments was about 0.8 as against about 0.6 for K. ivorensis seedlings. 11. In the growth in osmotic solution experiment growth was observed to decrease with decreasing osmotic potential. 12. leaf development in both species was greatest in Treatment A but this was more pronounced in K. senegalensis than in K. ivorensis. "Shen stress was imposed the leaf area was greatly reduced in both species, so that for K. senegalensis there was a significant reduction in Treatments B, C and D as compared with A. For K. ivorensis however differences between Treatments A and B, and C and D were not significant. Leaf development was however reduced in Treatments C and D as compared with Treatment A. 13» Relative growth rates obtained in the culture solution were higher at the same water stress than those in the soil experiment; for example Treatment A of K. senegalensis gave a rate of 0.31 g/g/®k in the fbnner treatment as compared with about 18 g/g/wk in the soil experiment. The corresponding rates were 18/g/g/wk and 15g/g/wk respectively for K. ivorensis seedlings. 14. The lower sensitivity of K. ivorensis to moderate stress in the root medium was observed up to -2.8 bars (Treatment B) osmotic potential. K. ivorgnsis showed higher sensitivity down to this potential. However the sensitivity at lower potential was reversed - K. senegalensis growing comparatively better than K. ivorensis. 15. Net assimilation rate followed the same trend as relative growth rate. In this case also there was no significant reduction in net assimilation 176University of Ghana http://ugspace.ug.edu.gh rate of seedlings of K. ivorensis in Treatments B as compared with A. 16. Root development was greater in K. senegalensis than in K. ivorensis. However the root to shoot ratio of the latter species was more stimulated vfoen osmotic potential was at -10.3 bars (Treatment D). 17. Diurnal pattern of leaf water status was studied by examining the relative water content (R.W. C.) leaf water potential (L.W.P.) and stem shinkage in relation to soil moisture stress. For R.W.C. and L.W.P. the four soil moisture treatments (see 4) were adopted. For stem shinkage studies, the soil moisture contents (S.M.C.) treatments were expressed as percentages water content values at field capacity, 18. Differences in S.W.C. and L.W.P. was observed to be small for seedlings in Treatments A, B and C of both species. Treatment D gave the lowest S.W.C. and L.W.C. values. 19. Iii general K. senegalensis maintained overall higher S.W.C. and lower L.W.P. values than K. ivorensis. 20. Diurnal stem shrinkage in It. senegalensis was observed to be greater than in K. ivorensis when soil moisture content was high; 100 to about 50/2. However around 27% soil moisture content, diurnal shrinkage of stem was greatly reduced in this species. 21. It. ivorensis stan shrinkage did not consistently follow any particular pattern. Thus on most occasions stem shrinkage was observed to be similar ’under all treatments. However on one occasion when both aerial and soil moisture conditions were severe, less shrinkage was observed in stems subjected to 27% S.M.Co University of Ghana http://ugspace.ug.edu.gh 178 22. Transpiration was also studied in relation to the four soil moisture treatments (see 4), both in the greenhouse and in the research room. The experiment was repeated with seedling growing in soil at -0.3 bars and in osmotic solution at -0.3 and -10.3 bars. The pot weighing technique was adopted. Transpiration rate for both species decreased with increase in moisture stress. 23. Higher transpiration rate was observed in K. senegalensis than in K. ivorensis seedlings both in the greenhouse and in the research room when the moisture stress was -0.3 to -0.8 bars. At -4.5 bars (Treatment D of soil experiment) transpiration per cm and day in the greenhouse of the former species was reduced to 29.3^ > of the rate in Treatment A (-0.3 bars). The corresponding reduction was to 47»3$> for the latter species. 24. In the research room the mean daily transpiration in Treatment D as percsitage of values in Treatment A were 84.6 and 81.8% respectively for K. senegalensis and K. ivorensis. 25* The effects of both matric and osmotic potentials of rooting medium on transpiration of K. senegalensis seedlings were observed to be similar. For K. ivorensis lower rates were recorded when osmotic solution was used as root medium than when..soil was, used. 26. Transpiration of K. ivorensis seedlings showed an immediate decrease when transferred from culture solution (-0.3 bars) to -10.3 bars osmotic solution, than did K. senegalensis seedlings. But while the rate of the former species remained steady, that of the latter species decreased gradually so that four hrs after the beginning of the e:xperiment the average University of Ghana http://ugspace.ug.edu.gh rate for the three e^erimental days recorded for the two species was 0.6/cm2/30 min. The reduction for K. senegalensis was to about 10% of that of the control (-0.3 bars), the corresponding reduction was to about 20% of the control for K. ivorensis. 27. Stomatal conductivity studied by the infiltration technique skwed that conductivity decreased vdth soil dryness. K. senegalensis leaves showed greater conductivity vihen soil moisture stress was -0.3 to -0.8 bars than K. ivorensis leaves. 28. High conductivity was observed in the mornings. Conductivity of leaves of K. senegalensis seedlings in soil at -0.3 to -0.8 bars generally started decreasing around 11.30 hrs while that of K. ivorensis showed decrease about 14.00 hrs. Some conductivity was recorded for both species after 15.00 hrs. When the moisture stress was -4.5 bars no conductivity was recorded for any of the species after 12.00 hrs. 29. The desorption curves for both species showed that the R.W.C. at -15 bars L.W.P. or the L.W.P. at 50% R.W,C. were 88 and 84/6 or -47 and -41 bars respectively for leaves of K. senegalensis and K. ivorensis seedlings. 30. R.W.C./L.W.P. curves for leaves of adult trees of the two species were very close together. Comparison of curves for adult tree leaves with those of the seedlings showed, tb&t for K. senegalensis the position of the curve at higher potentials (-44 bars) remained constant. But there was a a l i g h t u p w a rd s h i f t a t l o w e r p o t e n t i a l s f o r t h e a d u l t t r e e l e a v e s . On the o t h e r h a n d , t h e r e vjas a d i s t i n c t u p w a rd s h i f t i n the c u r v e f o r l e a v e s o f t h e a d u l t t r e e s o f K . i v o r e n s i s a s c o m p a re d w i t h t h a t o f t h e University of Ghana http://ugspace.ug.edu.gh 180 seedlings. y 31. The critical levels of leaf water potential at which tissue damage occurred were -81 and -59 bars respectively for K. senegalensis and K. ivorensis seedlings. The corresponding R.W. C. values were 29 and 32$ respectively for the former and latter species. 32. The R.W.C. at which stomatal closure occurred was about 88.0 and 81.0$ for K. senegalensis and K. ivorensis seedlings respectively. The corresponding R.W.C. values for adult trees in the Botanical G-arden were 90.7+1.5 snd 86.2+1.6 for K. senegalensis and K. ivorensis respectively. University of Ghana http://ugspace.ug.edu.gh 181 Appendix 1, Plate I Three months old seedlings of Khaya senegalen sis and K. ivorensis (Note the drip tips and large size of leaves of the latter species). The seedlings shown were taken from the soil moisture treatment experiments so that, A, B, C and D, denote soil tensions of -0.3, -0.4, -0.8 and -4.5 bars respectively. K. senegalensis University of Ghana http://ugspace.ug.edu.gh 182 Appendix 1, Plate I (continued) K. ivorensis University of Ghana http://ugspace.ug.edu.gh 183 Appendix 2 Table i The relationship between soil water potential (- bars) and soil moisture content dry weight) of the experimental soil, .'John Innes Posting Compost II. Soil water potential Soil moisture content (bars) dry weight) -0.3 23.1+1.6 -1.0 10.4+0.6 o 0 CM1 8.8+0.5 -3.0 7.9+0.5 -5.1 5.5+0.8 -7.1 5.0+0.8 -9.1 5.0+0.8 -11.2 4.910.8 University of Ghana http://ugspace.ug.edu.gh APPENDIX 2, GRAPH 1 THE REGRESSION OP PLAN3HETERED AREA ON LENGTH X BREADTH OP LEAVES OP SEEDLINGS OP KHAYA SENEGALENSIS (KS) AND K. IVORENSIS (Kl) GROWING ON SOIL AT THE EUTItt HARVEST. THE REGRESSION WAS DETERMINED POR EACH TREATMENT AT THE PINAL HARVEST. THE REGRESSION EQUATION OBTAINED ARE AS POLLOWS: TREATMENT ! V. STfraiALWfRTR K. IVORENSIS ....I ...... y = -O.25 + 0 .72 X y = 0.50 + 0.60 x B y s= 0 .79 + 0.66 x y a-0.37 + 0.64 x C y s *1.25 = 0.76 x y =-0.28 + 0.62 x D y = O .46 + O .67 x y =■*2.26 + 0.67 x University of Ghana http://ugspace.ug.edu.gh LE AF AR EA ( c m 2) 184 APPENDIX 2 , GRAPH I. University of Ghana http://ugspace.ug.edu.gh .APPENDIX 2, GRAPH 2 THE REGRESSION OF DRY WEIGHT ON PLMIMETEEED TOTAL LEAF AREA PHI SEEDLINGS OP KHAYA SMEGALENSIS (KS) AID K. ITORMSIS (Kl) GROWING IN SOIL. University of Ghana http://ugspace.ug.edu.gh 185 L E A F A REA 1 C M ) J A P P E N D IX 2 G R A P H 2 . University of Ghana http://ugspace.ug.edu.gh 186 Table ii Appendix 2 A breakdown of the total dry weights (g) at initial and final^ harvests into the weights for the main plant organs for seedlings of Khaya senegalensis and K. ivorensis under various soil moisture treatments; Treatments A, (-0.3); B, (-0.4); C (-0.8); D (-4.5) bars. K. senegalensis Plant organ Initial harvest 1 A Final harvest Treatments B C D Mean leaf weight per seedling (g) 0.383 1.185 1.237 O.864 0.855 Mean stem weight per seedling (g) 0.213 1.065 1.219 0.834 0.736 Mean root weight per seedling 0.181 1.806 2.010 1.496 1.379 Total plant dry weight 0.777 4.056 4.466 3.194 2.970 (continued next page) University of Ghana http://ugspace.ug.edu.gh 187 Appendix 2 Table ii ( c o n t i n u e d ) K. ivorensis Plant organ Initial Final harvest harvest A B C D Mean leaf weight per seedling (g) 0.221 0.712 0.850 0.722 0.656 Mean stem weight per seedling (g) 0.099 0.471 0.526 0.503 0.434 Mean root weight per seedling (g) 0.075 0.676 0.769 0.755 O.644 Total plant dry weight 0.395 1.859 2.145 1.980 1.734 University of Ghana http://ugspace.ug.edu.gh APPSJDIX 2, G-BAPH 3 THE HEGBBSSION OP PLMIMETERED J2E4 ON LENGTH X BRMETH OB' SBEELIN&S OP KHAYA SMSGALMSIS (KS) AMD £. ITOBENSlS (Kt) SHOWING- El CULTUHB SOLUTION, *" University of Ghana http://ugspace.ug.edu.gh LE A F AR EA (C M 2 ) 188 APPENDIX 2 GRAPH 3 University of Ghana http://ugspace.ug.edu.gh AFPMDIX 2, GRAPH 4 THE REGRESSION OP DRY WEIGHT ON PLMIMETEHKD TOTAL LEAF AEEA PER SEEDLING OF KHAYA S33SE&ALBNSIS (KS) AMD K. IVOBMSIS (EC) GROWING IN CULTURE SOLUTION. University of Ghana http://ugspace.ug.edu.gh PL AN T DR Y W EI GH T ( G J 189 TOTAL LEAF AREA PER SEEDLING TOTAL LEAF AREA PER SEEDLING ( CM)* APPENDIX 2 GRAPH 4 University of Ghana http://ugspace.ug.edu.gh 190 Table iii A breakdown of the total dry weights (g) at initial and final harvests into the weights for the main plant organs for seedlings of Kha.ya senegalensis and K. ivorensis grown in culture solution of different osmotic potential (bars). Treatments A, (-0.3); B (-2.8); C (-5.3); D, (-10.3) bars. Treatment A, consisted of culture solution alone; B, C and D consisted of culture solution plus polyethylene glycol. K. senegalensis Appendix 2 Plant organ Initial harvest A Pinal harvest Treatment B C D Mean leaf weight per seedling (g) 0.208 0.409 0.293 0.293 0.246 Mean stem weight per seedling (g) 0.129 0.216 0.218 0.209 0.144 Mean root weight per seedling (g) 0.102 0.294 0.273 0.2a 0.157 Total plant dry weight 0.437 0.919 0.784 0.743 0.547 (continued next page) University of Ghana http://ugspace.ug.edu.gh 191 Appendix 2 Table iii (continued) K. ivorensis Plant organ Initial Pinal harvest harvest A B C D Mean leaf weight per seedling (g) 0.077 0.208 0.146 0.116 0.117 Mean stem weight per seedling (g) 0.046 0.091 0.097 0.066 0.062 Mean root weight per seedling (g) 0.028 0.072 0.070 0.047 0.057 Total plant dry weight 0.151 0.371 0.313 0.299 0.236 University of Ghana http://ugspace.ug.edu.gh 192 Appendix 3, P la te IA Photograph o f the simple dendrometer used to measure stem shrinkage, in lo cation on seed ling . University of Ghana http://ugspace.ug.edu.gh Appendix 3» Plate IB 193 Hhotograph of the simple dendrometer used to measure stem shrinkage, showing details of galvanized metal plates, and pointer arrangement. A = Scale C = Galvanized iron plates E. Plant stem B = Metal wire (pointer) D = Expanded polystyrene University of Ghana http://ugspace.ug.edu.gh 194 Appendix 3» Plate IC Photograph of the simple dendrometer used to measure stem shrinkage showing d e ta ils of po in te r ly ing over s ca le . A = Scale C = Galvanized iron p la te s E = P lan t stem B = Metal wire D = Expanded polystyrene University of Ghana http://ugspace.ug.edu.gh APPENDIX 3, GRAPH I M EXAMPLE OP SHE REGRESSION OF STM DIMETER ON RECORD3N&S ON DMDEOMETER SCALE. University of Ghana http://ugspace.ug.edu.gh ST EM DI AM ET ER ( M M ) 195 RECORDING ON DENDROMETER SCALE APPENDIX 3 GRAPH I University of Ghana http://ugspace.ug.edu.gh 196 ' Appendix 3 (Tables i - v) Diurnal variation of leaf relative water content for Khava senegalensis and K. ivorensis seedlings in relation to soil moisture stress (bars) on several days, together with micro- climatic data. Treatments A (-0.3), B (-0.4), C (-0.8), D (-4.5) bars. Table i (15/1^70) Time of day (hrs) Relative water content {%) Microclimatic data K. A senegalensis B C D K. ivorensis A C D Relative humidity m Air tempera­ ture °C 06.00 95.3 97.0 94.1 94.6 95.3 93.3 95.0 87 22 08.00 94.1 93.2 95.3 95.4 93.1 93.7 90.0 76 25 10.00 93.1 93.1 95*4 93.0 91.9 93.4 92.5 51 30 12.00 94.9 91.5 93.8 92.5 91.5 92.1 90.5 60 33 14.00 93.8 93.1 94.4 92.7 92.4 92.6 91.3 48 30 16.00 95.0 94.9 94.9 92.7 92.7 93.7 91.0 56 30 18.00 95.0 95.2 93.9 93.6 93.6 92.1 93.4 69 26 20.00 94.2 92.5 96.7 96.3 90.7 94.8 92.7 76 25 22.00 97.8 95.5 95.6 96.3 94.8 95.0 93.4 80 24 University of Ghana http://ugspace.ug.edu.gh Appendix 3 Table i i (6/3/71) .197 Time of dey (hrs) Relative water content (%) data latic A K. senegalensis B C D A K. ivorensis B C D Relative humidity m Air temp °C. 06.00 98.6 96.3 96.1 95.1 96.9 95.8 94.5 95.0 92 23 08.00 92.6 92.6 93.9 94.4 93.6 91.6 93.0 93.3 90 24 10.00 96.6 95.9 94.7 94.4 89.3 94.3 95.0 92.0 72 28 12.00 96.6 93.9 91.8 91.2 93.0 92.3 91.2 89.9 63 30 14.00 90.8 92.8 91.3 93.6 89.8 89.5 91.4 90.1 53 33 16.00 89.3 92.6 92.9 93.5 88.4 91.9 90.5 91.2 60 32 18.00 95.0 94.8 92.1 94.6 94.1 94.8 93.6 92. k 84 28 20.00 98.0 95.7 94.9 95.4 97.6 94.0 97.2 93. i 90 27 22.00 99.8 97.0 93.6 96.6 99.2 95.6 93.3 94.3 97 25 University of Ghana http://ugspace.ug.edu.gh Table iii (16/3/71) Appendix 3 Time Relative water content (fo) Microclimatic or day (hrs) A K. senegalensis j B C D A K. ivorensis B C D Relative humidity l€\ Air temp. ° c 06.00 >VD ON • O 99.4 97.6 94.3 95.8 98.6 97.7 92.5 91 25 08.00 94.1 97.9 92.8 89.4 94.0 96.2 94.5 90.3 85 25 10.00 93.0 94.9 88.6 83.4 91.9 90.5 87.7 81.2 68 33 12.00 92.9 93.8 89.2 88.5 90.4 90.8 90.5 91.9 58 35 14.00 88.8 89.1 94.8 88.0 81.5 86.9 94.0 89.2 61 34 16.00 90.7 92.9 85.0 82.5 83.6 90.6 81.5 80.3 66 33 18.00 93.2 95.2 91.8 86.2 91.0 90.6 93.1 81.9 76 30 20.00 94.8 94.1 91.0 86.6 92.4 92.3 89.7 79.5 87 28 22.00 98.2 93.4 94.5 93.5 98.0 90.9 95.6 90.9 90 27 University of Ghana http://ugspace.ug.edu.gh 'fable iv (30/3/71) 199 Appendix 3 Time of day (h rs) R e la tive water content (%) M icroc lim atic A K . senegalensis B C D A K . ivo ren s is B C D data R e la tive humidity OS) A ir temp. °C 06.00 98.1 99.1 93.5 93.4 95.2 97.2 95.2 90.3 99 23 08.00 89.2 93.9 95.2 93.3 88.7 91.9 94.7 93.9 91 24 10.00 94.4 94.8 93.8 92.4 90.1 93.3 92.2 89.8 86 28 12.00 84.6 89.2 92.0 91.7 84.1 84.2 89.2 89.1 50 33 14.00 89.6 92.9 90.8 87.8 87.2 89.5 90,8 87.5 50 35 16.00 91.4 87.8 88.8 88.9 85.8 87.3 89.7 84.7 52 34 18.00 95.6 96.0 91.7 89.1 93.7 94.2 92.2 87.4 15 30 20.00 95.0 96.1 93.6 89.2 90.0 93.2 94.6 88.0 84 28^ 22.00 99.0 93.3 92.5 95.5 94.2 95.4 91.8 89.3 91 26 University of Ghana http://ugspace.ug.edu.gh Appendix 3 Table v (5/4/71) . 200 Time of day (hrs) Relative water content (fc) Microclimatic A K., senegalensis B C D K. A ivorensis B C D Relative humidity Air temp. °c. 06.00 98.4 98.1 95.3 90.4 97.3 98.9 96.® 87.8 98 25 08.00 95.1 91.5 93.9 81.8 91.4 87.4 90.2 78.6 87 31 10,00 89.6 91.0 88.2 83.6 84.4 89.1 94.4 84.6 51 36 12.00 92.0 89.7 90.0 80.2 87.9 89.6 89.6 84.3 50 36 14.00 96.4 95.7 52.8 80.0 93.3 91.5 90.6 80.6 53 36 16,00 96.0 93.2 96.? 76.3 93.8 93.8 97.3 72.0 65 36 18.00 97.8 95.8 94.7 82.1 94.7 93.9 94.4 82.8 76 31 20.00 95.7 94.3 95.9 83.8 93-3 95.0 94.5 74.9 86 29 22.00 96.2 97.1 96.5 86.0 94.3 94.5 96.0 83.8 99 26 University of Ghana http://ugspace.ug.edu.gh 201 Appendix 3 (Tables vi-x) Diurnal variation of leaf water potential for Khaya senegalensis and K. ivorensis seedlings in relation to soil moisture stress (bars), on several days, together vo.th microclimatic data, 'jjreatments; A (-0.3), B (-0.4), C (-0.8), D (-4.5) bars. Table vi (15/1^70) Time leaf water potential (bars) Microclimatic of day (hrs) A K. senegalensis B C D IC. A ivorensis C D Relative humidity (?0 Air temp. °C 06.00 -6.5 -4.5 -8.0 -7.5 -4.0 -6.5 -4.5 89 22 08.00 -8.0 -9.0 -6.5 -6.0 -6.1 -6.0 -9.5 76 25 10.00 -9.0 -9.0 -6.5 -9.0 -7.4 -6.5 -7.0 51 30 12.00 -6.5 -11.0 -8.0 -10.0 -8.0 -7.5 -9.0 60 33 14.00 -8.0 -9.0 -7.5 -9.5 -7.0 -7.0 -8.0 48 30 16.00 -6.5 -6.5 -7.0 -9.5 -6.6 -6.0 -16.0 56 30 18.00 -6.5 -6.5 -8.0 -8.0 -7.5 -6.0 -6.5 69 26 20.00 -8.0 -10.0 -5.0 -5.5 -8.5 -4.5 -7.0 76 25 22.00 -3.5 -5.0 -6.0 -5.5 -4.5 -4.5 -6.5 80 24 University of Ghana http://ugspace.ug.edu.gh (6/1/71) 202 Appendix 3 Table vii Time of day (hps) Leaf water potential (bars) Microclimatic A K. senegalensis B C D A K. ivorensis B C D us. ob. Relative humidity (2) Air temp. °C 06.00 -2.5 -5.5 -5.5 -6.5 -2.5 -3.5 -5.0 -4.5 92 23 08.00 -10.0 -10.0 -8.0 -7.5 -6.0 -8.0 -6.5 -6.5 90 24 10.00 -5.0 O•LPk1 -7.5 -7.5 -10.0 -5.5 -4.5 -7.5 72 28 12.00 -5.0 -8.0 -11.0 -■12.0 -6.5 -7.5 -8.0 -9.5 63 30 14.00 -12.0 -9.0 -12.0 -8.0 -9.5 -9.5 -8.0 -9.0 53 33 16.00 -14.5 -10.0 -10.0 -8.0 -11.5 -7£5 -9.0 -8.0 60 32 18.00 -6.5 -6.5 -11.0 -7.0 -5.5 -4.5 -5.5 1 -J • o 84 28 20.00 -3.0 -5.0 -7.0 -6.5 -2.0 -5.5 -3.0 -6.0 90 27 22.00 -1.0 -4.5 -8.5 -5.0 -0.8 -4.0 -6.5 -5.0 87 25 University of Ghana http://ugspace.ug.edu.gh 203 A ppendix 3 T a b le viii (16/3/71) Time of day (hrs) Leaf water potential (bars) Microclimatic A K. senegalensis B C D A K. ivorensis B C D aa-ca Relative humidity OS) Air temp. °C 06.00 -5.5 -1.5 -4.0 -7.5 -3.5 -1.5 -2.0 -7.0 • 91 25 08.00 -8.0 1 • VJ I 1 VO • O -14.0 -5.5 -3.5 -5.0 -9.0 85 25 10.00 -9.0 -6.5 -15.0 -21.0 -7.5 -9.0 -11.5 -18.0 68 33 12.00 -9.0 -8.0 -14.5 -15.0 -9.0 -8.5 -9.0 -7.5 58 35 14.00 -14.5 -14.0 -7.0 -16.0 -17.5 -12.0 -5.5 -10.0 61 34 16.00 -12.0 -9.0 -19.0 -22.0 -15.0 -8.0 -17.5 -19.5 66 33 18.00 -9.0 -6.5 -11.0 -18.0 -8.5 -8.0 -6.5 -17.0 76 30 20.00 -6.5 -8.0 -12.0 -17.5 -7.0 -7.0 -9.5 -19.0 87 28 22.00 -3.0 -8.5 -7.5 -8.5 o •CM1 -8.0 -4.0 -8.5 80 27 University of Ghana http://ugspace.ug.edu.gh 304 Appendix 3 Table ix (30/3/71) Time Leaf water potential (bars) Microclimatic of day (hrs) K. A senegalensis B C D A It. ivorensis B C D aaiia Relative humidity m Air temp. °C. 06.00 -4.0 -2.0 -8.5 -8.5 -4.0 -2.5 -4.5 -9.0 99 23 08.00 -10.5 -8.0 -6.5 -8.5 -10.0 -7.5 -5.0 -5.5 91 24 10.00 -9.0 -6.5 -8.0 -10.0 -9.0 -6.5 -7.5 -9.5 86 28 12.00 -15.0 -14.5 -11.0 -11.5 -15.0 -19.9 -10.3 -10.0 50 33 14.00 -12,0 -9.0 -12.5 -16.0 -12.0 -10.0 -8.5 -11.5 50 35 16.00 -13.0 -16.0 -14.5 -14.5 -13.0 -12.0 -9.5 -14.0 52 34 18.00 -6.0 -5.5 -11.0 -14.5 -6.0 -5.5 -7.5 -11.5 75 30 20.00 -7.5 -5.0 -8.5 -14.5 -9.5 -6.5 -5.0 -11.0 84 28 22.00 -2.0 -9.0 -10.0 -6,0 -5,5 -4.5 -7.5 -10.0 91 26 University of Ghana http://ugspace.ug.edu.gh 205 Appendix 3 Table x (5/4/71) Time of day (hrs) Leaf water potential (bars) Mieroclimatic K. senegalensis A B C D A K. ivorensis B C D data Relative humidity (fQ Air temp, °C 06.00 -2.5 -3.5 -6.5 -12.5 -3.0 -2.0 -3=2.5 -11.5 28 25 08.00 -7.0 -11.5 -2a.e -22.5 -8.0 -12,0 -9.5 -20.0 87 31 10.00 -14.0 -12.0 -15.5 -21.0 -14.5 -10.5 -5.0 -14.5 51 36 12.00 -10.5 -14.0 -13.0 -24.5 -11.0 -10.0 -10.0 -15.0 50 36 14.00 -5.0 -6.0 -9.0 -24.6 -6.5 -8.0 -9.0 -18.0 53 36 16.00 -5.5 -9.0 -4.5 -28.0 -5.5 -5.5 O•1 -25.5 65 36 18.00 -4.5 -6.0 -7.0 -22.5 -4.5 -5.5 -5.0 -16.0 76 31 20.00 -6.0 -7.5 -5.5 -20.5 -6.0 -4.5 -5.0 -25.0 86 29 22.00 -3.5 -4.5 -4.5 -18.0 ^5.5 -5.0 -3.5 -15.0 99 26 University of Ghana http://ugspace.ug.edu.gh 206 Diurnal variation of stem diameter (mm) in seedlings of Khaya Senegalen si a and K. ivorensis in relation to soil moisture content (expressed as percentage of the value at field capacity), during three soil drying-out cycles (Cycles I to III). Table xi Cycle I (12 to 17 May 1971) Appendix 3 Stables xi to xiii Time of day (hours) Stem diameter (mm) 93 K. senegalensis j Soil moisture c 64 51 27 K. ivorensis ontent (J5 field capacity) 93 64 51 27 06.00 13.1 13.2 12.4 13.7 11.8 11.7 11.4 12.2 08.00 13.0 13.0 12.2 13.6 11.6 11.6 11.2 12.1 10.00 12.8 12.8 12.0 13.5 11.5 11.4 10.9 12.1 12.00 12.7 12.7 11.7 13.4 11.5 11.4 10.8 12.0 14.00 12.6 12.6 11.7 13.4 11.5 11.4 10.8 12.0 16.00 12.6 12.7 11.7 13.4 11.5 11.4 10.8 12.0 18.00 12.7 12.7 11.7 13.5 11.6 11.4 10.9 12.0 20.00 - 12.9 11.9 13.5 - 11.6 11.0 12.0 22.00 13.0 13.0 11.7 13.5 11.8 11.7 11.1 12.1 - no record University of Ghana http://ugspace.ug.edu.gh 207 Appendix 3 Table xii Cycle n (18 to 21 May 197l) Time of Stem diameter (mm) day (hours) K. senegalensis j K. ivorensis Soil moisture content (% field capacity] 94 70 51 94 70 51 06*00 12.6 12.8 13.2 11.5 11.5 11.5 08.00 12.5 12.6 13.0 11.3 11.4 11.2 10.00 12.4 12.3 12.7 11.1 11 .1 11.0 12.00 12.3 12.2 12.5 11.0 10.8 10.9 14.00 12.3 12.1 12.4 11.0 10.8 10.9 16.00 12.3 12.2 12.4 11.1 10.8 10.9 18.00 12.4 12.4 12.4 11.2 11 .1 11.0 20.00 12.6 12.5 12.7 11.4 11.4 11.2 22.00 12.6 12.6 12.8 11.4 11.4 11.3 University of Ghana http://ugspace.ug.edu.gh 208 Cycle III (24 May to 3 June 1971) Appendix 3 Table xaii Time of day (hours) Stem diameter (mm) K. senegalensis j K. ivorensis Soil moisture content (/? field capacity) 93 64 50 27 93 64 50 27 06.00 12.8 12.9 12.7 Ha 4 11.8 11.9 11.8 11.7 08.00 12.6 12.7 12.7 11.3 11,6 11.6 11.7 11.5 10.00 12.3 12.3 12.5 11.2 11.3 11.5 11.6 11.3 12.00 12.1 12.1 12.2 11. 2 11.3 11.4 11.3 11.2 14.00 12.1 12.1 12.1 11.2 11.3 11.4 11.3 11.2 16.00 12.2 12*3 12.0 11.3 11.3 11.5 11.3 11.3 18.00 12.3 12.4 12.2 11.3 11.5 11.6 11.4 U.4 20.00 12.5 12.5 12.4 11.4 11.7 11.7 11.6 11.5 22.00 12.7 12.6 12.6 11.4 11.8 11.8 11.7 11.5 University of Ghana http://ugspace.ug.edu.gh 209 Transpiration rate of seedlings of Khaya senegalensis . and K. ivorensis an relation to soil moisture stress (Bars); A (-0,3), B (-0*4), C (-0,8), ■ D (-4*5) v together with mioronlimatic data. 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