DEVELOPING SUPERIOR CASSAVA [MANIHOT ESCULENTA (CRANTZ)] VARIETIES USING PARTIAL INBREDS By OKORO, PERPETUA (10359701) THIS THESIS/ DISSERTATION IS SUBMITTED TO THE UNIVERSITY OF GHANA, LEGON IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE AWARD OF DOCTOR OF PHILOSOPHY PLANT BREEDING DEGREE WEST AFRICA CENTRE FOR CROP IMPROVEMENT SCHOOL OF AGRICULTURE COLLEGE OF AGRICULTURE AND CONSUMER SCIENCES UNIVERSITY OF GHANA LEGON DECEMBER 2015 University of Ghana http://ugspace.ug.edu.gh ii DECLARATION I hereby declare that except for references to works of other researchers which have been duly cited, this work is my original research and that neither part nor whole has been presented elsewhere for the award of a degree. .................................................. Okoro, Perpetua STUDENT .................................................. Prof. Vernon Gracen Supervisor .................................................. Prof. Essie Blay Supervisor ................................................. Prof. I.K. Asante Supervisor ……………………………….. DR. C. N. Egesi Supervisor University of Ghana http://ugspace.ug.edu.gh iii ABSTRACT The research was conducted to develop cassava hybrids with superior traits. Two sets of elite S2 lines collected from the National Root Crop Research Institute (NRCRI) Umudike and the International Institute of tropical Agriculture (IITA) were used for the the research. The experiments were conducted at (IITA). The field experiment was conducted in an alpha lattice design. A component of the research involved a Participatory Rural Appraisal which was conducted to identify farmers’ preferences for cassava varieties were carried out in the three senatorial zones of Edo State. The Participatory Rural Appraisal was conducted through focus group discussion and semi-structured questionnaire administration. The survey revealed that farmers in Edo State desire varieties with high yield, high dry matter, and starch content. It also revealed that the farmers prefer their local varieties to improved varieties. Two sets of crosses were made between the S2 lines. The first involved random matings and was done at off season in November, 2012. The other was a 6x6 diallel mating design done in September, 2013 to determine combining ability, degree of heterosis and gene action influencing the yield and yield component traits. Much of the genetic variation identified among the S2 lines was non-additive in nature. High heritability in some of the traits indicated high breeding value in the S2 lines. Sufficient useful genetic variation was present in the lines that could be exploited for high fresh root yield and dry matter. There was significant heterosis for yield and dry matter content. For the first group of crosses, the highest mid-parent and better parent heterosis for root number was recorded in the hybrids TMS14-001-03 with 147% increase and TMS14-091-08 with 122 % increase. For fresh root yield, the highest positive mid-parent and better parent heterosis were recorded for the same hybrid TMS14-001-07 with 350.08% for MPH and University of Ghana http://ugspace.ug.edu.gh iv with 276% for BPH. The best MPH for dry matter content was recorded for the hybrid TMS14-035-5 with 18.3% and the same hybrid for BPH with 13.24%. For harvest index, the best hybrids for MPH was TMS14 091-04, with 39.2% and TMS14-035-06 with 33.9% increase respectively. Eight hybrids recorded superior dry matter content and two hybrids recorded superior fresh root yield compared to the parents and checks. For the second group, ten hybrids had high fresh root yield, three hybrids had high dry matter, and seven hybrids had high total carotene content making a total of 20 superior hybrids. Significant heterosis was observed. The highest MP and BP heterosis for fresh root yield was seen in the cross IBA102888 X NRIS2109-10. The highest for BP heterosis for DM was seen in the cross IBA102923 X NRIS2109-14. MP heterosis was as high as 204.68%, 63.98% and 89.77%, and BP heterosis was as high as 176.50%, 51.31% and 75.32% for the three traits, respectively. Five hybrids had high levels of heterosis and specific combining ability better than the best parent and the best check (farmers’ variety) used in the study. University of Ghana http://ugspace.ug.edu.gh v DEDICATION To my husband – Ifesinachi To my children – Somtoochukwu and Ifesinachi To the blessed memories of our late mothers – Mrs Anne Udu and Mrs Ucha Okoro University of Ghana http://ugspace.ug.edu.gh vi ACKNOWLEDGMENTS I give praise, worship and adoration to the King of Kings for His mercies throughout the period of this study. I am indebted to you my Lord for grace, mercy and favour. Special appreciation goes to AGRA for sponsoring this study through the scholarship grant to WACCI. I appreciate the management and staff of WACCI under the leadership of Prof. Eric Danquah for selecting me to be a beneficiary of this scholarship. Thank you. I wish to express my sincere thanks to my supervisors: Professors Vernon Gracen, Asante Isaac K, Essie Blay and Drs Egesi Chiedozie and Peter Kulakow for their patience, diligent guidance, constructive criticism and supervision. I appreciate Dr Kulakow for his benevolence, trainings given and guidance at IITA for the period of this study—Dr, God bless you. I appreciate the support of Dr Ismail Y. Rabbi, Dr Elizabeth Parkes, Dr Paul Ilona, Dr A.G.O. Dixon, Mrs Sylvia Oyinlola and Mr Peter Iluebbey. Worthy of mention is the support I received from the staff of the Cassava Breeding Unit, IITA, Ibadan, Ikenne and Ubiaja stations. Names are numerous to mention here but I sincerely appreciate you all. I appreciate the entire staff of IITA training unit, God bless you. I appreciate the effort of late (Dr) Ken Nwosu of NRCRI, Umudike who believed in me and supported me by writing a recommendation letter to WACCI on my behalf. I sincerely appreciate my siblings (Kate, Stella, Elizabeth, Jude, Pius, Veronica, Mary and Uche), your support, love and care for me and children during the period of this study are boundless. Thank you. I appreciate my father-in-law, Elder Major (Rtd) Dr. Okoro, O.U. – daddy, your support, I appreciate. God bless you. My late mother-in-law (Mrs Ucha Okoro), death did not allow you to see the outcome of your support and prayers for the University of Ghana http://ugspace.ug.edu.gh vii successful completion of this study. Amaama, rest in peace. Worthy of mention are my in- laws (Nnenna, Nkechi, Azu, Uka, Osgood and Chibuzo) for their support, love and prayers. I appreciate my father-Chief Aloysius O. Udu you always believed in me, daddy thank you for your support, prayers and encouragement. I sincerely appreciate the efforts of Dr Ceballos Hernan during my proposal presentation your induction and guidance was instrumental to the outcome. I appreciate the support of Dr Beatrice Ifie, Mr Bakare, Prof Nokoe, Otene, Uche Okeke and Nofou for the assistance rendered on data analysis. I appreciate Dr Damian Njoku for his assistance in making this a reality. My cohort 4 colleagues (Sissoko, Priscilla, Nofou, Mahamadou, Fafa, Maureen and Dorcas), your assistance and support all the way are appreciated. Thank you all. I also appreciate the assistance received from Cohort 8 colleagues… thank you all. To my big brother Rev Father Ukpai your prayers and encouragement can not be measured. To my brothers in the Lord Rev Father Orji, Rev Father Damian, Rev Father Dominic, Rev Father Ajayi and others numerous to mention. God bless you all. I appreciate friends and colleagues (Emilia, Nkechi, Jacy, Ngalamu, Abraham, Ijeoma, Kumba, Isata, Sam and Toba), Africa Rice unit, the Bioscience unit, and many more too numerous to mention whose support I have enjoyed in this journey--- God bless you. My husband, Ifesinachi Okoro, you are really appreciated for your sacrifices. You filled the gap for our sons during my long absence from home. I appreciate your support and encouragement. To our sons, Somtochukwu and Ifesinachi, you guys have been wonderful, thank you. Mummy loves you. University of Ghana http://ugspace.ug.edu.gh viii TABLE OF CONTENTS DECLARATION ....................................................................................................................... ii ABSTRACT .............................................................................................................................. iii DEDICATION ........................................................................................................................... v ACKNOWLEDGMENTS ........................................................................................................ vi TABLE OF CONTENTS ........................................................................................................ viii LIST OF TABLES .................................................................................................................. xvi LIST OF FIGURES ................................................................................................................. xx LIST OF PLATES .................................................................................................................. xxi LIST OF ABBREVIATIONS ................................................................................................ xxii CHAPTER ONE ........................................................................................................................ 1 1.0 GENERAL INTORDUCTION ........................................................................................... 1 CHAPTER TWO ....................................................................................................................... 7 2.0 LITERATURE REVIEW ................................................................................................... 7 2.1 Origin, History and Botany of Cassava .......................................................................... 7 2.1.1 Origin and evolution of cassava ........................................................................... 7 2.1.2 Taxonomy ................................................................................................................ 7 2.1.3 Genetics and cytology of cassava......................................................................... 8 2.1.4 The stem ............................................................................................................... 9 2.1.5 The leaf.................................................................................................................... 9 2.1.6 The inflorescence ................................................................................................... 10 University of Ghana http://ugspace.ug.edu.gh ix 2.1.7 The fruit .............................................................................................................. 11 2.1.8 The seed.............................................................................................................. 11 2.1.9 Storage roots, tuberous roots, thickening and bulking ............................................ 12 2.2 Importance of cassava ................................................................................................... 13 2.2.1 Cassava production in Nigeria ........................................................................... 15 2.2.2 Utilization of Cassava Products in Nigeria ........................................................ 16 2.2.3 Cassava producing states in Nigeria................................................................... 17 2.2.4 Cassava Varieties: .............................................................................................. 19 2.5 Cropping systems .......................................................................................................... 20 2.6 Genetic diversity ............................................................................................................ 20 2.6.1 Genetic diversity of cassava ............................................................................... 21 2.7 Genetic distance ............................................................................................................ 22 2.8 Cassava pests and diseases............................................................................................ 23 2.9 Genetic improvement in cassava .................................................................................. 25 2.10 Hybridization and selection in cassava ......................................................................... 25 2.11 Breeding ....................................................................................................................... 26 2.12 Breeding procedures ..................................................................................................... 27 2.12.1 Germplasm collection and evaluation ................................................................ 27 2.12.2 Source population ............................................................................................... 27 2.12.3 Seed production .................................................................................................. 28 University of Ghana http://ugspace.ug.edu.gh x 2.12.4 Seed germination and transplanting ................................................................... 29 2.12.5 Breeding scheme ................................................................................................ 30 2.13 Yellow root cassava ..................................................................................................... 30 2.14 Environmental variation ............................................................................................... 31 2.14.1 Genetic variation ................................................................................................ 32 2.14.2 Genotype by environment (G x E) interactions ................................................. 33 2.14.3 Heritability ...................................................................................................... 34 2.15 Dry matter content ........................................................................................................ 35 2.15.1 Accumulation and partitioning of dry matter ..................................................... 35 2.15.2 Estimation of dry matter content ........................................................................ 36 2.16 Mating design. .............................................................................................................. 37 2.16.1 North Carolina Design I ..................................................................................... 38 2.16.2 North Carolina Design II ....................................................................................... 38 2.16.3 North Carolina Design III .................................................................................. 39 2.16.4 Diallel Design ..................................................................................................... 39 2.16.5 Half-diallel mating design .............................................................................. 40 2.16.6 Analysis of half-diallel mating design with missing crosses ............................. 40 2.16.7 Half-diallel analysis with missing crosses using mixed model procedure in SAS programme. ............................................................................................................... 41 2.16.8 Combining ability ............................................................................................... 42 University of Ghana http://ugspace.ug.edu.gh xi 2.17 Inbreeding .................................................................................................................... 42 2.17.1 Inbreeding depression ........................................................................................ 43 2.17.2 The genetic basis of inbreeding depression........................................................ 43 2.18 Heterosis ....................................................................................................................... 44 2.18.1 Types of heterosis............................................................................................... 44 2.18.2 Methods for Estimation of Heterosis ................................................................. 45 2.19 Participatory Rural Appraisal (PRA) ............................................................................ 45 CHAPTER THREE .................................................................................................................. 47 3.0 EVALUATION OF FARMERS’ PERCEPTION OF DIFFERENT CASSAVA VARIETIES AND DESIRED CHARACTERISTICS FOR PREFERRED VARIETIES47 3.1 INTRODUCTION ........................................................................................................ 47 3.2 MATERIALS AND METHODS .................................................................................. 49 3.2.1 Description of the study areas ............................................................................ 49 3.2.2 SAMPLING PROCEDURES AND DATA COLLECTION ............................ 51 3.2.3 Structured Survey ............................................................................................... 51 3.2.4 Participatory Rural Appraisal (PRA) ................................................................. 52 3.2.5 Data collection and analysis ............................................................................... 53 3.3 Results ........................................................................................................................... 53 3.3.1 Bio-data and Socio-economic characteristics of Respondents ........................... 53 3.3.2 Farmers’ major objective for cassava production and customer patronage ....... 57 University of Ghana http://ugspace.ug.edu.gh xii Farmers’ major objective for cassava production and customer patronage are presented in ......................................................................................................................................... Table 3.3.............................................................................................................................. 57 3.3.3 Percentage of cassava harvest sold and difficulties experienced in the sale of cassava. ..................................................................................................................... 59 3.3.6 Farmers Desirable traits to be incorporated into cassava breeding programmes 64 3.3.7 Major constraints to cassava production ............................................................ 68 3.3.8 Problems encountered in getting improved varieties ......................................... 69 3.3.10 Knowledge, perception and planting of improved varieties .............................. 71 3.3.11 Source of information on improved cassava varieties ....................................... 72 3.3.12 Farmer participation in field days ...................................................................... 74 3. 3.13 Labour demand .............................................................................................. 75 3. 3.14 Diseases and Pests affecting Cassava plants in the study area ....................... 76 3.3.15 Gender differences in cassava production .......................................................... 79 3.3.16 Other crops planted with cassava in the study area ............................................ 80 3.4 Discussion ..................................................................................................................... 81 3.5 Conclusions and Recommendations ............................................................................. 85 CHAPTER FOUR .................................................................................................................... 90 4.0 PHENOTYPIC AND YIELD CHARACTERIZATION OF THE S2 PARENTS AND THEIR DERIVED F1 PROGENIES ................................................................................. 90 4.1 Introduction .................................................................................................................... 90 University of Ghana http://ugspace.ug.edu.gh xiii 4.2 MATERIALS AND METHODS ................................................................................... 93 4.2.1 Plant material .......................................................................................................... 93 4.2.2 Seed germination ..................................................................................................... 94 4.2.3 Experimental Design (Preliminary Yield trial) ....................................................... 94 4.2.4 Data collection ........................................................................................................ 95 4.2.5 Data analysis ........................................................................................................... 97 4.2.5.2 Principal Components Analysis ................................................................................... 97 4.4 Results ............................................................................................................................ 99 4.4.1 Agro- morphological data ....................................................................................... 99 4.4.2 Principal Component Analysis .............................................................................. 101 4.4.3 Pearson Correlation Analysis ................................................................................ 103 4.4.4 Heterosis Estimation ............................................................................................. 104 4.4.5 Cluster Analysis .................................................................................................... 105 4.4.6 Average performance of the best 20 and worst 20 genotypes ranked by yield ..... 112 4.5 Discussion .................................................................................................................... 114 4.5.1 Agro- morphological data ..................................................................................... 114 4.5.2 Principal Components Analysis ............................................................................ 115 4.5.3 Pearson Correlation Analysis ................................................................................ 116 4.5.4 Mid parent and better parent Heterosis ................................................................. 117 4.5.5 Cluster Analysis .................................................................................................... 118 University of Ghana http://ugspace.ug.edu.gh xiv 4.6 Conclusion .................................................................................................................. 118 CHAPTER FIVE .................................................................................................................... 120 5.0 COMBINING ABILITY AND HETEROSIS IN THE ELITE S2 CASSAVA LINES USING HALF DIALLEL MATING DESIGN ............................................................................ 120 5.1 Introduction .................................................................................................................. 120 5.2 Materials and Methods ................................................................................................. 123 5.2.1 Germplasm source and progeny Development ..................................................... 123 5.2.2 Seed germination ................................................................................................... 123 5.2.3 Experimental design for Preliminary Yield Trial.................................................. 124 5.2.4 Data collection ...................................................................................................... 125 5.3 Results .......................................................................................................................... 129 5.3.1 Estimation of combining ability ............................................................................ 129 5.3.2 Gene action and relative importance of general and specific combining ability .. 129 5.3.3 Heritability ............................................................................................................ 130 5.3.4 General combining ability ..................................................................................... 131 5.3.5 Specific combining ability .................................................................................... 132 5.3.6 Heterosis ................................................................................................................ 133 5.4 Discussion .................................................................................................................... 139 5.4.1 Estimation of combining ability variances ............................................................ 139 5.4.2 Heritability ............................................................................................................ 139 University of Ghana http://ugspace.ug.edu.gh xv 5.4.3 General combining ability ..................................................................................... 140 5.4.4 Specific combining ability .................................................................................... 140 5.4.5 Gene action and relative importance of general and specific combining ability .. 141 5.4.6 Heterosis ................................................................................................................ 142 5.5 Conclusions ................................................................................................................. 143 CHAPTER SIX ...................................................................................................................... 144 6.0 GENERAL CONCLUSION AND RECOMMENDATION ........................................... 144 6.2 RECOMMENDATIONS ................................................................................................. 146 BIBLIOGRAPHY .................................................................................................................. 148 APPENDICES ....................................................................................................................... 179 University of Ghana http://ugspace.ug.edu.gh xvi LIST OF TABLES Table 2.1: Levels of cassava production from 1990-2003 (tonnes) .................................. 15 Table 2.2: Animal Feed Rations using Cassava Meal ...................................................... 16 Table 2.3: Cassava Production by Zone 2000-2002 (tonnes) ........................................... 18 Table 3.1: Location and elevations of the study areas ...................................................... 51 Table 3.2: Bio-data and Socio-economic characteristics of respondents ......................... 55 Table 3.3: Farmers’ major objective for cassava production and Customer patronage .... 58 Table 3.4: Percentage of Cassava sold in a Cropping season, difficulties experienced in the sales of cassava and Reasons for difficulties experienced in sales ............................ 60 Table 3.5: Farming Practices of Cassava Farmers in Edo State ....................................... 62 Table 3.6: Problems encountered in getting improved varieties ....................................... 70 Table 3.7: Farmers’ perception on involvement in cassava research ............................... 71 Table 3.8: Knowledge, Perception and planting of improved varieties ............................ 72 Table 3.9: Source of information and planting materials of improved cassava varieties . 73 Table 3.10: Participation in field days .............................................................................. 75 Table 3.11: Labour demand .............................................................................................. 76 Table 3.12: Diseases and Pests affecting Cassava plants in the study area ..................... 78 Table 3.13: Gender differences ......................................................................................... 80 Table 4.1: Genetic material for study ............................................................................... 93 Table 4.2: Agro-ecological characteristics of the locations where evaluation was performed, 2014/2015 ................................................................................................ 95 University of Ghana http://ugspace.ug.edu.gh xvii Table 4.3: Mean square values for plant height (cm), cassava mosaic disease and cassava bacterial blight ......................................................................................................... 100 Table 4.4: Mean square values for root number, fresh root yield total carotene content, top yield, dry matter content and harvest index ............................................................. 101 Table 4.5: Eigenvalues of the principal component........................................................ 102 Table 4.6: Principal component analysis from ten traits showing their contribution to the total variation among 87 S2F1 hybrids in cassava genotypes .................................. 103 Table 4.7: Phenotypic correlation on plant height, cassava mosaic disease, cassava bacterial blight, root number, fresh root yield (t/ha), total carotene content, top yield (t/ha), dry matter content and harvest index ............................................................ 106 Table 4.8: Mean performance of parents, F1 hybrids and percentage increase over mid- parent (relative heterosis) and better parent (heterobeltiosis) for root number and fresh root yield. ................................................................................................................. 107 Table 4.9: Mean performance of parents, F1 hybrids and percentage increase over mid- parent (relative heterosis) and better parent (heterobeltiosis) for dry matter content and harvest index ............................................................................................................ 108 Table 4.10: Mean performance of parents, F1 hybrids and percentage decrease over mid- parent (relative heterosis) and better parent (heterobeltiosis) for root number and fresh root yield. ................................................................................................................. 109 Table 4.11: Mean performance of parents, F1 hybrids and percentage decrease over mid- parent (relative heterosis) and better parent (heterobeltiosis) for dry matter content and harvest index. ........................................................................................................... 110 Table 4.12: Average performance of the best 20 genotypes ranked by yield ................. 113 University of Ghana http://ugspace.ug.edu.gh xviii Table 4.13: Average performance of the worst 20 genotypes ranked by yield .............. 114 Table 5.1 Genetic material for study............................................................................... 123 Table 5.2: Agro-ecological characteristics of the locations where evaluation was performed, 2014/2015 .............................................................................................. 125 Table 5.3: Performance of the best 5 F1 hybrids between S2 Cassava Lines over three reps and two locations ..................................................................................................... 126 Table 5.4: Mean square values and estimates of genetic components for cassava mosaic disease severity, (mcmds) dry matter (dm) and fresh yield, (fyld t/ha), top yield (tyld t/ha) and total carotene (tc) Ibadan. ......................................................................... 129 Table 5.5: Mean square values and estimates of genetic components for cassava mosaic disease severity, (mcmds) dry matter (dm) and fresh yield, (fyld t/ha), top yield (tyld t/ha) and total carotene (tc) Ikenne. ......................................................................... 130 Table 5.6: Mean and GCA effects of genotypes for dry matter (dm) and fresh yield, (fyld t/ha), and total carotene (tc) for Ibadan and Ikenne ................................................. 132 Table 5.7: Mean and SCA effects of the F1 crosses for fresh yield, (fyld t/ha), dry matter (dm) and total carotene (tc) for Ibadan .................................................................... 134 Table 5.8: Mean and SCA effects of the F1 crosses for fresh yield, (fyld t/ha), dry matter (dm) and total carotene (tc) for Ikenne .................................................................... 135 Table 5.9: Mean and SCA effects of the F1 crosses for fresh yield, (fyld t/ha), dry matter (dm) and total carotene (tc) for Ikenne .................................................................... 136 Table 5.10: Mid-parent heterosis for fresh yield, dry matter and total carotene content 137 University of Ghana http://ugspace.ug.edu.gh xix Table 5.11: better-parent heterosis for fresh yield, dry matter and total carotene content ................................................................................................................................. 138 University of Ghana http://ugspace.ug.edu.gh xx LIST OF FIGURES Figure 2.1: Map of Nigeria showing 23 cassava growing areas (IITA, 2004). ................ 19 Figure 3.1: Map of Edo State showing the surveyed areas ............................................... 50 Figure 3.2: List of Cassava Varieties most preferred and specific to l location ........... 63 Figure 3.3: Ranking of farmers desired cassava traits to be incorporated into cassava breeding programme .................................................................................................. 65 Figure 3.4: Cassava Varieties most preferred for Dry Matter Content by location .......... 66 Figure 3.5: Cassava Varieties most preferred for Starch Content by location .................. 67 Figure 3.6: Major constraints to cassava production ........................................................ 68 Figure 3. 7: List of other Crops planted with Cassava in Edo .......................................... 80 Figure 3. 8: List of other Crops planted with Cassava in Edo Central ............................. 81 Figure 3. 9: List of other Crops planted with Cassava in Edo South ................................ 81 Figure 4.1: Dendrogram of the 100 genotypes showing the relationship between 87 S2F1 cassava and their parents ......................................................................................... 111 University of Ghana http://ugspace.ug.edu.gh xxi LIST OF PLATES Plate 3.1: Cross section of the respondents in Irrua Edo Central senatorial zone ............ 87 Plate 3.2: Cross section of the respondents in Uromi Edo Central senatorial zone ......... 87 Plate 3.3: Cross section of the respondents in Ayogwiri Edo North senatorial ............... 88 Plate 3.4: Cross section of the respondents in Igbogiri Edo South senatorial zone ......... 88 Plate 3.5: Cross section of the respondents in Iguomokhoa Edo South senatorial zone .. 89 Plate 3.6: Cross section of the respondents in Iguomokhoa Edo South senatorial zone. . 89 University of Ghana http://ugspace.ug.edu.gh xxii LIST OF ABBREVIATIONS ACMV African Cassava Mosaic Virus AFLP Amplified Fragment Length Polymorphism ANOVA Analysis of Variance BPH Better-Parent Heterosis CBB Cassava Bacterial Blight CBSD Cassava Brown Streak Disease CBSV Cassava Brown Steak Virus CIAT International Centre for Tropical Agriculture CMD Cassava Mosaic Disease CMGs Cassava Mosaic Geminiviruses DM Dry Matter DNA Deoxyribonucleic Acid EACMCV East Africa Cassava Mosaic Cameroon Virus EACMMV East Africa Cassava Mosaic Malawi Virus EACMV East Africa Cassava Mosaic Virus EACMV-Ug East Africa Cassava Mosaic Virus-Ugandan Variant EACMZV East Africa Cassava Mosaic Zanzibar Virus ESADP Edo State Agricultural Development Programme ESTs Expressed Sequence Tags FAO Food and Agriculture Organization GCA General Combining Ability GEI Genotype and Environment Interaction H2 Broad Sense Heritability h2 Narrow Sense Heritability IFAD International Fund for Agricultural Development IITA International Institute of Tropical Agriculture MPH Mid-Parent Heterosis NGOs Non-Governmental Organizations NRCRI National Root Crop Research Institute PCA Principal Component Analysis PRA Participatory Rural Appraisal RAPD Random Amplified Polymorphic DNA RFLP Restriction Fragment Length Polymorphism SACMV South Africa Cassava Mosaic Virus SAS Statistical Analysis System SCA Specific Combining Ability SCARs Sequence Characterized Amplified regions SGD Specific Gravity Determination SSR Simple Sequence Repeats University of Ghana http://ugspace.ug.edu.gh 1 CHAPTER ONE 1.0 GENERAL INTRODUCTION Manihot Mill. is a genus of the spurge family Euphorbiaceae, sub family Crotonoidae and the tribe Manihotae. Euphorbiaceae is a dicotyledonous family with 334 genera and 8,900 species of which cassava and castor plants belong. The roots of Manihot cultigens are tuberous and rich in carbohydrate while the roots of the wild species are fibrous and slender. All Manihot species are native to the tropical regions of the America with centers of diversity in Brazil and Mexico (Nassar, 2004; Ospina and Ceballos, 2012). Of all the 98 species, Manihot esculenta Crantz (cassava) is the most important being the only commercially cultivated species in the tropics and subtropics. Cassava is the fourth most important food after rice, wheat and maize and is a fundamental component in the diet of millions of people (FAO/IFAD, 2000). Cassava is one of the most accepted root crops grown in Africa, especially Nigeria (Ajani and Onwubuya, 2013) It is a starchy root crop that feeds about 800 million people in the developing countries (FAO, 2002) and it is estimated that 250 million people in Sub Saharan Africa derive half of their daily calories from cassava (FAO, 2006). Scott et al., (2000) estimated that for the year 1993, annual production of cassava was about 172.4 million tons with a value of $US9.31billion. Nigeria is currently the world’s largest producer of cassava, with an estimated 45 million tons per year (IITA, 2007; FAOSTAT, 2012). Since 1980, the expansion of cassava production in Nigeria has been relatively steady, gaining momentum during the period 1988-1992 following the release of improved IITA varieties that boosted farm production (IITA, 2007). Brazil is the second largest producer (Nassar et al., 2002; FAO, 2005). Africa University of Ghana http://ugspace.ug.edu.gh 2 alone accountes for 51% of production (FAOSTAT, 2008). Approximately 57% of cassava production is used for human consumption, 32% for animal feed and industrial purposes. (Bellotti et al., 1999). One of the best methods to increase cassava production to serve as a main staple, food security and a cash crop in Nigeria is by developing of superior varieties that have fixed agronomic traits and are resistant to diseases, pest and drought (Ugorji, 1998). Like most crop breeding activities, cassava genetic improvement begins with the assembling and evaluating a broad germplasm base, followed by production of new recombinant genotypes derived from selected elite clones. Genetic diversity study is a tool geared towards harnessing the genetic variability that exists in available germplasm. (Hurtado et al., 2008). The success of a breeding programme depends greatly on the knowledge of genetic diversity that exists in available germplasm (Meredith and Bridge, 1984). The knowledge of genetic distances of gene pools in a breeding programme is useful; it creates a better understanding of germplasm organization and efficient parental selection. Genetic diversity can be assessed by a number of methods including morphological data and DNA-based data (Mohammadi and Prasanna, 2003). DNA based molecular markers reveal polymorphisms and is extensively used in various fields of plant breeding and germplasm management. These markers can identify many genetic loci simultaneously with excellent coverage of the entire genome. They are phenotypically neutral and can be applied at any developmental stage (Jones et al., 1997). Crop production effort is geared towards optimum yield. Optimizing yield can be achieved through agronomic practices and/or through breeding. To get maximum yield with best quality combinations are the aims of breeding programmes. The use of heterosis for getting University of Ghana http://ugspace.ug.edu.gh 3 high yield with improved quality has been largely employed in cross-pollinated crops, mainly cereals. The economic importance of heterosis has been proven over the years for large increases in hybrid production for crops like maize, rice, onions, cotton, alfalfa, tobacco and Ethiopian mustard (Suwarno et al., 2014; Bagheri and Jelodar 2010; Pavlović et al., 2011; Muhammad et al., 2014; Tucak et al., 2012; Gixhari and Sulovari, 2010; Teklewold and Becker 2006) and can be gainfully explored in cassava breeding scheme. The studies on the inheritance of quantitative and qualitative traits of cassava have been reported (Easwari et al., 1995; Easwari and Sheela 1998; Pérez et al., 2005; Chavez et al., 2005). Unikrishnan et al., (2004) assessed hybrid vigour for root yield over better-parent values and root yield performance was associated with heterosis for yield components. In chilli pepper Khadi (1983) observed high heterosis for vitamin C content and found additive gene effect was more predominant than non-additive gene effect, also Nair et al., (1986) reported increased heterosis for ascorbic acid content in the hybrids. Similarly, Mtunda (2009) found that 23 out of top 30 best performing cassava progenies were obtained from a cross between a local Tanzanian and exotic variety cassava germplasm. In addition, Friedrichs (2009) found heterosis in yield, seed size and height for Glycine max. Tuhina-Khatun et al., (2010) also reported on superior relative heterosis and heterobeltiosis in wheat. In order to develop cassava varieties that will meet farmers’ diverse preferences and fit into the different cropping systems, it is important to adopt a participatory rural approach where farmers will be involved at all stages of variety development. Morris and Bellon (2004), inferred that farmers in Participatory approach evaluate varieties developed by plant University of Ghana http://ugspace.ug.edu.gh 4 breeders in their fields using their own management practices, provide germplasm, identify agronomic traits to be improved, suggest the selection criteria and help to set the breeding objectives, for instance, participatory rural appraisal has been used in eastern Ethiopia to identify selection criteria for beans varieties, which was based on yield and yield components (Assefa et al., 2005). Muhinyuza et al., (2012) reported about the identification of preferred traits and potato production constraint in Rwanda using PRA. Similarly, Tumuhimbise et al., (2012) indentified a selection criteria and farmers’ perception on early bulking storage in cassava in East and Central Uganda. Kamble (2014) identified PRA as an efficient tool for participatory policymaking, planning and development process for local farmers. Manu-Aduening et al., (2007) used PRA to describe the characteristics needed for cassava varieties in Ghana and reported that farmers preferred cassava varieties that had early growth and vigour to suppress weeds, early maturity, high yield, good cooking quality for making fufu are suitable for intercropping. This would ensure that improved varieties meet farmers’ needs and fit into their cropping systems and environments, improving prospects for adoption. Food security is a challenging task globally and more so in the developing countries in the face of increasing population, climate change and limited resources. Conventionally this is met by more inputs from pesticides, herbicides, fertilizers, and irrigation to increase yield. Farmers who cannot afford these farm inputs always resort to acquiring more land to meet the increasing demand for food. Cassava has limited genetic diversity and is a vegetatively propagated perennial shrub, so improving it requires innovative genetic enhancement methods. Also many cultivars do University of Ghana http://ugspace.ug.edu.gh 5 not flower readily as a result cassava is almost exclusively cultivated clonally by stem cuttings. The genetic improvement of cassava could benefit from introducing inbred lines to exploit heterosis, enhance back-cross scheme for fundamental trait introgression, maintained and improved successful allele combination (Ceballos et al., 2012). Each time pollination is made in cassava it makes the hybrids highly heterozygous therefore, sending the breeders back to starting point. Identification of the true value of each family and progenitor is difficult in cassava breeding unlike the cereals where within family variation is absent in the first generation of crosses. (Ceballos et al., 2015) The development of inbred lines in an out crossing crop like cassava with long period of maturity requires between 9-10 generations of selfing to attain the expected level of homozygosity and many cassava varieties show severe inbreeding depression upon long selfing Kawuki et al., (2009). Developing a hybrid that allows frequency of desirable allele, better understanding of the inheritance of key traits especially fresh root yield in cassava needs different approach in cassava breeding programme. However, literature on exploiting heterosis in cassava with inbred lines or partial inbred is scarce. In a heterozygous clonal plant like cassava, to produce improved crop varieties that are resistant to major pests and diseases, high yielding, and can meet sufficient nutritional requirements, a more practical option is the use of available genetic diversity, through a hybrid breeding strategy that maximizes heterosis. The vegetative propagation of cassava is an advantage to exploit heterosis because once a trait or few traits are fixed it will be perpetuated clonally by small holder farmers. University of Ghana http://ugspace.ug.edu.gh 6 Despite the advances made in cassava breeding for traits such as tolerance/resistance to major diseases, the crop’s yield has remained the same in the major producing areas of the tropics (Kawano, 2003). The obvious reason is the geographical limitation of germplasm diversity. Kawano (2003) stated that 0.5% of 5263 accessions held in CIAT gene bank are from Africa. Similarly, less than 1% of Latin American materials are found in IITA breeding program. The primary goal of this research was to determine farmers’ perception of different cassava varieties and preferred characteristics for cassava varieties, and develop inbred lines which will allow for specific hybrid combinations to be identified in cassava and to determine the performance of hybrids. Specific Objectives:  Objective 1: To determine farmers’ perception of the available cassava varieties and their preffered characteristics for cassava varieties.  Objective 2: To evaluate the performance of some S2 parents and their derived F1 progenies.  Objective 3: To determine combining ability and heterosis in the hybrids of the (S2) lines using half diallel mating design University of Ghana http://ugspace.ug.edu.gh 7 CHAPTER TWO 2.0 LITERATURE REVIEW 2.1 Origin, History and Botany of Cassava The Manihot genus contains 98 species among which the production of latex and cyanogenic glucosides is common (Rogers and Fleming, 1973; Bailey, 1976). These species are classified into 19 taxonomic sections (Rogers and Appan, 1973) varying from trees in the section Glazioviannae to nearly acaulascent subshrubs in the section Stipularis. Other sections such as Tripatitae and Gracilis are perennial subshrubs with large woody roots whose stems frequently die back to the root crown in response to dry periods or fires (Nassar, 1980). 2.1.1 Origin and evolution of cassava All 98 species of the Manihot genus are native to the Neotropics from where cassava was introduced to other regions of the world (Rogers and Appan, 1973). The origin of cultivated cassava is still unclear. Allem (2002) propounded that cultivated cassava originated in South America (Olsen and Schaal, 2001; Allem, 2002). 2.1.2 Taxonomy Cassava belongs to the Euphorbiaceae family, which is made up of about 7200 species, characterized for their notable development of lactiferous vessels, themselves made up of secretory cells called lacticifers. These produce the milky secretion, or “latex”, that characterizes the plants of this family. Plant architecture varies enormously within this family, ranging from arboreal types such as rubber (Hevea brasiliensis) to shrubs, also of economic importance, such as the castor-oil plant (Ricinus communis). Also representing University of Ghana http://ugspace.ug.edu.gh 8 this family are numerous weeds, ornamental plants, and medicinal plants. A highly significant genus of this family is Manihot to which cassava belongs. Although all species of the genus can cross with each other, evidence suggests that, in nature, they are reproductively isolated. About 98 species have been described in this genus, of which only cassava (Manihot esculenta Crantz) has economic importance and is cultivated. Perhaps more than 100 common names now exist for this species, owing to its spread throughout the tropical world by early traders. In Latin America, it is usually known either as yuca (Spanish) or as mandioca (Portuguese). In Brazil, sweet cassava (aipim) is distinguished from bitter cassava (mandioca). Other names in different languages include manioc, manioca, tapioca, and mhogo (Cock, et al., 1979). 2.1.3 Genetics and cytology of cassava Very little is known of either cassava genetics or cassava cytogenetics. The basic chromosome number in the Euphorbiaceae family is usually 8, although this may vary between 6 and 11. About 50% of Euphorbia species are polyploid (Léon, 1976). Although cassava is frequently considered as a polyploid species, analyses conducted during diakinesis and metaphase I indicate the presence of 18 small and similar bivalents in cassava (Hahn et al., 1979). Univalents, trivalents, and late bivalent pairings have also been observed in cassava. The plant is therefore a functional diploid, that is, 2n = 2x = 36 (De Carvahlo and Guerra 2002; Nassar et al., 2008). Magoon et al., (1969) have suggested that certain portions of the genome may be duplicated and, therefore, cassava may in fact be a segmental allotetraploid. Cassava is a perennial shrub. It is monoecious, that is, a single plant may carry both male and female flowers, but these are separated from each other. Cassava plant has sympodial University of Ghana http://ugspace.ug.edu.gh 9 branching and variable plant height, ranging between 1 and 5 m, although maximum height usually does not exceed 3 m. 2.1.4 The stem Stems are particularly important in cassava, as they are the means by which the species is propagated vegetatively or asexually. Lignified parts of the stem, commonly called stakes or cangres (cuttings), serve as “seed” for the crop’s commercial production. The mature stem is cylindrical, with a diameter that varies from 2 to 6 cm and coloring that may be silvery gray, purple, or yellow. Both stem diameter and color vary significantly with plant age and, obviously, with variety. Stems are formed by the alternation of nodes and internodes. The oldest parts may show protuberances, which mark, within the nodes, the position that leaves had initially occupied. The node is that place where a leaf joins the stem. Inserted into the node are the leaf petiole, an axillary bud protected by a scale, and two lateral stipules. The length of internodes in the principal stem is highly variable and depends, not only on the variety, but also on other factors such as plant age, drought, thrips attacks, and available soil fertility. The stem provides a lasting record of the history of the plant’s development, enabling one to deduce the conditions and events that had influenced it. 2.1.5 The leaf Leaves are the organs in which photosynthesis mostly occurs, transforming radiant energy into chemical energy. Leaves are caducous, that is, with age they senesce, and fall from the plant as it develops. The total number of leaves produced by the plant, their longevity, and University of Ghana http://ugspace.ug.edu.gh 10 photosynthetic capacity are varietal characteristics, which are profoundly influenced by environmental conditions. Leaves are simple, consisting of the leaf blade and petiole. The blade is palmate with variable number of lobes, usually, odd, ranging between 3 and 9. Lobes measure between 4 and 20 cm long and between 1 and 6 cm wide. The central lobes are larger than the lateral ones. Lobe shape can be classified in different ways, with variable number of categories. A simple classification distinguishes three types of lobes: linear or straight, obovate, and pandurate. Intermediate types also exist, encouraging the development of other classification systems to qualify such characteristics. Leaf size is a typical characteristic of each cultivar, although it varies with environmental conditions. Leaves produced in the first 3 to 4 months of the plant’s life are larger than those produced after the fourth month. For example, in variety M Col 72, the average leaf area at 4 months old is about 250 cm2 ; at 7 months, it is 130 cm2 ; and at 10 months (harvest), only about 90 cm2. 2.1.6 The inflorescence Not all cassava varieties flower under the same environmental conditions. The environment greatly influences the induction of flowering. Cassava undergoes cross pollination, which makes it a highly heterozygous plant, with each individual being a hybrid. Pollination is typically carried out by insects. Self-pollination is prevented by the female flowers of a raceme opening first before the male flowers of that same raceme. This phenomenon is known as protogyny. However, occasionally, the male and female flowers of different racemes on a single plant may open simultaneously. When this happens, self-pollination may naturally occur. Cassava “flowers” are produced in inflorescences. The basic arrangement of flowers is the raceme where the female flowers occupy basal positions and University of Ghana http://ugspace.ug.edu.gh 11 the male the distal ones. The latter are smaller and usually more numerous than the female ones. Frequently, panicles are also produced, that is, from the botanical viewpoint, a raceme of racemes develops. In such cases, a principal raceme exists, which is composed of secondary racemes. 2.1.7 The fruit Once the female flower has been pollinated, fruit begins to form from the ovary. Fruit maturation requires about 3 months to complete. The fruit is a dehiscent capsule that is trilocular, and ovoid to globate, with a diameter of 1.0 to 1.5 cm and six longitudinal, narrow, and prominent ridges. Cross-sections of the developing fruit show a series of clearly discernible tissues: epicarp, mesocarp, and endocarp. As the seed matures, the epicarp and mesocarp dry up. The endocarp, which is ligneous, opens abruptly when the fruit is mature and dried, releasing and dispersing seeds to a certain distance. During dehiscence tissues separate both, throughout the mid-vein of each fruit loculus and between the separations themselves. 2.1.8 The seed While cassava seed is not important in reproduction and commercial multiplication, it has immeasurable value for plant breeding, as only through sexual reproduction can new, genetically superior, cultivars be developed. The seed is ovoid-ellipsoid in form and measures about 1 cm long, 6 mm wide, and 4 mm thick. The seed coat is smooth, coffee- colored, and mottled gray. In the upper part, especially of new seed, the caruncle is found. This structure is lost once the seed falls to the ground. At the other end of the seed, opposite the caruncle, a small cavity is found. A slender suture leaves from the caruncle and finishes University of Ghana http://ugspace.ug.edu.gh 12 in this basal cavity. The seed coat is the outermost part of the seed. Immediately inside the seed coat is the endosperm, which is formed of polyhedral parenchymatous cells that protect and nourish the embryo, itself located in the central area of the seed. Within the endosperm are found the cotyledons and embryonic axis that gives rise to the new plant after germination. The embryo is made up of the two cotyledonous leaves, plumule, hypocotyl, and radicle. The cotyledonous leaves and endosperm occupy almost the entire interior of the seed; they are white, elliptical, and carnose. 2.1.9 Storage roots, tuberous roots, thickening and bulking The principal characteristic of cassava roots is their capacity for starch storage, which is the reason why, so far, it is the plant organ that has the greatest economic value. However, not all roots produced ultimately become storage organs. When the plant grows from sexual seed, a primary root develops and then, several secondary ones. Apparently the primary root always evolves into a tuberous root, and is the first to do so. If the plant grows from a stake, the roots are adventitious, forming at the lower end of the stake, which produces a callus, and form buds in that part of the stake that is buried in the soil. The roots initially form a fibrous system but, later, some begin thickening and become tuberous roots. The number of tuberous roots is determined, in most cases, by the plant’s early growth. Although root density is low, penetration into the soil is deep. This is a highly relevant characteristic, as it contributes to the plant having the capacity to endure prolonged droughts. Fibrous cassava roots can reach depths of up to 2.5 m. The plant absorbs water and nutrients through the fibrous roots, a capacity that is lost when they become tuberous. Morphologically and anatomically no differences are found among fibrous and tuberous roots. Starch accumulation begins, the direction of root growth changes from longitudinal University of Ghana http://ugspace.ug.edu.gh 13 to radial. As mentioned above, tuberous roots come from secondary enlargement of fibrous roots. This means that the root system first penetrates the soil while they are thin and only begin thickening after penetration. 2.2 Importance of cassava Cassava (Manihot esculenta Crantz) is a tropical root crop in the tropics consumed by over 600 million people in Africa, Asia and Latin America. It is the third most important source of calories in the tropics after rice and maize (Fauquet and Tohme, 2008). The crop is crucial for both food security and income generation. In Asia and Latin America, cassava serves as livestock feed, an industrial raw material, and a source of food (Ceballos et al., 2012). Fauquet and Tohme (2008) described it as the second most important source of calories, an inexpensive food, and emerging cash crop. Cassava is known to have the highest carbohydrates contents among the staple crops (Coursey, 1973). In sub-Saharan Africa, cassava is mainly a subsistence crop grown by small-scale farmers and it feeds over 200 million people daily (Madeley, 1993). World production of cassava root was estimated to be 184 million in 2002, rising to 230 million tonnes in 2008. (FAOSTAT, 2011) The majority of production in 2002 was in Africa, where 99.1 million tonnes were grown; 51.5 million tonnes in Asia; and 33.2 million tonnes in Latin America and the Caribbean. Nigeria is the world's largest producer of cassava. However, based on the statistics from the FAO of the United Nations, Thailand is the largest exporting country of dried cassava, with a total of 77% of world export in 2005 followed by Vietnam, with 13.6%, Indonesia (5.8%) and Costa Rica (2.1%). Worldwide cassava production increased by 12.5% between 1988 and 1990, in 2010, the average yield of cassava crops worldwide was 12.5 tonnes per hectare. The most productive University of Ghana http://ugspace.ug.edu.gh https://en.wikipedia.org/wiki/Africa https://en.wikipedia.org/wiki/Asia https://en.wikipedia.org/wiki/Latin_America https://en.wikipedia.org/wiki/Caribbean https://en.wikipedia.org/wiki/Nigeria 14 cassava farms in the world were in India with a nationwide average yield of 34.8 tonnes per hectare in 2010 (Adams et al., 2009). The cassava plant gives the highest yield of carbohydrate per cultivated area among crop plants, except for sugarcane and sugar beets (FAOSTAT, 2011) Cassava plays a particularly important role in agriculture in developing countries, especially in sub-Saharan Africa, because it does well on poor soils and with low rainfall, and because it is a perennial that can be harvested as required. Its wide harvesting window allows it to act as a famine reserve and is invaluable in managing labor schedules. It offers flexibility to resource-poor farmers because it serves as either subsistence or a cash crop. (FAOSTAT, 2011: Adjebeng-Danquah et al., 2012). Nweke et al., (2002) stipulated that the bulk of cassava production is consumed as food. Cassava is described as a ‘classic food security crop’ (DeVries and Toenniessen, 2001), cassava offers several advantages: it provides a decent harvest under erratic rainfall conditions and degraded soils, it grows well under marginal conditions where few other crops could survive, and allows farmers to keep the roots stored in the ground until when needed (El-Sharkawy, 1993) It provides a flexible harvesting date or extended harvesting period. University of Ghana http://ugspace.ug.edu.gh 15 2.2.1 Cassava production in Nigeria Nigeria grows more cassava than any other country in the world. The production of cassava is in the hands of numerous smallholder farmers located primarily in the south and central regions of Nigeria. A significant population of cassava growers in Nigeria has made the transition from traditional production systems to the use of high-yielding varieties and mechanization of processing activities (Nweke et al, 2002). According to Berry (1993), Nigeria and Zaire possess both large and small scale farms on which cassava is grown by full-time and part time farmers. In these farming areas, an average of about 45 percent of cassava field were cultivated for commercial purposes, but this varied from 0 to 100 percent (Nweke, 1989). Table 2.1: Levels of cassava production from 1990-2003 (tons) University of Ghana http://ugspace.ug.edu.gh 16 FAO (2004) provided statistics of cassava production of three countries, Nigeria, Cameroun and Togo, for the period 1990 to 2003 (Table 2.1). The data shows that there was increased cassava production in the three countries with Nigeria leading levels. 2.2.2 Utilization of Cassava Products in Nigeria Cassava contains about 92.2 percent carbohydrates and 3.2 percent protein in its dry matter, and is said to have high energy content. It has a capacity of substituting up to 44 percent maize in pig feed without any reduction in the performance of pigs. Okeke (1998) also reported that in compounding feed for pigs, broilers, pullets and layers, cassava meal plays a significant role. Eagleston et al., (1992) provided evidence as contained in table 2 that the whole cassava plant, boiled root, cassava root meal, chips and pellets could be used in compounding livestock feed (Table 2.2). The roots could be dried, ground and fed to ruminants and it could be used as substitutes for maize in poultry feed. Table 2.2: Animal Feed Rations using Cassava Meal Types of feed Percentage cassava meal Cautius Maximum Broiler starter 5 10 Broiler finisher (4 wks) 10 20 Chick starter 5 10 Pullet starter 10 25 Layer 25 40 Piglets 5 10 Pigs (8-18 wks) 10 25 Source: Eagleston et al (1992) University of Ghana http://ugspace.ug.edu.gh 17 Furthermore, cassava starch, cassava flour, cassava juice and fermented cassava are now used in industries (Terry et al., 1983; Ene, 1992; Olomu, 1995). For instance, cassava starch is used in making products such as biscuits, bread and derivatives such as sagos and sauce. Cassava starch has also been industrially modified to provide products with physical and chemical properties for specific applications, including the preparation of jelly, thickening agents, gravies, custard powders, baby food, glucose and confectioneries (Ene, 1992). Apart from being used in a variety of paste products such as spaghetti and macaroni, cassava flour has been identified to be useful in the manufacture of cassava beer in the brewery industry (Olomu, 1995). In addition, Terry et al., (1983) noted that since the rapid escalation of energy cost, especially liquid fuel prices, considerable attention has been given to cassava as a source of ethanol with particular example in Brazil, where enormous effort had been put into production of alcohol using sugarcane and cassava as biological resources. 2.2.3 Cassava producing states in Nigeria Expansion of cassava production has been relatively steady since 1980 with an additional push between the years 1988 to 1992 owing to the release of improved IITA varieties The North Central zone produces over 7 million tonnes of cassava a year (1999 and 2002). South South produces over 6 million tonnes a year while the South West and South East produce less than 6 million tonnes a year. Production from the North West and North East is 2 and 0.14 million tonnes respectively (Table 2.3). University of Ghana http://ugspace.ug.edu.gh 18 Table 2.3: Cassava Production by Region in Nigeria from 2000-2002 (tonnes) Region 2000 2001 2002 South West 4993380 5663614 5883805 South South 6268114 6533944 6321674 South East 5384130 5542412 5846310 North West 2435211 2395543 2340000 North Central 7116920 7243970 7405640 North East 165344 414533 140620 Total 26363099 27521016 27938049 Source: (PCU, 2004) On a per capita basis, North Central is the highest producing state at 0.72 tonnes/per person in 2002, followed by South East (0.56), South South (0.47), South West (0.34), North West (.10) and North East (0.01). National per capita production of cassava is 0.32 tonne/per person. Benue and Kogi states in the North Central Zone are the largest producers of cassava (IITA, 2004; FAOSTAT, 2012) University of Ghana http://ugspace.ug.edu.gh 19 Figure 2. 1: Map of Nigeria showing 23 cassava growing areas (IITA, 2004). 2.2.4 Cassava Varieties: In the early mid twentieth century when cassava was at the rural food staple stage in Nigeria, farmers relied on farmer- to-farmer transfer of varieties until 1940 in Nigeria. These varieties (cultivars) were originally introduced by The collaborative study on cassava in Africa (COSCA). Cassava varieties planted by the farmers were mostly the sweet type that could be eaten without processing but they gave low yields and were susceptible to pests and diseases. Farmers replaced several of the sweet cassava varieties with the bitter varieties (Nweke, et al., 1994). University of Ghana http://ugspace.ug.edu.gh 20 2.5 Cropping systems According to Alves (2002) cassava is intercropped with short or long season staple crops. When grown particularly as a food crop it is produced under a low input and low output system (Leihner, 2002). In the Americas and Africa cassava is intercropped with maize and legumes (Mutsaers et al., 1993; Alves, 2002). One third of cassava grown in the world is reported to be intercropped, to minimize the risk of crop failure (Cock, 1985) (Kizito et al., 2007). Traditionally farmers practice mixed cropping in Nigeria. Cassava is intercropped with other food crop staples to maximize the use of the land and also to ensure food security (Francis, 1990; Okai, 2001; Dapaah et al., 2003; Manu-Aduening et al., 2006; Baafi and Sarfo-Kantanka, 2008). Production and processing are mostly done by women, and they produce it as food and also process it into ‘garri’ ‘tapioca’ and other products (Al- Hassan, 1989; Nweke et al., 1994). 2.6 Genetic diversity An understanding and clear knowledge of the distribution of genetic diversity and relationship among individuals, populations and gene pools is important for efficient management of germplasm collections and breeding programmes (Manjarrez-Sandoval et al., 1997; Geleta, 2003; Yang et al., 2006) and the potential performance would be useful for all phases of crop improvement. Genetic variability and genetic diversity of a taxon are of great importance to plant geneticists, breeders and taxonomists (Prince et al., 1995). In populations, the genetic composition and genetic diversity are derived from wild parents. This has been influenced by evolutionary processes such as mutation, recombination, genetic drift, migration, natural selection (Hartl and Clark, 1997) and adaptation to a range of environments. An understanding of the genetic diversity is the first step to harness the University of Ghana http://ugspace.ug.edu.gh 21 genetic variability in the germplasm (Hurtado et al., 2008). The success of a breeding programme depends greatly on the genetic diversity that exists in available germplasm (Meredith and Bridge, 1984). Studies on cassava based on DNA sequence and SSR marker data revealed that genetic variation found in cassava is a sub-set of that found in its putative progenitor (Olsen and Schaal, 2001). The evaluation of genetic diversity among adapted or elite germplasm provided estimates of genetic variation among segregating progenies during pure line development (Manjarrez-Sandoval et al., 1997) and also from the degree of heterosis in the progenies of parental combinations (Barbosa-Neto et al. 1997; Cox and Murphy 1990; Geleta, 2003). The assessment of genetic diversity within and between populations is routinely performed at the molecular level using various laboratory-based techniques such as allozyme or DNA analysis, which measure levels of variation directly. Genetic diversity may also be assessed using morphological, and biochemical maker and molecular makers. 2.6.1 Genetic diversity of cassava The genetic distances within a population afford a better comprehension of germplasm organization and efficient parental selection during genotype sampling (Meredith and Bridge, 1984). It also has implications on the choice of parents for crosses and gene introgression from exotic germplasm. The use of DNA-based markers has contributed to cassava breeding and genetics through the understanding of the phylogenetic relationships in the genus (Fregene et al., 1994; Roa et al., 2000; Olsen and Schaal, 2001) and facilitates the assessment of the genetic diversity and origin (Beeching et al., 1993, Second et al., 1997; Okai, 2001; Elias et al., 2001; Mkumbira et al., 2003; Kizito et al., 2007). It has also helped with the development of genetic maps and identification of quantitative trait loci University of Ghana http://ugspace.ug.edu.gh 22 for traits of importance (Fregene et al., 1997; Jorge et al., 2000; 2001; Mba et al., 2001; Okogbenin and Fregene, 2002; 2003; Lokko et al., 2005; Ojulong, 2006; Okogbenin et al., 2006). Other molecular markers used in cassava breeding include single nucleotide polymorphisms (SNPs) identified from whole genome scans, Deletion Amplified Regions Tags (DArTs) and Expressed Sequence Tags (ESTs) (Hurtado et al., 2008; Kawuki et al., 2009). Molecular markers have been successfully used to recommend cultivars for a given region (Vieira et al., 2007). It also helps breeders to concentrate their breeding efforts on the most promising combinations (Ceballos et al, 2004; Brennan and Martin, 2007; Betrand et al., 2008). Crop plants are distinguished by classical morphological descriptors which are highly subject to environmental influences. 2.7 Genetic distance Nei and Li (1979) defined genetic distance as that measure that accounts for the extent of the gene differences between cultivars, as measured by allele frequencies at sample loci while the genetic relationship among individuals and the populations can be measured by similarity of any number of quantitative characters (Souza and Sorrels, 1991). Genetic distance measurements are indicators of relatedness among populations or species and are useful for reconstructing the history and phylogenetic relationships among such groups. There are two basic approaches for measuring genetic distance, these are the cluster analysis and the parsimony analysis and they represent the genetic and phylogenetic relationship, respectively. The data input for this analysis involves numerical or a combination of different variables provided by a range of markers that can be used to measure genetic distance. This includes pedigree data, morphological traits, isozymes and more recently DNA-based markers such as restriction fragment length polymorphism University of Ghana http://ugspace.ug.edu.gh 23 (RFLP), amplified fragment length polymorphism (AFLP), simple sequence repeats (SSR), random amplified polymorphic DNA (RAPD), sequence characterized amplified regions (SCARs) and several others. The molecular markers are recognized as significant tools to enhance plant resource conservation management. This provides a means to accurately estimate the genetic diversity and structure for species of interest (Hamrick and Godt, 1997). 2.8 Cassava pests and diseases African farmers recognize pests and diseases as important production constraints (Ndunguru et al., 2005). Arthropod pests including the cassava green mite (Mononychellus tanajoa Bonder), cassava mealybug (Phenaccocus manihoti Matile-Ferrero), and whitefly (Bemisia tabaci Gennadius) and (Bemisia afer Priesner and Hosny) which pose serious damage to the crop, affect the final yield (IITA, 2000). The cassava green mite mainly causes direct physical damage whilst the whitefly is primarily important as a virus vector. Cassava mealybug and green mite can be controlled by effective classical biological control. Some of the major diseases of economic importance is cassava mosaic disease (CMD) caused by cassava mosaic geminiviruses (CMGs) (Geminiviridae; Begomovirus), cassava bacterial blight (CBB) caused by Xanthomonas axonopodis f.sp. manihoti) which is the most important non-virus disease (Lozano, 1975) and cassava brown streak disease (CBSD) caused by cassava brown streak virus (CBSV) (Potyviridae; Ipomovirus) (Hillocks and Jennings, 2003; Nichols, 1950). Yield losses of susceptible varieties due to CMD have been reported to range from 20 to 95% (Hahn et al., 1979). Unlike CMD, symptoms of CBSD may be found on the roots as brown/yellow, corky necrosis in the starch-bearing tissue, making severely affected roots unfit for consumption (Hillocks et al., 2001). Cassava brown streak disease can decrease the root weight of susceptible cultivars University of Ghana http://ugspace.ug.edu.gh 24 by up to 70% (Hillocks et al., 2001). Mtunda et al. (2003) recorded yield losses due to CBSD of up to 64% in Muheza district, Tanzania. These diseases are the biggest threats to the crop’s health and productivity. Recent studies (Ndunguru et al., 2005) have shown the presence of six distinct cassava mosaic geminiviruses (CMG) species found to infect cassava in Africa: African cassava mosaic virus (ACMV), East African cassava mosaic virus (EACMV), East African cassava mosaic Cameroon virus (EACMCV), East African cassava mosaic Malawi virus (EACMMV), East African cassava mosaic Zanzibar virus (EACMZV) and South African cassava mosaic virus (SACMV). The report indicate that much variation exists in the CMGs including the evidence that certain CMGs when present in mixtures, employ pseudo-recombination or re-assortment strategies and recombination at certain hot spots such as the origin of replication resulting in the emergence of new viruses with altered virulence (Ndunguru et al., 2005). For instance, the severe CMD designated East African cassava mosaic virus-Ugandan variant (EACMV-Ug) which is currently devastating cassava in east and central Africa is a recombination of ACMV and EACMV. Additionally, small satellite DNA molecules (satDNA II and III), which seem to spread with CGMs and have proved to increase disease severity and break resistance in some of the CMD resistant varieties, have also been discovered (Ndunguru et al., 2005). University of Ghana http://ugspace.ug.edu.gh 25 2.9 Genetic improvement in cassava Genetic improvement begins in cassava, with the assembly and evaluation of broad based germplasm (Ceballos et al., 2004; Poehlman and Sleper, 1995; Hahn et al. 1979). The source populations with high frequencies of genes associated with desirable characters are acquired, followed by the production of new recombinant genotypes derived from selected elite clones (Ceballos et al., 2004; Hahn et al., 1979). Selected genotypes from the initial germplasm evaluation normally enter the hybridization scheme, followed by selection of superior clones in the segregating population (Kawano, 2003; Poehlman, 1987). 2.10 Hybridization and selection in cassava The recombination of genes occurs as a result of sexual reproduction (Poehlman, 1987). Since the cassava parent genotypes are highly heterozygous, the selection of suitable parents for hybridization is one of the most important steps in a hybridization programme. Parents are generally selected on the basis of their known performance as varieties and as parents in hybridization programmes. However, selection based on phenotypic performance alone is not a sound procedure (Hallauer and Miranda, 1988), since phenotypically superior lines may yield poor recombinants in the segregating population. Hence, it is necessary that the parents are chosen on the basis of genetic value (Singh, 2003). The performance of a genotype in hybridization programmes depends on its effectiveness in transmitting heredity characteristics to its offspring and it’s combining ability (Falconer and Mackay, 1996). If general combing ability (GCA) is more important, a small number of parents with good GCA should be used in hybridization programmes. On the other hand, when specific combining ability (SCA) is important, a large number of parents should be used to produce a large number of the F1 families (Singh, 2003; University of Ghana http://ugspace.ug.edu.gh 26 Poehlman, 1987). Knowledge of the clones to be used as parents is very important to enhance effective hybridization. Improvement through hybridization comprises: selection of parents; production of F1 progeny; and selection of superior clones (Singh, 2003). Crossing in cassava is relatively easy (Kawano, 1980; Kawano et al., 1978). Clonally propagated crops are generally improved by crossing two or more desirable clones, followed by selection in the F1 progeny. Crossing can occur by controlled pollination, carried out manually to produce full-sib families, or in polycross nurseries where open pollination results, in half-sib families (Ceballos et al., 2004). 2.11 Breeding The goal in cassava breeding is to develop varieties which combine high and stable yields with good quality characteristics relevant to the ways in which the crop is utilized in specific regions. The objectives of a cassava breeding program usually include: high yield in terms of dry matter per unit of land area per unit time, resistance to the major diseases prevalent in target areas (for example, ACMV, CBB and CAD) and resistance to the major insect pests in target areas (for example, CM and CGM). Improved quality in terms of local consumption requirements (for example, low cyanide and mealy varieties in areas where the roots are boiled and eaten without further processing). Adaptability to environmental conditions and cropping systems in target areas and improved plant characteristics in terms of canopy and roots are important to farmers. University of Ghana http://ugspace.ug.edu.gh 27 2.12 Breeding procedures 2.12.1 Germplasm collection and evaluation The most important tasks in any cassava breeding are the acquisition and selection of superior breeding material. In Africa, there is considerable variability among the local germplasm collections. There are two reasons for this. Firstly, some of the materials flower and set seed freely, and new cultivars are established from volunteer seedlings; because cassava is a cross-pollinated crop, continuing recombination and generation of variation occur from outcrosses of genetically heterozygous cultivars. Secondly, spontaneous mutation may give rise to additional genetic variation, although this has not been proven. Many of the local cultivars flower well. However, some flower only to a limited extent ('shy flowering') and others do not flower at all under normal growing conditions; this makes their exploitation in a breeding program rather limited (Ceballos et al., 2012). Both the clones developed by a breeding programme and those from exotic introduction in seed form need to be evaluated in order to identify their potential as breeding materials or as varieties in terms of their agronomic characteristics. The agronomic characteristics include resistance to diseases and pests, characteristic plant architecture, yield, tuber quality, cyanide content, adaptation to agroecological zone and any additional locally important traits. The germplasm may be conserved as clones in field plots, as meristem tips in vitro, and or as seeds in low temperature and humidity conditions. 2.12.2 Source population The source population for improvement is made up of genotypes which have genes associated with desirable characteristics. The population may be improved through cyclic recombination and selection procedures while retaining a high degree of genetic variability. University of Ghana http://ugspace.ug.edu.gh 28 Conventional methods of creating source population can also be used by making crosses between two selected parents. 2.12.3 Seed production The female flowers are large, and open first; the male flowers are small, and usually open about 1 week after the female flowers. Under normal conditions the stigma remains receptive for up to 24 hours after the opening of the flower and dried pollen remains viable for about 6 days under controlled conditions. Both the stigma and pollen are sticky, and pollination is easily carried out by honey bees. Structurally and functionally, therefore, cassava flower is well adapted to cross-pollination. In the northern hemisphere, cassava usually flowers from July to January, with a peak between September and November. In the southern hemisphere, it usually flowers from January to July, with a peak between March and May. The time of flowering, however, depends to a large extent on rainfall distribution, day-length and temperature. In general, there is a vegetative phase of 1 to 4 months in most cultivars that flower under natural conditions, making it important to plant cassava at least 4 months before the peak flowering period. In order to synchronize the flowering periods of different cultivars or clones, parental genotypes should be planted every 2 to 3 months because flowering of an individual plant usually lasts for more than 2 months. For pollination by hand, pollen is collected early in the morning before 10:00 hour and pollination made before 13:00 hour. Both male and female flowers that are at the point of opening are used. When the anthers are mature, they change from green to yellow. This change in color is a useful indication of when pollen can be collected. Pollination can be done by hand using the male flower after removing the perianth or, for mass pollination, University of Ghana http://ugspace.ug.edu.gh 29 by using an applicator. The applicator can be made from a stick with the tip covered with an adhesive piece of velvet-like material to which the pollen will readily adhere. Several flowers can be pollinated without recharging the applicator. If the applicator is to be used for other pollen parents, it should be sterilized; this is done by dipping it into alcohol before using it for new parents. The pollinated flowers are bagged with cloth to protect them against bees or other insects carrying foreign pollen and the bags are removed 5 days later. Seeds mature about 70 to 90 days after pollination. Fruits from pollinated plots are collected in cloth bags hung on cassava plants for each variety or clone and left there until they shatter, releasing hybrid seeds which are ready for germination. 2.12.4 Seed germination and transplanting Cassava seeds have very short dormancy. Seeds germinate quickly at optimal soil temperatures between 30 to 35oC and moisture regimes. Seeds may be sown in peat pellets, jiffy pots or plastic bags arranged on nursery beds during the dry season. During the first 3 weeks, the nursery beds are irrigated twice daily, in the mornings and afternoons, thereafter, they are irrigated at regular intervals until the transplanting stage. If irrigation is not possible, seeds can be planted soon after the first rain. The seeds germinate from 10 to 30 days after planting and are ready for transplanting when they are from 15 to 20cm high because cassava seedlings are weak and grow slowly, weed control is very important at the early stages of growth to offset competition. University of Ghana http://ugspace.ug.edu.gh 30 2.12.5 Breeding scheme Breeding in cassava takes quite a number of years to develop a superior cassava hybrid. However, the International lnstitute of Tropical Agriculture (IITA) has cassava breeding scheme which may be modified to suit local conditions (IITA, 1990). 2.13 Yellow root cassava Most of the cassava landraces cultivated in Nigeria, have white fleshed roots, with little or no pro-vitamin A. In 2005, National Root Crops Research Institute (NRCRI), Umudike, Nigeria acquired some yellow cassava genotypes with improved agronomic traits from International Institute of Tropical Agriculture (IITA), in Nigeria. These genotypes, especially those of orange fleshed roots, were bred as a tool for the global fight against vitamin A deficiency in areas that lack vitamin A rich foods materials (NRCRI, 2008). The deficiency of vitamin A is a serious public health problem in many parts of the world, as it causes eye damage, which when severe, can result in blindness, especially in children. The consumption of carotene rich foods is the most effective intervention for vitamin A deficiency. Since cassava is a major staple food crop in Nigeria the use of yellow-fleshed cassava varieties will be nutritionally preferable. Yellow pigmented cassava root is known to be cultivated in a limited way in Colombia, Philippines, Jamaica and some African countries. Previous studies have shown that yellow root cassava varieties tend to have low dry matter content (Akinwale et al., 2010) which is associated with poor cooking quality (Vimala et al., 2008). Although some yellow landraces have been identified in Amazonia in Brazil (Ferreira et al., 2008; Nassar et al., 2009); most breeding populations are white fleshed root. Wide variation exists in root pigmentation within the global yellow root germplasm, with a range from pale yellow through orange to pink (Nassar et al., 2007). University of Ghana http://ugspace.ug.edu.gh 31 This variation in root pigmentation is associated with wide variation in carotenoid contents within the germplasm (Sanchez et al., 2006). The availability of yellow root cassava (Sanchez et al., 2006; Nassar, 2007) offers a different perception on nutritional benefits associated with the crop. Enhanced content of β-carotene (provitamin A) in yellow root cassava (Chavez et al., 2007; Sanchez et al., 2006) provides great opportunity to sustainably address vitamin A malnutrition through deployment of provitamin A cassava varieties where the crop is a major staple (Makokha and Tunze, 2005; Nassar and Ortiz, 2010). However, the global efforts towards breeding cassava for high β-carotene content are only recent with low progress registered towards deployment of carotene rich varieties to farmers (Ross Welch and Robin Graham, 2004), is attributable to the negative association between β–carotene and dry matter (Vimala et al., 2008; Akinwale et al., 2010). 2.14 Environmental variation The field is often heterogeneous with respect to plant growth factors such as moisture, light, nutrients, and temperature (Ceccarelli and Grando, 1991). Environmental variation is normally difficult to control because it is nonheritable. Environmental variation is usually associated with environmental conditions prevailing on the site where the crops are grown (Ceccarelli and Grando, 1991; Annicchiarico and Perenzin, 1994). Some of these conditions, such as population density, plant to plant competition can be controlled by use of agronomic practices, others like rainfall, wind, and temperature cannot be controlled. University of Ghana http://ugspace.ug.edu.gh 32 2.14.1 Genetic variation Genetic or heritable variation is the differences attributed to genes that encode specific traits, and can be transmitted from one generation to the other Acquaah (2009). Since genes are expressed in an environment, the degree of expression of a heritable trait is impacted by its environment, some more so than others. A phenotype (P), defined as the characteristic that is observed, is as a result of a combination of its genetic constitution, called the genotype (G), and the environment (E) and a component attributed to the interaction between the genetic and environmental components (G x E). This is usually expressed as: Phenotype = Genotype + Environment + G x E (Falconer and Mackay, 1996; Sleper and Poehlman, 2006; Brown and Caligari, 2009). From this equation for phenotypic expression, it follows that any variation seen in the phenotype is due to variation in the factors resulting in the phenotype. The relationship can then be presented as: VP = VG + VE + VG x VGE. Where: VP = Phenotypic variation, VG = Genotypic variation, VE = Variation as a result of the environment, VGxE = variation due to genotype x environment interaction effects Genotypic variation is generally divided into two components, which are additive and nonadditive components (Falconer and Mackay, 1996; Sleper and Poehlman, 2006; Brown and Caligari, 2009). Additive variation is due to the cumulative effect of alleles on all gene loci influencing a trait, and is usually of most value in a crop improvement programme (Falconer and Mackay, 1996). Non-additive variation is divided into dominance variation, caused by the interaction of specific alleles at a gene locus, and epistatic variation, caused by the interaction among gene loci (Falconer and Mackay, 1996). The non-additive variation is normally given little attention since only the additive component of genetic University of Ghana http://ugspace.ug.edu.gh 33 variation is heritable (Falconer and Mackay, 1996; Sleper and Poehlman, 2006; Brown and Caligari, 2009). Genetic or heritable variation in nature originates from gene recombination, modifications in chromosome number, and mutations (Falconer and Mackay, 1996). 2.14.2 Genotype by environment (G x E) interactions A genotype x environment interaction may be defined as a change in the relative performance of a character of two or more genotypes measured in two or more environments (Bowman, 1972). Interactions may therefore involve changes in rank order for genotypes between environments and changes in the absolute and relative magnitude of the genetic, environmental and phenotypic variances between environments. These changes in rank order and in variances have important implications for the breeder in designing selection programmes. The selection of locations for the evaluation of quantitative character is important for the plant breeder, and involves a number of considerations. Given the complexity of quantitative traits, many different lines or crosses must be carefully analyzed over different years and environments to unravel important components of gene interaction (Baiyeri et al., 2008). Theoretically, lack of a significant interaction of genotypes with locations, years, or location x year indicates that a test at one location during one year would be sufficient to identify genotypes with superior genetic potential. The similarity in relative performance of genotypes can be determined by the magnitude of the genotype x location interaction computed by a standard analysis of variance (Frey and Horner, 1957). Wide fluctuations in the rank performance of genotypes at test locations suggest that it may be desirable to develop genotypes for different locations through independent selection and testing programmes. University of Ghana http://ugspace.ug.edu.gh 34 Cassava is prone to significant genotype by environment interaction (GEI) (Kvitschal et al., 2006; Ssemakula and Dixon, 2007; Lebot, 2009). Ssemakula and Dixon, (2007) and Aina et al., (2009) have reported that cassava genotypes tested in contrasting environment revealed strong (GEI) on fresh root yield (FRY). Tan and Mak (1995) reported that genotype by environment interaction (GEI) effects were significant for commercial storage root number, harvest index (HI), (FRY), starch and cyanide content. Although significant, their effects were smaller than the genotype effects, except for commercial storage root number and fresh root yield. They found only cyanide content exhibited a linear G x E relationship with the environment. 2.14.3 Heritability Heritability is the degree of resemblance between progenies and their parents. It is the most important genetic parameter on which various breeding strategies are hinged. The information on heritability of agronomic characters is considered as a prerequisite for the execution of breeding plan. There are two main measures of heritability broad-sense (H2) and narrow-sense (h2) heritabilities. Narrow-sense heritability estimation does not involve dominance and epistasis variances and it is more useful in breeding programmes. There are many approaches for estimating heritability. It may be estimated by the parent-offspring regression approach, or by comparing full-sibs. Analysis of variance is commonly used in estimating heritabilities, as well as correlation and regression. (Holland et al., 2003) Higher heritability for a trait is important for the prediction of its breeding value and genetic gain. Harvest index and resistance to diseases are important and highly heritable traits in cassava breeding programmes (Kawano, 1978) indicating that they are transmitted relatively easily to progenies. University of Ghana http://ugspace.ug.edu.gh 35 2.15 Dry matter content Dry matter content of cassava varies from one accession to another and ranged between 17% and 47% with the majority lying between 20% and 40% (Teye et al., 2011); values above 30% are considered high. The dry matter of the tuberous roots has become an important character for the acceptance of cassava by researchers and consumers. Heritability for DM in cassava is relatively high; 0.87 broad sense heritability and 0.51 – 0.67 narrow sense heritability (Kawano et al., 1987). Estimation of DM and starch content in cassava is based on the principle of a linear relationship between specific gravity with DM and or starch content. Percentage DM = 158.3x – 142. 2.15.1 Accumulation and partitioning of dry matter Dry matter production and partitioning is an important determinant of storage root yield in cassava and could be an important selection criterion in breeding programmes for enhanced yield. Total dry matter production is a good estimator of the degree of adaptation of a genotype to the environment in which it is grown (Kamara et al., 2003). Differences in total dry matter accumulation in genotypes reflect differences in photosynthetic rates (Kamprath et al., 1982). Cassava genotypes that produce high dry matter also produce high leaf area index and root yield. Partitioning of dry matter is particularly important in cassava because the crop has simultaneous development of leaves, stems and storage roots and supply of assimilate is partitioned between these parts (Cock, 1985; Ekanayake et al., 1996). This results in a delicate balance between shoot and storage root growth for maximum yield (Ramanujam, 1985). Generally, genotypes that allocate higher proportion of dry matter to storage roots University of Ghana http://ugspace.ug.edu.gh 36 than the stems and leaves give highe