Microsett and Slip Propagation of Dioscorea Rotundata Seed Yam

Abstract

The microsett technique involves the use of 2-10g setts comprising two size-ranges: a lower 2-5g and an upper 5.Ol-lOg. Preliminary experiments were undertaken to determine the ideal presprouting medium, moisture regime of the latter, presprouting environment, and sett size of the microsetts derived from the white yam variety, TDr 131. The upper 5.01-10g sett-class presprouted in open-air, raised beds covered with low palm-frond shade, using fresh sawdust at an initial moisture content of 76%, i.e. lOOOg of dry sawdust mixed with 31 of water was ideal. The acceleration and synchronisation of sprouting of the microsetts in the nursery prior to transplanting, by means of mineral nutrients were also investigated. The most practically feasible method of accelerating the sprouting of the microsetts was the tail removal technique. This involved plucking off the cormous structure bearing an apical bud from the head region of the tuber and separating the distal-1 / 3 region or tail from the head and middle portions of the tuber, herein also referred to as "head" at 3 weeks-after-harvest f (WAH). The resultant tuber parts were soaked in a nutrient mixture of 1500 parts per million (ppm) Urea, 50ppm Florel (ethephon) and lOppm Ferrous sulphate. The nutrient mixture elicited highly significant sprouting compared to the control such that at 17, 18 and 21 WAH there were no real differences between the heads and tails. The objective was to accelerate the sprouting of the tails so as to synchronize it with the heads. Consequently, microsetts from the tails could behave as heads when presprouted. The nutrient mixture application and the tail removal operation were undertaken at 3 WAH, i.e during TP I, on the bases of studies on changes in some macro- and micro-nutrient levels in the tuber with time-after-harvest. The trends in percent total nitrogen (% TN) were most spectacular : there was a decline between the 3rd and 5th weeks and a rise thereafter in the peripheral 1 .6 -1 .9cm portion of the tuber. The % TN values in the head and tail region microsetts were similar, whilst those of the middle were markedly low. These suggested a bipolar internal redistribution within the tuber. The head and tail regions of the the tuber were largely sinks, whilst the middle was largely a source. Furthermore, movement of total nitrogen was probably directed out of the peripheral 1.6 - 1.9 cm tissues into the inner ground tissues from 3 - 5 WAH and towards the periphery from the 5 WAH onwards. The nutrient mixture was thus supplied at 3 WAH on the assumption that the greatest demand for nitrogen must be at this period. The trends in % TN and hence crude protein, potassium and iron were similar. On the strength of this suggested nutrient redistribution, the tuber dormancy stage of yam ontogeny was considered as comprising a "true dormancy" sub-phase, followed by a "biochemically non-dormant tuber" sub-phase. The latter stage was assumedly indicated by the rise in % TN at 5 WAH. Consequently, the reported progressive development of the meristeiaatic layer probably starts at 5 WAH and continues till the bud on the cormous structure at the head of the tuber, becomes visibly active: the external indication that dormancy is naturally over. The slip propagation technique entails the use of the cormous structure at the head of the tuber and the associated shoot that arises from it after natural tuber dormancy release. The non-green, achlorophyllised slips derived from tubers of the white yam variety, TDr 603, stored in the dark, showed high morphogenetic ability. This was attributed to probably phytochrome-mediated responses. The direct field planting of the freshly plucked non-green slips is agroriomically ideal. A seed yam.production package based on microsetts and non-green slips is proposed.