Global Journal of Science Frontier Research, G: Bio-Tech & Genetics, Volume 22 Issue 2

Fig. 1: The structures of primary nodal complex-PNC, showing external sprouting process at two different levels of germination locus Structurally, PNC provides a vascular connection between the developing vine shoots and the mother tuber, the store reserve and later when the vine becomes established and start photosynthesizing and the mother tuber gives a way for new developing tuber, the vestigial PNC again connects the developing tuber and the photosynthesizing vine shoot. Primary nodal complex therefore, serves as the physical link between the new plant, its perennation and storge organs in an annual growth habit. Anatomically, there appears to be a direct meristematic continuity between the primary thickening meristem of the mother tuber, the apical meristem of new vine shoot, the PNC meristem and the primary thickening meristem of the new developing tuber being produced from PNC (24). If the PNC or the head corm as it is also called is analogous with tuberous tissue produced by the embryonic hypocotyl and axillary tissue in vine cuttings as earlier suggested (24), then this meristematic continuity might be traced back to the rooting stem cutting and the seedling which is broken only by the mature tissue separating meristematic cells of the axillary tissue in the germinating stem cutting from vascular cambium of the stem. Thus, the yam tuber is unique is among organs of vegetative propagation because it does not contain a pre-formed bud, but has a layer of meristematic cells within the tuber cortex with the potentials of generating new plants. These cells also represent a remarkable meristematic continuity between plants of different generations with the PNC as the central organ in the continuity. Therefore, it is important that more research attention should be given to this organ through more intensive studies on the improvement yam species, particularly in the development of systems for producing cheap planting material by its multiplication which will ameliorate the impact of seed yam scarcity in yam production system and also significantly reduce long period of tuber dormancy by manipulating the meristems in the PNC at which germination also originate. Such research should also aim at building on the “tuber milking” technique of traditional yam farmers in West Africa. b) Phases of dormancy in Yams Dormant yam tubers are unique and in contrast to other crops such as; onion ( Allium cepa ) ococoyam ( Colocasia esculenta L) and potatoes ( Solanum tuberosum ) in some ways. Yams do not have any internal or external apical shoot buds or sprouts, but have a layer of meristematic cells below the surface of the tuber (24). Onwueme, (27) and Wickham et al , (24) have shown that at the resumption of active growth, shoot apical bud formation begins in this meristematic cell layer, long before any external shoot bud/ sprout is visible on the tuber surface. Implying that the processes which culminate on the surface appearance of tuber shoot bud start long before the physical appearance of shoot bud. According to Ile et al, (28) dormancy in yam tubers occurs in phases: the long phase I of dormancy (the period from tuber physiological maturity to the formation of tuber germinating meristem-TGM, which is up to 200 days). Phase II, this is the period from TGM to the initiation of foliar primordium-IFP, which is about 40 days long. Thirdly a short Phase III; the period from IFP to the physical appearance of shoot bud (ASB) on the surface of the tuber, which is only about 10 days. Shortening the period under Phase I would be useful in developing yam genotypes with reduced period of dormancy. The two key approaches that have been suggested for solving the problem of dormancy in yam are: (1) induction of early sprouting through the prevention/inhibition of the initiation of dormancy in yam tuber such that shoot growth/sprout can resume soon after tuber formation. (2) Shortening of the duration of dormancy such that shoot growth/sprouting can resume soon after physiological maturity (180-200 days after vine emergence). From the Ile, et al., (28), it is clear that a promising approach to solving the problem of yam tuber dormancy is one that is targeted at the long phase one the TGM which also coincides with the duration of 1 Year 2022 5 © 2022 Global Journals Global Journal of Science Frontier Research Volume XXII Issue ersion I VII ( G ) Physiological and Molecular basis of Dormancy in Yam Tuber: A Way Forward towards Genetic Manipulation of Dormancy in Yam Tubers endo-dormancy that is controlled as stated earlier by internal physiological and genetic factors. This phase is