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

harvested in the month of August (about 180 days after planting) or November (about 270 days after planting), most of such tubers do not resume shoot growth/sprouting until about 210 days or 150 days after harvest respectively. The long waiting for the resumption of sprouting (dormancy), imposes the need for prolonged storage of seed tubers, restricts planting to once per annum, exposes up to 40% of highly-valued tubers to loss (due to pests and diseases during the compulsory storage period), exposes whole seed tubers to unplanned consumption, and these in turn contribute to scarcity of tubers especially during the planting season and consequently increased the inputs cost of yam production, (12). The cost of planting material (seed yams) alone constitutes about 40% of the total cost of yam production (13, 14). Tuber dormancy is the major cause of the prolonged inability of ware or seed tubers to sprout. Harvested tubers remain dormant; incapable of developing an internal shoot bud or external shoot bud/sprout for 150 to 210 days depending on the date of harvest, species, and growing and storage environmental conditions (15, 16). Thus, making it impossible to have more than one crop cycle per year and thereby limiting the crop production, productivity, tuber availability and the rate of genetic improvement through breeding (17, 18). The mechanisms controlling yam tuber dormancy are not well understood, though, some studies have made valuable efforts towards elucidating the mechanisms. The objective of this review is to summarize available information on physiological mechanisms of yam tuber dormancy while adapting novel studies on other crops on genetic mechanisms of tuber dormancy. We present insights and future perspective on research for increased food security, income generation and improved livelihoods. II. D efinition of D ormancy According to Lang et al , (19) dormancy is a temporary growth arrest of any plant part containing a meristem. It is an inherent plant physiological mechanism that regulates the timing of sprouting of affected plant parts (20). Dormancy period can also be defined as the period of reduced endogenous metabolic activity during which the tuber shows no intrinsic bud growth, although it retains the potential for future growth. It is highly influenced by genetic and evolutionary constituent and also affected by environmental factors such as; temperature, moisture, oxygen and CO 2 content of the storage atmosphere (21). Dormancy has been classified into three categories based on the factors that influenced growth arrest (19). These include: Endodormancy (this is a deep dormancy during which growth arrest is influenced by internal physiological and genetic factors within the meristem), Para-dormancy (this occurs due to growth arrest by physiological factors external to the meristem) and Eco-dormancy (growth is stopped by unfavorable external or environmental factors). The consequence of dormancy is severe on yam production and production system, because the duration of dormancy is very long; as much as 270 days, depending on the time of tuber harvest and definition of the start of dormancy (22). a) Yam tuber regenerative organ and its relevance in tuber dormancy induction During seedling development, the embryonic hypocotyl is the site of tuber induction (23). After the occurrence of adventitious root growth from the developing tuber, all other further proceeds by the diageotropic or plagiotropic lobing of the original hypocotyl bulge (24). In similar fashion, during the establishment of new plants from vine cutting, adventitious roots arise from the axillary tissue in association with the axillary bud, and not directly from the stem, while tuber induction proceeds as a result of the lobing of this axillary tissue (24, 25). Regenerative activity in whole tuber and tuber pieces begins with the production of shoot apical meristem with the associated primary nodal complex (PNC) on which induction of shoots, roots and tubers are initiated. It is on the basis of this fact that the hypocotyl of the seedling, the axillary tissues of the stem cutting, and the PNC of germinating tuber were assumed to be analogous and it is suggested that there is a common ontogeny in the tuber induction of every regenerative part of Dioscorea species plants (seeds, vine cutting, tuber piece) (24). This ontogeny is characterized by the production of an organ of renewed growth in the tissue subtending the stem apex. It is on this basis that organ of renewed growth in tuber has been designated as the PNC. Burkill (26) opined that it was during the evolution of the edible Dioscoreas that the thickening and lobing of the ancestral rhizome gave way to a well development tuber system. With the loss of axillary buds of the rhizome, the primary thickening meristem became the site of renewed shoot growth during tuber dormancy release. This activity leads majorly to the production of a shoot and a modidfied node (PNC) which is the vestige of the ancestral rhizome and is the site of roots and tuber origin in plants. However, it has been reported that every node of Dioscorea including the cotyledonary node of the germinating seedling and calyptral node of the germinating tuber have the capacity to produce the vestigial rhizome-PNC (24). It seems that during phylogenic partial separation of Dioscorea from perennation to annual crops, the perennation potentials of the degenerated rhizome were retained in the vestigial PNC, while the new developing tuber assumes the storage role. Figure 1 shows different stages of shoot emergence from primary nodal complex. © 2022 Global Journals 1 Year 2022 50 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