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

not influenced by environmental cues, implying that they are strictly controlled by physiological/genetic factors. c) Induction and duration of dormancy in yam tuber There are two contrasting schools of thoughts (scenarios) on the induction/development of dormancy in yam tubers (22). Scenario A postulates that dormancy commences during tuber maturity or vine senescence/onset of the dry season and end at sprouting. In contrast, scenario B, opines that dormancy commences much earlier during the early tuber development, and ends with sprouting. This section highlights on these scenarios and their effects on: (1) the accuracy and consistency in the duration of dormancy often presented, (2) the design of research targeted at reducing yam tuber dormancy duration, (3) the timing of treatment application, and (4) the extent to which the length of the dormant period can be reduced. i. Scenario A Scenarios A is consistent with the long-standing definition; that dormancy is an adaptive mechanism developed for survival in adverse weather conditions, in this case, the dry season of the tropics. Also, in agreement with this scenario are the results of some published findings (15, 29, 30) which showed that there is a slowing down of metabolic activities in tubers with the start of the dry season. For instance, tubers that are harvested at vine senescence exhibit a reduced rate of respiration, and reduced starch and sugar metabolism. They contain high concentrations of growth-inhibiting substances, etc., with the reverse occurring at the end of dormancy/resumption of sprouting. It is important to note that in most of these studies, the experimental tubers were harvested at the attainment of tuber maturity or at best only a few days before this stage and the period covered is until the visible end of dormancy (sprouting). As such, the studies have provided information only on changes occurring from the defined time of harvest until sprouting. Based on the definition in scenario A, therefore, the duration of dormancy can range from 50 to 150 days, even for the same variety, this is largely inconsistent. Some reasons for such wide variation relate to the ambiguous nature of the terms; tuber maturity and sprouting, which consequently allows the use of varied dates of tuber harvest and varied signs of sprouting. Hamadina, (22), investigated how these factors, as well as differences in species/varieties, and poorly defined/poor knowledge of environmental conditions in postharvest storage, can result in an inconsistent in duration of dormancy. The findings of this study concluded that the duration of dormancy is long and highly variable, and the variability in the duration of dormancy highlights the need for researchers to define terms clearly and describe all conditions experienced by tubers during storage and the growing season. There is no evidence that the variability in the duration of dormancy within varieties of D. rotundata “indigenous” to distinct agroecological zones in Nigeria, is due to inherent adaptation to their agroecology of origin/latitude of origin. Tubers, in spite of perceived differences in their agroecology of origin, tend to sprout at about the same time if grown and stored in similar environmental conditions. The growing and storage conditions/agro-ecologies are important factors affecting the duration of dormancy with the effects being as long as 20 days. Based on the effects of exogenous PGRs on the duration of whole tuber dormancy as well as the effects of physical and environmental factors, it is clear that whole tuber dormancy, in the context of scenario A, can be shortened only by about 30 days using agronomic approach. ii. Scenario B Some researchers have hypothesized that tuber dormancy does not begin when tubers reach agronomic maturity or leaf/vine senescence, But rather much earlier during early tuber development. This school of thought holds that dormancy begins sometime during tuber development and ends before sprouting (15, 31). This second group suggests that there is a ‘true” dormancy period (endo-dormancy) that starts during tuber development and ends well before sprouting, being marked by the onset of activity in the meristematic region that leads to the formation of the internal shoot bud (21, 27). Only a few studies have been carried out within the context of this school of thought. Although the reason for this is not clear, it is supposed that scenario B has been unattractive, probably due to the fact that it implies that actively growing and developing tubers exhibit dormancy and sampling growing underground tubers for analysis may be a tedious task. Another reason may be because it implies that yam tuber dormancy (observed in whole harvested tubers) may not arise simply due to the effects of adaptation to a prevalent or impending adverse environmental condition (such as the advent of cold periods in temperate regions and the dry season in tropical regions.) and thereby highlighting the fact that genetics is much involved in the mechanisms regulating tuber dormancy. The consequence of limited research in this area implies that the factors that affect the initiation and duration of dormancy are not clearly understood and evidence that elucidates its control mechanism(s) is more than eco- physiological factors as suggested by scenario A. Again, Hamadina (22) findings concluded that; Dormancy commences much earlier, during tuber initiation and development, rather than later. The duration of this dormancy is much longer than its estimation under Scenario A and covers a larger part if not all of the period of dormancy. The difference in the duration of dormancy/timing of sprouting among © 2022 Global Journals 1 Year 2022 52 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 landraces of D. rotundata is not related simply to