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

position which concluded that genetics and storage condition instead of only environmental and eco- physiological factors are regulating the duration of endo- dormancy is more accurate from the findings of Hamadina (22). Hence, it could be concluded that accurate definition of the commencement of dormancy induction is when supply of metabolites and photosynthates from source sites (photosynthesizing vegetative parts) to sink (tuber; the storage organ) is terminated. And this always coincide with the commencement of leaves/vines senescence, indicating that tuber filling has stop, thus marking the end of tuber growth and development. A growing tuber, even though it lacks differentiated meristematic cells and is incapable of sprouting, cannot be described as a dormant tuber either as scenario B postulated because the hormonal machinery involve in tuber growth and development are also sprout growth inhibitors and as such will not permit sprouting of growing tuber. d) Anatomical basis of yam tuber dormancy The cellular anatomical structure of yam tuber is quite striking and it influences every physiological process in the tuber. Studies on cellular anatomical structure of yam tuber have revealed distinct fragmented pattern which was linked to tubers ability to germinate and grow new plants (32). Generally, in cellular anatomic structure of yam tuber there are five major identifiable regions and additional one region conspicuous only in D. alata (33). Each of these anatomical regions performs one or more cellular physiological functions that drive the whole plant phenotype including determination of yam tuber dormancy. The anatomical structure is organized in the following layers: (1) Cork layers comprising of primary and secondary cork layers. It is an average of 5.05 layers of suberized corky cells tangentially elongated and disposed in radial series, the inner layers being often compacted against a basic cambial layer. It performs mainly the functions of mechanical protection and providing firm frame, as well as regulating respiratory and hydric economy of food stores. (2) Cambium layer; this is located at the outer cortex and sub-apical meristem and it performs the function of sensor cells by relaying information through and fro from the inner cortex to the outer region. (3) Cortical parenchyma; this layer contains more or less tangentially elongated cells with secretory and raphide cells. It serves mainly as the store house, packs of grana and starch grains. (4) Procambium; This is an area of sheath and lengthening cells like layer. It is involved in the tissue conductivity (24). Fig. 3: Hypothetical anatomical structure of dormant yam tuber. Showing different anatomical cell layers and their physiological functions, revealing cell blocks as the anatomical locus for dormancy induction in yam tuber Procambium layer also contains individual cell blocks that function as organizing pole structure and determine growth and morphogenesis. they can be compared to the undifferentiated embryo of some conventional seeds, and are located in the main part of tuber cellular anatomic structure of mature tuber between harvest and germination, where they maintain a kind of embryonic dormancy and also determine other organogenetic processes(24). In fact, this particular layer has been described as a centre for species diversity and site for growth initiation and multiplication in any tuber. Hence, the individual cell blocks in procambium can be described as cellular anatomical locus for dormancy trait. According to Wickham, et al (24) the general presence of generative cell blocks offers histological basis for the structural and dynamic analysis of the tuber multiplication. In D. alata procambium layer equally contains thick layer of distinct sclerenchymatous cells which is the main centre of specie specific diversity. According to Boureau, (34) it contains something like the “transfusion tissue” which play the role of water conservation in the tubers of D. alata, and their cell wall punctuation and the vascular bundles vicinity are good for its function. It was also suggested to be the factor that conferred wide ecological adaptation and dispersion potentials on D. © 2022 Global Journals 1 Year 2022 54 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