Effect of Planting Season on Phenology and Accumulated Heat Units in Relation to Yield of White Yam in the Tropical Wet- and-Dry Climate

Field experiment was carried out to study the effect of phenology and accumulated heat units of white yam (Dioscorea rotundata) in different seasons. White yam was cultivated during early (T1) and late (T2) cropping seasons of 2011 and 2012. Rainfall-potential evapotranspiration (P-PE) model according to Cocheme and Franquin (1967) was followed to determine planting dates. The observations on days required for attainment of different phenological stages viz., establishment, vine elongation, vegetative, bulking and senescence were recorded. Heat units such as Growing Degree Days (GDD), Photo Thermal Units (PTU), Helio Thermal Units (HTU), Relative Temperature Disparity (RTD), Heat Unit Efficiency (HUE), Photo Thermal Index (PTI) and seasonal efficiency were worked for different seasons of white yam and related to different phenological stages of growth and yield. The results revealed that among the different indices, GDD, HTU, PTU and RTD does not influence the phenolgical stages of growth whereas HUE, PTI and seasonal efficiency had great influence on yield of white yam. Thus, the indices such as HUE, PTI and seasonal efficiency are seem to be effective in taking into account and expressing the effect of varying ambient temperature on the duration between the phenological events for comparing the white yam response to the ambient temperature between different phenological stages.