Diurnal and seasonal variations in the photosynthetic performance and chlorophyll fluorescence of cassava 'Rayong 9' under irrigated and rainfed conditions

Diurnal photosynthesis responses of cassava cultivar Rayong 9 ('RY9') three months after planting, grown in a field conditions under irrigated and rainfed conditions, were evaluated during the rainy, cool, and hot seasons. Under the mild conditions of the rainy and cool seasons, net photosynthetic rates (P-N) increased in parallel with light intensity and attained the maximum at 13.00 or 11.00 h. In the hot season, P-N attained the prominent peak at 9.00 h, after which stomatal conductance decreased rapidly coordinated with declining P-N and nonphotochemical quenching was enhanced. Photosynthetically active radiation was the major factor influencing P-N in the rainy and cool seasons, whereas vapor pressure deficit was the major factor in the hot season. 'RY9' adapted extremely well in this climate because the maximal quantum yield of PSII photochemistry recovered fully in the evening even under the rainfed conditions in the hot season.