| Summary: | Photosynthesis performance during early vegetative growth is an important physiological trait determining yield of cassava, but limited information is currently available for the tropical savanna climate of Asia. Diurnal photosynthesis and chlorophyll fluorescence of the three-month-old plants of four commercial cassava genotypes (Rayong 9, RY9; Rayong 11, RY11; Kasetsart 50, KU50 and CMR38-125-77) grown under irrigation, were investigated in three seasons i.e., rainy, cool and hot. The mean daily net photosynthetic rate (Pn) across genotypes in the rainy season (11.75 µmolCO<sub>2</sub>/m<sup>2</sup>/s) was significantly lower than that in the cool season (14.60 µmolCO<sub>2</sub>/m<sup>2</sup>/s). Daily mean Pn in the hot season was 14.32 µmolCO<sub>2</sub>/m<sup>2</sup>/s. In the rainy season, maximum photochemical quantum yield of PSII (F<sub>v</sub>/F<sub>m</sub>) and effective quantum yield of PSII photochemistry (Φ<sub>PSII</sub>) were significantly higher than the other seasons, while electron transfer rate (ETR) and non-photochemical quenching (NPQ) were significantly lower. Genotypic variation was observed during the hot season in which RY11 had the highest and CMR38-125-77 the lowest mean daily Pn. The prominent mechanism to avoid damages from stress during afternoon in the hot season was to reduce leaf temperature by enhancing transpiration for RY11; to close stomata early for RY9, and to increase NPQ for CMR38-125-77.
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