Seasonal Variation in Diurnal Photosynthesis and Chlorophyll Fluorescence of Four Genotypes of Cassava (Manihot esculenta Crantz) under Irrigation Conditions in a Tropical Savanna Climate

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 µmolCO2/m2/s) was significantly lower than that in the cool season (14.60 µmolCO2/m2/s). Daily mean Pn in the hot season was 14.32 µmolCO2/m2/s. In the rainy season, maximum photochemical quantum yield of PSII (Fv/Fm) and effective quantum yield of PSII photochemistry (ΦPSII) 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.

DIURNAL PHOTOSYNTHESIS AND STARCH DEPOSITION IN LEAVES OF FRUITING AND DEFLOWERED RED RASPBERRY CANES

On four dates during the 1991 growing season, gas exchange rates were measured on the same middle leaflets every 3 h from 7am-10pm from deflowered (DF) and fruiting (F) red raspberry (Rubus idaeus L. cv. “Meeker”) canes. Concurrently, the adjacent side leaflets were sampled for anatomical starch determination. The dates corresponded to the late anthesis/early green fruit, early red fruit, late red fruit, and post fruit maturity stages of the growing season. For all dates, CO2 assimilation (A) was highest from 7-10am, lowest at 4pm, and increased at 7pm. Overall A peaked during fruit development. Leaves of F canes had greater A than leaves of DF canes during fruit development, but rates were similar after fruit maturity. Starch accumulation in leaf cross-sections generally followed the diurnal pattern observed for A. Starch appeared heaviest from 7am-lpm and often showed an increase from 7-10pm. Leaves from DF canes generally had a greater accumulation of starch. Seasonally, leaf starch from F canes appeared greatest at late anthesis, decreased during fruit development and was very low post fruit maturity. Leaf starch in DF canes appeared greatest at the late anthesis and late red fruit stages. DF leaves had greater dry weight accumulation than F leaves during the red fruit stages. A Western blot showed that Rubisco levels as a percentage of total soluble protein were higher during fruit development and decreased after fruit maturity.