Effect of moderately-high temperature stress on photosynthesis and carbohydrate metabolism in tomato (Lycopersico esculentum L.) leaves

Changes in the production and metabolism of photosynthate in tomato leaves were investigated after plants were treated to moderately high temperature stress for 8 h. Plants grown continuously at 25°C served as the controls. Compared to the controls, photosynthetic activity decreased in plants exposed to 35°C for ≥2 h. The net rate of photosynthetic (Pn) and the limitation of stomatal conductance (Ls) decreased, but stomatal conductance (Gs), intercellular CO2 concentration (Ci) and the rate of transpiration (Tr) increased. These results suggested the decreased in Pn under 35°C stress was caused mainly by non-stomatal restriction. In parallel with the decline in photosynthesis, the activities of sucrose-metabolizing enzymes and the contents of carbohydrate of plants that had been exposed to 35°C also changed. The invertase activities increased first then decreased but the activities of sucrose synthase (SS) and sucrose phosphate synthase (SPS) increased continuously in stressed plants. The contents of fructose and glucose decreased but sucrose content increased compared with controls. It may result from the effects of 35°C on photosynthesis and sucrose-metabolizing enzymes activities that provoked the changes in carbohydrate contents in tomato leaves.   Key words: Photosynthesis, carbohydrate, sucrose-metabolizing enzymes.