Growth at moderately elevated temperature alters the physiological response of the photosynthetic apparatus to heat stress in pea (Pisum sativum L.) leaves

The impact of heat stress on the functioning of the photosynthetic apparatus was examined in pea (Pisum sativum L.) plants grown at control (25 degreesC; 25 degreesC-plants) or moderately elevated temperature (35 degreesC; 35 degreesC-plants). In both types of plants net photosynthesis (P-n) decreased with increasing leaf temperature (LT) and was more than 80% reduced at 45 degreesC as compared to 25 degreesC. In the 25 degreesC-plants, LTs higher than 40 degreesC could result in a complete suppression of P-n. Short-term acclimation to heat stress did not alter the temperature response of P-n. Chlorophyll a fluorescence measurements revealed that photosynthetic electron transport (PET) started to decrease when LT increased above 35 degreesC and that growth at 35 degreesC improved the thermal stability of the thylakoid membranes. In the 25 degreesC-plants, but not in the 35 degreesC-plants, the maximum quantum yield of the photosystem II primary photochemistry, as judged by measuring the F-v/F-m ratio, decreased significantly at LTs higher than 38 degreesC. A post-illumination heat-induced reduction of the plastoquinone pool was observed in the 25 degreesC-plants, but not in the 35 degreesC-plants. Inhibition of P-n by heat stress correlated with a reduction of the activation state of ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco). Western-blot analysis of Rubisco activase showed that heat stress resulted in a redistribution of activase polypeptides from the soluble to the insoluble fraction of extracts. Heat-dependent inhibition of P-n and PET could be reduced by increasing the intercellular CO2 concentration, but much more effectively so in the 35 degreesC-plants than in the 25 degreesC-plants. The 35 degreesC-plants recovered more efficiently from heat-dependent inhibition of P-n than the 25 degreesC-plants. The results show that growth at moderately high temperature hardly diminished inhibition of P-n by heat stress that originated from a reversible heat-dependent reduction of the Rubisco activation state. However, by improving the thermal stability of the thylakoid membranes it allowed the photosynthetic apparatus to preserve its functional potential at high LTs, thus minimizing the after-effects of heat stress.