Inhibition and acclimation of photosynthesis to heat stress is closely correlated with activation of ribulose-1,5-bisphosphate carboxylase/oxygenase

Increasing the leaf temperature of intact cotton (Gossypium hirsotum L.) an d wheat (Triticum aestivum L.) plants caused a progressive decline in the l ight-saturated CO2-exchange rate (CER). CER was more sensitive to increased leaf temperature in wheat than in cotton, and both species demonstrated ph otosynthetic acclimation when leaf temperature was increased gradually. Inh ibition of CER was not a consequence of stomatal closure, as indicated by a positive relationship between leaf temperature and transpiration. The acti vation state of ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco), which is regulated by Rubisco activase, was closely correlated with tempera ture-induced changes in CER. Nonphotochemical chlorophyll fluorescence quen ching increased with leaf temperature in a manner consistent with inhibited CER and Rubisco activation. Both nonphotochemical fluorescence quenching a nd Rubisco activation were more sensitive to heat stress than the maximum q uantum yield of photochemistry of photosystem II. Heat stress led to decrea sed 3-phosphoglyceric acid content and increased ribulose-1,5-bisphosphate content, which is indicative of inhibited metabolite flow through Rubisco. We conclude that heat stress inhibited CER primarily by decreasing the acti vation state of Rubisco via inhibition of Rubisco activase. Although Rubisc o activation was more closely correlated with CER than the maximum quantum yield of photochemistry of photosystem II, both processes could be acclimat ed to heat stress by gradually increasing the leaf temperature.