High-temperature effects on photosynthetic processes in temperate and tropical cereals

High temperature limits productivity of crops in many regions of the world, but the physiological basis of tolerance to the stress is not known. Our o bjective was to determine the relationship between photosynthetic responses to high temperature at the whole-plant level and sensitivity of light reac tions, the most labile systems, to brief heat treatment. Wheat (Triticum ae stivum L.), a C-3 species adapted to cool environments; rice (Oryza saliva L.), a C-3 species adapted to warm environments; and millet [Pennesitum gla ucum (L.) R. Br.], a C-4 species adapted to hot environments, were compared . Photosynthesis was measured in plants grown at 22, 32, or 42 degrees C, a nd light reactions were measured in protoplasts, chloroplasts, and thylakoi ds isolated from seedlings grown at 27 degrees C and treated in vitro at 22 , 32, and 42 degrees C. Leaf photosynthesis of millet and rice increased fr om 22 to 32 degrees C and then decreased to 42 degrees C, whereas in wheat it was highest at 22 degrees C and decreased as temperature increased. Phot osynthetic rates of protoplasts and chloroplasts from all species decreased after being treated in vitro over the same temperature range. Photosystem II (PSII) activity declined steadily in protoplasts, chloroplasts, and thyl akoids of millet and rice from 22 to 42 degrees C but decreased abruptly in organelles of wheat from 32 to 42 degrees C. The results suggest that diff erences in photosynthetic responses to high temperature are associated with light reactions, and extreme sensitivity of wheat may be attributable to i njury to PSII.