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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 objective was to determine the relationship between photosynthetic responses to high temperature at the whole‐plant level and sensitivity of light reactions, the most labile systems, to brief heat treatment. Wheat ( Triticum aestivum L.), a C 3 species adapted to cool environments; rice ( Oryza sativa L.), a C 3 species adapted to warm environments; and millet Pennesitum glaucum (L.) R. Br., a C 4 species adapted to hot environments, were compared. Photosynthesis was measured in plants grown at 22, 32, or 42°C, and light reactions were measured in protoplasts, chloroplasts, and thylakoids isolated from seedlings grown at 27° C and treated in vitro at 22, 32, and 42°C. Leaf photosynthesis of millet and rice increased from 22 to 32°C and then decreased to 42°C, whereas in wheat it was highest at 22°C and decreased as temperature increased. Photosynthetic 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 thylakoids of millet and rice from 22 to 42°C but decreased abruptly in organelles of wheat from 32 to 42°C. The results suggest that differences in photosynthetic responses to high temperature are associated with light reactions, and extreme sensitivity of wheat may be attributable to injury to PSII.
Al‐Khatib et al. (Fri,) studied this question.