| Effects of high temperatures (35℃/ 22℃, 42℃/ 29℃) on photosynthetic apparatus and photoprotective mechanisms were investigated in cucumber leaves(Cucumis sativus L. Jinyou No.3) by measurements of gas exchange parameters, chlorophyll fluorescence, membrane permeability, antioxidative enzyme activities, chlorophyll contents, the ultrastructure of chloroplast and mitochondria. The main results are as follows:1. The photosynthesis was inhibited under the heat stresses, but there were differences in the reasons when plants under the 35℃/22℃and 42℃/29℃. Under the high temperature stress of 42℃/29℃, the inhibition of photosynthesis was resulted from nonstomatal limitations, but stomatal limitations lead to inhibition of photosynthesis when plants were treated with 35℃/22℃.2. Light requirement and CO2 requirement in Cucumber leaves were changed under high temperature stress.3. Fv/Fm, qP, Fv′/Fm′andФPSⅡchanged slightly when plants were treated under 35℃/22℃, but decreased remarkably under 42℃/ 29℃and could not resumed after the plants had been resumed for three days.4. The contents of chlorophyll was reduced considerably under high temperature stress, but Chl a/b was increased. The contents of caro increased at 35℃/22℃, but decreased at 42℃/ 29℃and could not reach comparison level after the plants had been resumed for 3 days.5. The ultrastructure of chloroplast was hardly changed under high temperature stress, the envelope of chloroplast was partially injured and the thylakoids of grana was loose. The number of plastoglobulus increased while the starch grain decreased. The ultrastructure of mitochondria was more stable than that of chloroplast.6. Pr, Pr/Pn, NPQ increased under high temperature stress, the activity of SOD decreased, but POD activity increased, which could dissipate the excess excitation energy and protect photosynthetic apparatus.
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