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Winter wheat (Triticum aestivum L. cv Norin No. 61) was grown at 25degreesC until the third leaves reached about 10 cm in length and then at 15degreesC, 25degreesC, or 35degreesC until full development of the third leaves (about 1 week at 25degreesC, but 2-3 weeks at 15degreesC or 35degreesC). In the leaves developed at 15degreesC, 25degreesC, and 35degreesC, the optimum temperature for CO2-saturated photosynthesis was 15degreesC to 20degreesC, 25degreesC to 30degreesC, and 35degreesC, respectively, The photosystem II (PS II) electron transport, determined either polarographically with isolated thylakoids or by measuring the modulated chlorophyll a fluorescence in leaves, also showed the maximum rate near the temperature at which the leaves had developed. Maximum rates of CO2-saturated photosynthesis and PS 11 electron transport determined at respective optimum temperatures were the highest in the leaves developed at 25degreesC and lowest in the leaves developed at 35degreesC. So were the levels of chlorophyll, photosystem I and PS II whereas the level of Rubisco decreased with increasing temperature at which the leaves had developed. Kinetic analyses of chlorophyll a fluorescence changes and P700 reduction showed that the temperature dependence of electron transport at the plastoquinone and water-oxidation sites was modulated by the temperature at which the leaves had developed. These results indicate that the major factor that contributes to thermal acclimation of photosynthesis in winter wheat is the plastic response of PS II electron transport to environmental temperature.