On Midday Depression of Photosynthesis in Wheat Seedlings

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Abstract
Leaves were sampled at intervals from early morning to evening from wheat seedlings grown in different seasons, and their photosynthetic activities were determined under the constant and normal measurement conditions. In summer, the photosynthetic activity was depressed from the early morning to midday and was less than 1/2 that obtained early in the morning. In the afternoon the depressed activity slightly recovered. In plants grown in spring, autumn and winter, photosynthetic activity was constant all day without the midday depression observed in summer. The midday depression of photosynthesis in summer was not observed when the plants were defoliated on the day before the measurement of photosynthesis, leaving only the 2nd leaves. The cause of the depression could be ascribed neither to the stomatal closure, water deficiency nor sugar accumulation in leaves.

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