On Midday Depression of Photosynthesis in Wheat Seedlings

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 continuously depressed from the early morning to midday, and reached less than half that obtained early in the morning. In the afternoon the depressed activity slightly recovered. In the plants grown in spring, autumn and winter, the photosynthetic activity kept constant levels all day, and the midday depression observed in summer was not recognized.
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 second leaves. The cause of the depression could be ascribed neither to the stomatal closure, water deficiency nor accumulation of sugars in leaves.
The author owes his thanks to Prof. Y. MURATA, Faculty of Agriculture, University of Tokyo, for going through manuscript, and to late M.S. I. FURUHATA for his technical assistance.