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Xu Da-quan, Li De-yao, Shen Yun-gang (Shen, Y. K.)(Shanghai Institute of Plant Physiology, Academia Sinica) and Liang Guo-an(Shijiazhuang Institute of Agricultural Modernization, Academia Sinica)  
The photosynthesis of flag leaves of wheat measured with an infrared gas analyser displayed significant 'midday depression' on bright sunny days in early summer under field conditions. The net photosynthetic rate increased rapidly for a few hours after sunrise, reached a peak at about 10 A. M. which was followed by a marked decrease until about 2 P. M., then increased again and reached a second but lower peak at about 4 P. M. The midday depression of photosynthesis observed could not be explained by the accumulation of photosynthates in the wheat leaves, because the content of photosynthetic products such as sucrose and starch was not higher at noon than at eigher 10 A. M. or 4 P. M. This deduction was confirmed by the experiment in which after blocking the export of photosynthates from leaves by killing leaf sheaths with boiling water, the net photosynthetic rates of the leaves declined only slightly, although the content of photosynthates was much higher in treated leaves than that in the control. The midday depression of photosynthesis was significantly correlated with the increase in stomatal resistance of the leaves, the rise of temperature, and, to a lesser extent, the decline of atmospheric CO_2 concentration. It is possible that increase in stomatal resistance was mainly due to the increase of saturation deficit caused by rather low relative humidity and high temperature at noon. This explanation was supported by the fact that an increase in relative humidity of ambient air by enclosing the plants with wet cheese cloth stimulated substantially the net photosynthetic rate of the leaf and eliminated the midday depression. A diagram has been suggested on the basis of the findings, in which the mechanisms by which strong solar radiation at noon leads to midday depression are depicted.
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