Inhibition of Photosynthesis by Carbohydrates in Wheat Leaves

The rate of net CO(2) assimilation of mature wheat (Triticum aestivum L.) leaves in ambient air (21% O(2), 340 microbars CO(2)) declined with time of illumination at temperatures lower than 25 degrees C, but not at higher temperatures, and the rate of decline increased when maintained in air with higher CO(2) concentration (700-825 microbars). In this latter case, the decline in the rate of net CO(2) assimilation also occurred at high temperatures. Stomatal conductance also declined with time in some cases and stomata became more sensitive to CO(2), but this was not the primary cause of the decrease in CO(2) assimilation because internal partial pressure of CO(2) remained constant. Treatments which reduced the rate of translocation (e.g. lower temperatures, chilling the base of the leaf) produced a marked decline in CO(2) assimilation of leaves in atmospheric and high CO(2) concentrations. The decreased net CO(2) assimilation was correlated with carbohydrate accumulation in each case, suggesting end product inhibition of photosynthesis. Analysis of CO(2) assimilation in high carbohydrate leaves as a function of intercellular CO(2) partial pressure showed reduction in the upper part of the curve. The initial slope of this curve, however, was not affected. Photosynthetic rates in the upper part of this curve generally recovered after a short period in darkness in which carbohydrates were removed from the leaf. The stimulation of net CO(2) assimilation by 2% O(2) (Warburg effect), and the apparent quantum yield, decreased after several hours of light.