Gross O-2 evolution and uptake by attached, drought-stressed leaves of wheat (Triticum aestivum) were measured using a O-16(2)/O-18(2) isotope technique and mass spectrometry. The activity of photosystem II, determined from the rate of O-16(2) evolution, is only slightly affected under drought conditions. During drought stress, net CO2 uptake decreases due to stomatal closure, whereas the uptake of O-18(2) is stimulated. The main O-2-consuming reactions in the light are the Mehler-peroxidase (MP) reaction and the photorespiratory pathway. From measurements of the rate of carbon flux through the photorespiratory pathway, estimated by the analysis of the specific radioactivities of glycolate, we conclude that the rate of photorespiration is decreased with drought stress. Therefore, the O-2 taken up in the light appears to be preferentially used by the MP reaction. In stressed leaves, 29.1% of the photosynthetic electrons are consumed in the MP reaction and 18.4% drive the photorespiratory pathway. Thus, overreduction of the electron transport chain is avoided preferably by the MP reaction when drought stress restricts CO2 reduction.
