EVIDENCE FOR THE CONTRIBUTION OF THE MEHLER-PEROXIDASE REACTION IN DISSIPATING EXCESS ELECTRONS IN DROUGHT-STRESSED WHEAT

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) isoto pe technique and mass spectrometry. The activity of photosystem II, de termined from the rate of O-16(2) evolution, is only slightly affected under drought conditions. During drought stress, net CO2 uptake decre ases due to stomatal closure, whereas the uptake of O-18(2) is stimula ted. The main O-2-consuming reactions in the light are the Mehler-pero xidase (MP) reaction and the photorespiratory pathway. From measuremen ts of the rate of carbon flux through the photorespiratory pathway, es timated by the analysis of the specific radioactivities of glycolate, we conclude that the rate of photorespiration is decreased with drough t stress. Therefore, the O-2 taken up in the light appears to be prefe rentially used by the MP reaction. In stressed leaves, 29.1% of the ph otosynthetic electrons are consumed in the MP reaction and 18.4% drive the photorespiratory pathway. Thus, overreduction of the electron tra nsport chain is avoided preferably by the MP reaction when drought str ess restricts CO2 reduction.