INVOLVEMENT OF ACTIVE OXYGEN SPECIES IN PHOTOINHIBITION OF PHOTOSYSTEM-II - PROTECTION OF PHOTOSYNTHETIC EFFICIENCY AND INHIBITION OF LIPID-PEROXIDATION BY SUPEROXIDE-DISMUTASE AND CATALASE

Superoxide dismutase and catalase were observed to prevent the loss of photosynthetic electron transport activity and the variable chlorophyll fluorescence in wheat photosystem II (PSII) preparations exposed to strong visible light at 25 and 5-degrees-C. A simultaneous increase in the rate of lipid peroxidation in PSII membranes was also observed which was significantly inhibited in the presence of superoxide dismutase and catalase. Our results strongly suggest the generation of active species such as O2-, H2O2 and OH' in the PSII complex during photoinhibition at ambient temperature and low temperatures. The protection of PSII functions against strong-light-induced damage by the antioxidant enzymes was, however, not complete, which might suggest the existence of an oxygen-independent mechanism operating simultaneously.