THE INVOLVEMENT OF PHOTOSYSTEM-II-GENERATED H2O2 IN PHOTOINHIBITION

The involvement of H2O2 generated by photosystem II (PSII) in the process of photoinhibition of thylakoids with a functional oxygen-evolving complex (OEC) was investigated. The rate of photoinhibition was decreased to the rate of loss of activity in the dark when bovine Fe-catalase was present during the photoinhibitory illumination. Photoinhibition was accelerated for both Cl--depleted and Cl--sufficient thylakoids when KCN was present to inhibit the thylakoid-bound Fe-catalase. We propose that these preparations become photoinhibed by reactions with H2O2 produced via oxidation of water by the Cl--depleted OEC and by reduction of O2 at the Q(B) site when PSII is illuminated without an electron acceptor.