Mechanism of photoinhibition in pea thylakoids: effects of irradiance level and pH

Abstract

The photosynthetic apparatus can be damaged by light energy in the process of photoinhibition. The target of this photoinhibition is mainly photosystem II (PSII). The mechanism leading to photoinhibitory damage is not yet known. We have characterized photoinhibition by measuring the photoinactivation of electron transport rates using the electron acceptors silicomolybdate and ferricyanide at different irradiance levels and different pH values. The effects of light on the donor side of PSII were measured with silicomolybdate, the effects on the acceptor side were measured with ferricyanide. We observed that photoinactivation of both donor and acceptor side of PSII are light dose-dependent, donor and acceptor side inactivation being independent processes. The donor side of PSII is less sensitive to photoinhibition than the acceptor side. The difference in pH dependence of donor and acceptor side photoinactivation leads us to propose that light-induced release of bicarbonate from PSII is a primary event leading to photoinhibition. In addition, we report that a photoinhibitory treatment increases the proton permeability of thylakoid membranes. This increase seems to be related to the presence of inactivated PSII reaction centers. It is suggested that radicals formed by inactivated PSII reaction centers causing lipid peroxidation are responsible.