Photosystem-II damage and repair cycle in chloroplasts: what modulates therate of photodamage in vivo?

Organisms that rely on oxygenic photosynthesis are subject to the effects o f photo-oxidative damage, which impairs the function of photosystem-II (PSI I), This phenomenon has the potential to lower rates of photosynthesis and diminish plant growth. Experimental evidence shows that the steady-state ox idation-reduction level of the primary quinone acceptor (Q(A)) of PSII is t he parameter that controls photodamage under a variety of physiological and environmental conditions. When Q(A) is reduced, excitation energy at PSII is dissipated via a charge-recombination reaction. Such non-assimilatory di ssipation of excitation generates singlet oxygen that might act to covalent ly modify the photochemical reaction center chlorophyll. Under steady-state photosynthesis conditions, the reduction state of Q(A) increases linearly with irradiance, thereby causing a correspondingly linear increase in the p robability of photodamage, It is concluded that there is a low probability that photodamage will occur when Q(A) is oxidized and excitation energy is utilized in electron transport, and a significantly higher probability when Q(A) is reduced in the course of steady-state photosynthesis.