Comparison of chloride-depleted and calcium-depleted PSII: The midpoint potential of Q(A) and susceptibility to photodamage

Photosystem II has been studied in membranes in which O-2 evolution was inhibited by depletion of either chloride or calcium ions. It has been shown earlier [Krieger, A. and Weis, E. (1992) Photosynthetica 27, 89-98] that depletion of calcium ions results in a 150-mV up-shift of the midpoint redox potential (Em) of Q(A)/Q(A)(-) (the protein-bound plastoquinone which acts as an electron acceptor). Here it is shown that chloride depletion has no effect on the Em of Q(A)/Q(A)(-). It is also demonstrated that chloride-depleted PSII is more susceptible than Ca2+-depleted PSII to damage by light. This extra susceptibility to light in Cl--depleted PSII is eliminated when the artificial electron acceptor DCPIP is present during illumination. These observations are consistent with the hypothesis that the up-shifted Em of Q(A)/Q(A)(-) in Ca2+-depleted PSII results in a protection of the reaction centre from damage by light by changing the dominant charge recombination pathway to one which does not involve formation of the P680(+)Ph(-) radical pair, the P680 triplet and singlet oxygen [Johnson, G.N., Rutherford, A.W. and Krieger, A. (1995) Biochim. Biophys. Acta 1229, 202-207].