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 wa s inhibited by depletion of either chloride or calcium ions. It has be en 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 p lastoquinone which acts as an electron acceptor). Here it is shown tha t chloride depletion has no effect on the Em of Q(A)/Q(A)(-). It is al so demonstrated that chloride-depleted PSII is more susceptible than C a2+-depleted PSII to damage by light. This extra susceptibility to lig ht in Cl--depleted PSII is eliminated when the artificial electron acc eptor DCPIP is present during illumination. These observations are con sistent 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 p air, the P680 triplet and singlet oxygen [Johnson, G.N., Rutherford, A .W. and Krieger, A. (1995) Biochim. Biophys. Acta 1229, 202-207].