Photoconsumption of oxygen in photosystem II preparations under impairment of the water-oxidizing complex.

Oxygen consumption in photosystem II (PSII) preparations in the light was 2 micromol O2/h per mg Chl at weakly acidic and at neutral pH values. It increased fourfold to fivefold at pH 8.5-9.0. The addition of either artificial electron donors for PSII such as MnCl2 or diphenylcarbazide, or diuron as an inhibitor of electron transfer from QA, the primary bound quinone acceptor, to QB, the secondary bound quinone acceptor of PSII, resulted in a decrease in oxygen consumption rate at basic pH to value close to ones measured at pH 6.5. Such additions did not affect oxygen consumption at lower pH values. The induction of variable chlorophyll fluorescence yield in the light differed greatly at pH 6.5 and 8.5. While at pH 6.5 the fluorescence yield, after an initial fast rise almost to Fmax, only slightly decreased, at pH 8.5 after such a rise it dropped promptly to a low value. The additions of the artificial electron donors at pH 8.5 resulted in the induction kinetics close to that observed at pH 6.5. These data indicate impairment of electron donation to P680+ that could be caused by damage to the water oxidation system at basic pH values. In experiments with PSII preparations treated with Tris to destroy the water-oxidizing complex, photoconsumption of oxygen in the entire pH region was close to the values in untreated preparations at basic pH. In untreated preparations the rate of light-induced oxygen consumption decreased in the presence of catalase, which decomposes H2O2, as well as in the presence of electron acceptor potassium ferricyanide. From these data it is suggested that the light-induced oxygen consumption in PSII is caused by two processes, by an interaction of O2 with organic radicals, which were formed due to oxidation of components of the donor side of this photosystem (proteins, lipids, pigments) by cation-radical P680+, as well as by oxygen reduction by still unidentified components of PSII.