Comparative study of the water oxidizing reactions and the millisecond delayed chlorophyll fluorescence in photosystem II at different pH

Water splitting activity, the multiline EPR signal associated with S2-state of the CaMn4-cluster and the fast and slow phases of the induction curve of the millisecond delayed chlorophyll fluorescence from photosystem II (PSII) in the pH range of 4.5–8.5 were studied in the thylakoid membranes and purified PSII particles. It has been found that O2 evolution and the multiline EPR signal were inhibited at acidic (pK ∼ 5.3) and alkaline (pK ∼ 8.1) pH values, and were maximal at pH 6.0–7.0. Our results indicate that the loss of O2 evolution and the S2-state multiline EPR signal associated with the decrease of the millisecond delayed chlorophyll fluorescence only in alkaline region (pH 7.0–8.5). Possible correlations of the millisecond delayed chlorophyll fluorescence components with the donor side reactions in PSII are discussed.