Primary reactions of photosystem II at low pH. 2. Light-induced changes of absorbance and electron spin resonance in spinach chloroplasts.

The effects of lowering the pH on Photosystem II has been studied by measuring changes in absorbance and electron spin resonance in spinach chloroplasts. At pH values around 4 a light-induced dark-reversible chlorophyll oxidation by Photosystem II was observed. This chlorophyll is presumably the primary electron donor of system II. At pH values between 5 and 4 steady state illumination induced an ESR signal, similar in shape and amplitude to signal II, which was rapidly reversed in the dark. This may reflect the accumulation of the oxidized secondary donor upon inhibition of oxygen evolution. Near pH 4 the rapidly reversible signal and the stable and slowly decaying components of signal II disappeared irreversibly concomitant with the release of bound manganese. The results are discussed in relation to the effects of low pH on prompt and delayed fluorescence reported earlier (van Gorkom, H.J., Pulles, M.P.J., Haveman, J. and den Haan, G.A. (1976) Biochim. Biophys, Acta 423, 217-226).