DISSIPATION OF EXCITATION-ENERGY BY PHOTOSYSTEM-II PARTICLES AT LOW PH

The effect of decreasing the pH on the properties of Photosystem II particles isolated from pea and spinach has been investigated. Between pH 6.5 and 4.0 there was a decrease in quantum efficiency of oxygen evolution that was not associated with a proportional closure of reaction centres, as determined by the fluorescence coefficient, q(P). This decrease in the quantum yield of Photosystem II appeared to be due to two separate mechanisms. Between pH 6.5 and 5.0, the decrease in quantum yield was not associated with any change in fluorescence yield or in the ratio of F(v)/F(m); the decrease was greater at limiting light than in saturating light and may be caused by a cyclic electron transfer that is associated with the destabilisation of the oxidised intermediates of the oxygen-evolving complex and an accelerated rate of Q(A)- oxidation. Below pH 5.0 the decrease in efficiency is correlated with a quenching of chlorophyll fluorescence. This quenching was light-dependent and required oxidising conditions; redox titrations indicated that a component with a midpoint oxidation reduction potential of +405 mV at pH 4.0 needed to be oxidised if the low pH-dependent quenching was to be observed. The relationship between these effects of low pH and the 'down-regulation' of Photosystem II observed under physiological conditions is discussed.