The quantum yield of photoinhibition is the same in flash light and under continuous illumination - implications for the mechanism

Abstract

The action spectrum of Photosystem II (PSII) photoinhibition peaks in ultraviolet-C (UV-C) and has a low-efficiency tail in visible light. We show that the action spectra of anaerobic and aerobic photoinhibition are similar, indicating that the photoreceptor of photoinhibition is the same in both cases. The similarity of these two spectra also shows that photoinhibition is caused by a constituent of PSII. This constituent is not PSII antenna, as photoinhibition showed no sign of light saturation even if the photoinhibitory light was so intense that chlorophyll excitations decayed mostly by annihilation. We suggest that the photoreceptor of both UV and visible-light-induced photoinhibition is the manganese of oxygen evolving complex (OEC). The absorption spectra of Mn(III) and Mn(IV) complexes indeed closely resemble the action spectrum of photoinhibition. Furthermore, an artificial electron donor can partially compensate for photoinhibitory damage induced by strong visible light or UV light, indicating that OEC is the primary target of photoinhibition in both cases. We also show that photoinhibitory loss of oxygen-evolving activity is accompanied by the release of one manganese ion per PSII to the lumen. We conclude that excitation of manganese causes its release from OEC, and the inactive OEC renders the remaining PSII reaction centre susceptible to oxidation by P680+. The rate of photoinhibition depends on the delay between laser pulses because the S1 state of OEC contains the target manganese. The mechanism explains a number of earlier observations, including the independence of the quantum yield of photoinhibition on light intensity.