Molecular Events of Photoinhibitory Inactivation in the Reaction Centre of Photosystem II

Abstract

The events of photoinhibition were examined at the molecular level. The evidence presented here suggests that the primary event of photoinhibition involves inactivation of photosystem II reaction centre function. Chlorophyll fluorescence emission and excitation spectra and fluorescence induction transients were suppressed after photoinhibitory treatment. Such fluorescence is directly dependent on photosystem II photochemistry. In addition, photoinhibition caused a decline in charge separation measured by the absorbance change at 320 nm arising from the light-induced reduction of the primary acceptor of photosystem II, Q_A. Inhibition of this parameter is indicative of damage to a component involved in primary photochemistry. That the effect of high light treatment could not be correlated with any loss of the D1 protein supports the suggestion that a cofactor involved in primary photochemistry is the initial site of photoinhibition. It is possible that D1 may eventually be lost as a result of such damage. The reaction centre chlorophyll of photosystem II, P680, is suggested to be the cofactor involved. Possible mechanisms of damage are discussed with reference to the prosthetic components of the reaction centre of photosynthetic bacteria.