In whole cells of the wild type of Synechocystis 6714 exposed to high irradiance (Hl) the electron transfer to Qb was inhibited before the transfer to Qa In the mutant AzV with two point mutations in the Qg pocket (Ala 251 Val and Phe 211 Ser) the inhibition of electron transport to Qb had about the same rate as in the wild type but the transport to Qa was inhibited faster in the mutant. In the mutant also the repair of photosystem 2 (PS 2) reaction centres was slower after a moderate photoinhibitory damage. In contrast degradation of the Dl protein in the course of photoinhibition in vitro was slower with thylakoids from the mutant than with those of the wild type. In CMamydomonas cells photoinhibited at low temperature (5 °C) and in spinách thylakoids under anaerobic conditions a fast inhibition of electron transfer developed in PS 2 simultaneously with a State characteiized by a high level of F^. This high fluorescent statě disappeared and electron transport activity was restored in the dark without protein synďiesis when photoinhibited samples were soon transferred to higher temperature (25 °C) or to aerobic conditions, respectively. On the other hand, the irreversible photoinhibitory damage characteiized by the dechne of was slowed down by lowering of the temperature in all the objects tested.
Plants have developed various photoprotective mechanisms to resist irradiation stress. One of the photoprotective mechanisms described in the literature for LHC2-containing organisms involves a down-regulation of photosystem (PS) 2 occurring simultaneously with the build-up of a proton gradient across the thylakoid membrane (ΔpH). It is often correlated with deepoxidation of xanthophylls located in LHC2. In Rhodophyta instead of LHC2, the peripheral antenna of PS2 consists of a large extramembrane complex, the phycobilisome (PBS), which transfers its excitation to the core antennae of PS2 composed of the CP43 and CP47 protein-chlorophyll complexes and there is no xanthophyll cycle. In the red alga Rhodella violacea a ΔpH-dependent chlorophyll (Chl) a fluorescence quenching can be formed. We characterised this quenching, studied the effects of various irradiances and inhibitors. Under photoinhibitory conditions, the ΔpH-dependent Chl fluorescence quenching exerts a photoprotective role and delays the kinetics of photoinhibition. It is the first time that such a photoprotective mechanism is described in PBS-containing organisms. and M. Ritz, K. V. Neverov, A.-L. Etienne.