Post-illumination-related loss of photochemical efficiency of Photosystem II and degradation of the D1 protein are temperature-dependent

Photosystem II (PSII) photochemical efficiency (chlorophyll fluorescence ratio Fv/Fm) was recorded in vivo in Synechocystis 6803 during high light illumination and during a subsequent shift of the cells to darkness. A continuing decrease in the Fv/Fm ratio was observed even after the cells were transferred to darkness, provided the temperature was high enough. The decrease in the PSII efficiency after the shifting of the cells to darkness correlated directly with the loss of the D1 protein under different temperatures, suggesting that temperature-dependent proteolysis of the D1 protein in darkness induces the loss of PSII photochemical efficiency under these conditions. Furthermore, the amount of FtsH protease was found to increase during the high light treatment. This observation suggests that the synthesis of the FtsH protein is a light-regulated process and that this protease most probably has a key role in an efficient degradation of the D1 protein even under post-illuminative conditions, provided the temperature is high enough to prevent the initial reversible steps of photoinhibition.