PRIMARY EVENTS OCCURRING IN PHOTOINHIBITION IN SYNECHOCYSTIS-6714 WILD-TYPE AND AN ATRAZINE-RESISTANT MUTANT

Exposure of photosynthetic organisms to a light intensity higher than that needed to saturate photosynthesis causes the inhibition of Photosystem II activity (photoinhibition). We induced photoinhibition in cells of Synechocystis 6714 strains by exposure to light intensities between 1000 and 4000 μE.m-2.s-1. Fluorescence, thermoluminescence and oxygen measurements were used to follow the inhibition of electron transfer in Photosystem II centers. We demonstrated that, in oxygen evolving Photosystem II centers, the electron transfer from QA to QB was not slowed down. Photoinhibited samples presented a normal oscillation pattern of oxygen yield. The rate of deactivation of S2 state was similar in control and photoinhibited cells. This indicates that the QA QB equilibrium is not modified in the centers which evolve oxygen. By studying the kinetics of the decrease of the amplitudes of the thermoluminescence Q and B bands in Synechocystis 6714 we confirmed our previous conclusions drawn from electron transfer measurements: the inhibition of electron transfer to QB is faster than the inhibition of electron transfer to QA. We conclude that a succession of two different states of Photosystem II centers is produced during the process of photoinhibition: (1) A state in which the electron transfer is inhibited between QA and QB. (2) an inactivated low fluorescent state where Q-A formation is also inhibited. Restoration of the photoactive fluorescent Photosystem II requires de novo synthesis of D1. Our main conclusion is that the first damage during photoinhibition in cyanobacteria cells is at the level of the QA to QB electron transfer step. © 1990.