PHOTOINACTIVATION OF THE ELECTRON-TRANSFER THROUGH PHOTOSYSTEM-2 AS STUDIED BY TIME-RESOLVED FLASH ABSORPTION-SPECTROSCOPY

Primary events in the photoinactivation of photosystem (PS) 2 membrane fragments by low and high ''visible light'' irradiance (17 to 1700 W m-2) and UV-B irradiation (90 W m-2) were analyzed by measuring flash- induced absorption changes at 830 nm that reflect transient formation of P680+ and Pheo-. Following results were obtained: (1) Irradiation w ith ''visible light'' under aerobic conditions affects the PS 2 electr on transfer at two different sites: (a) within the PS 2 reaction centr e by impairment of primary charge separation (P680 Pheo Q(A) --> P680Pheo-Q(A)), and (b) on the PS 2 donor side by inhibition of the electr on transfer from Y(Z) to P680+. (2) In PS 2 membrane fragments with in tact O2-evolution the primary charge separation is the most sensitive target of the photoinhibition by ''visible light''. The UV-B irradiati on, however, affects predominantly the oxygen-evolving complex or the electron transfer from the oxygen-evolving complex to Y(Z)ox. (3) Susc eptibility of the P680 Y(Z) segment to photoinhibition by ''visible li ght'' is drastically increased in the samples with lifetimes of Y(Z)ox and P680+ having been significantly prolonged by Tris-treatment. Susc eptibility of the primary charge separation to photoinhibition, howeve r, is not dependent on the lifetimes of P680+ and Y(Z)ox.