FLUORESCENCE QUENCHING BY PLASTOQUINONE IN AN OXYGEN-EVOLVING PHOTOSYSTEM-II-ENRICHED PREPARATION

The chlorophyll fluorescence of an oxygen-evolving photosystem-II-enriched preparation was dramatically quenched by the inhibitor 3-(3,4-dichlorophenyl)-1,1-dimethyl urea (DCMU). Evidence was provided that the quenching was not due to an inhibition by DCMU on the donor side of photosystem II (PSII) as previously suggested. It was attributed instead to oxidized plastoquinone which formed traps for the excitation energy in the pigment bed. Nevertheless, the quenching by plastoquinone was not uniform in at least two aspects. Studies on fluorescence spectra showed that oxidized plastoquinone more effectively quenched certain chlorophyll species responsible for fluorescent emission. Fluorescence induction measurements in the presence of DCMU revealed that one type of photosystem II (PSIIbeta) giving rise to the slow exponential rise in the induction curve was preferentially quenched by oxidized plastoquinone, as compared with the type of photosystem II (PSIIalpha) which was responsible for the major sigmoidal rise.