Influence of DCMU and ferricyanide on photodamage in photosystem II

The effect of strong illumination of thylakoid membranes was studied under a range of conditions. Under anaerobic conditions, the relatively small quenching of the maximum fluorescence (Fmax) is accompanied by a large increase of the initial fluorescence (F0), which is partially reversible. Changes in the extent of the QA-Fe2+ and chlorophyll triplet EPR signal during anaerobic photoinhibition were consistent with double reduction of QA [as reported by Vass et al. ((1992) Proc. Natl. Acad. Sci. U.S.A. 89, 1408-1412)]. When illumination was done in the presence of 3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU) or ferricyanide, no change occurred in F0 while Fmax was quenched. The quenching of Fmax occurred more rapidly than the loss of oxygen evolution, and they were both irreversible. In the presence of ferricyanide, the percentage of inhibition of oxygen evolution was larger than the decrease in the extent of the QA-Fe2+ signal, indicating that damage of the donor side occurred. In the presence of DCMU, a decrease of the QA-Fe2+ EPR signal occurred which corresponded to the inhibition of oxygen evolution and to an increase of the triplet EPR signal, indicating a possible overreduction of QA. However, these changes were less marked in the DCMU-treated samples than in the sample without additions and occurred despite the quenching of Fmax. These results suggest that strong illumination of thylakoids, in the presence of DCMU, results in a slower formation of stable forms of reduced QA, thereby allowing the occurrence of side-path reactions leading to Fmax quenching.(ABSTRACT TRUNCATED AT 250 WORDS)