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 (F-max) is accompanied by a large increase of the initial fluorescence (F-0), which is partially reversible. Changes in the extent of the Q(A)(-)Fe(2+) and chlorophyll triplet EPR signal during anaerobic photoinhibition were consistent with double reduction of Q(A) [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 F-0 while F-max was quenched. The quenching of F-max 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 Q(A)(-)Fe(2+) signal, indicating that damage of the donor side occurred. In the presence of DCMU, a decrease of the Q(A)(-)Fe(2+) 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 F-max. These results suggest that strong illumination of thylakoids, in the presence of DCMU, results in a slower formation of stable forms of reduced Q(A), thereby allowing the occurrence of side-path reactions leading to F-max quenching. From the fluorescence quenching data, these side-path reactions seem to occur under all the conditions studied with the exception of samples illuminated under anaerobic conditions without additions. To rationalize these observations, we propose that, under the latter conditions, relatively rapid overreduction of the reaction center results in the inhibition of charge separation and hence the prevention of F-max quencher formation. When overreduction of Q(A) is slowed down (by the presence of electron acceptors, oxygen, or DCMU), then side-path reactions occur resulting in the irreversible fluorescence quenching and irreversible damage. These two effects may be related to overoxidation of PS II components.