Light-Induced Oxidation-Reduction Reactions of Photosystem II in Dichlorophenyl-dimethyl Urea (DCMU) Inhibited Thylakoids

Illumination o f thylakoid membranes in the presence o f 3-(3',4'-dichlorophenyl)-1,1-dimethyl urea (D C M U ) causes the reduction o f the primary quinone acceptor QA o f photosys­ tem II (PS II) and the storage o f a positive charge on the donor side o f the photochemical reac­ tion center. These oxidation-reduction reactions are accompanied by characteristic changes o f absorbance in the ultra-violet region o f the spectrum. The PS II-related absorbance difference spectra (250 -350 nm) were compared in control and hydroxylamine-treated thylakoid mem­ branes, and in thylakoids suspended in the presence o f carbonyl cyanide-/7-(trifluoromethoxy)phenylhydrazone (FCCP). The light minus dark difference spectra were dominated by the QA minus QA difference spectrum. Qualitatively, the three spectra were identical in the 300 -350 nm region, however, they showed distinct differences in the 2 5 0 -3 0 0 nm region. The latter arose because o f different contributions from the donor side o f PS II in the thylakoid membrane o f the three samples. The result suggested that FCCP acts as the ultimate electron donor in DCM U-poisoned chloroplasts. Therefore, the absorbance difference spectrum in the presence o f FCCP reflected a contribution from the QA minus QA component only. Deconvolu­ tion o f the absorbance difference spectra o f control and hydroxylamine-treated thylakoids yielded difference spectra attributed to the oxidation o f a component on the donor side o f PS II. This component did not conform with the known Mn(III) —■> Mn(IV) transition. Rath­ er, it indicated the oxidation o f a modified form o f Mn in the presence o f D C M U , probably a Mn(II) —> Mn(III) transition. The results are discussed in terms o f the use o f DCM U-poisoned thylakoid membranes in the quantitation o f the primary quinone acceptor QA by spectrophotometric approaches.