Inhibition of photosystem 2 (PS2) reducing side in pea chloroplasts by 10-* M DCMU [3-(3,4-dichlorophenyl)-l,l-dimethylurea], resulting from its binding at the secondary quinone acceptor site is manifested by suppression of millisecond time range delayed fluorescence (DF). In concentrations hi^er than 10-^ M DCMU binds to another site in PS2 and inhibits the oxidizing side, thus leading to suppression of DF with ti/2 = 2 s. Based upon DF measurements, DCMU-affinity of the second site was increased upon chaotropic action of oleic acid, while addition of polyethylene glycol that stabilizes polypeptides lead to preferential decrease in aífinity of the main binding site. The afFmities of the two sites differred also in their dependence upon pH of the mediiun. Thus both binding sites can háve high or low aíiitities to DCMU depending on the statě of their microenvironment. A lower affmity to DCMU of the site responsible for inhibition of the oxidizing side is probably related to its more rigid conformation.
Senescence-induced changes in the xanthophyll cycle activity and chlorophyll (Chl) fluorescence parameters were compared in detached barley (Hordeum vulgare L.) leaf segments kept for 6 d in darkness or under continuous " white light" (90 μmol m-2 s-1). Before detachment of the leaf segments, the plants were grown at periodic regime [12 h light (90 μmol m-2 s-1)/12 h dark]. The de-epoxidation state of the xanthophyll cycle pigments (DEPS) in the leaf samples was determined immediately (the actual DEPS), after 1 h of dark-adaptation (the residual DEPS), and during 14 min of a high-irradiance (HI) exposure (500 μmol m-2 s-1) (HI-induced DEPS). In the light-senescing segments, senescence was delayed pronouncedly compared to dark-senescing ones as the Chl content, the photosystem 2 photochemistry, and electron transport processes were highly maintained. Further, the actual DEPS increased, probably due to the increased mean photon dose. The HI-induced increase in the DEPS was stimulated in the light-senescing segments, whereas it was slowed down in the dark-senescing ones. However, after the 14 min HI-exposure of the dark-senescing segments the HI-induced DEPS was not markedly lower than in the mature leaves, which indicated the maintenance of the xanthophyll cycle operation. and M. Špundová, K. Strzałka, J. Nauš.