Effects of α-benzyl-α-bromo-malodinitrile on the primary electron acceptor of Photosystem II in spinach chloroplasts

Reactions at the reducing side of Photosystem II in spinach chloroplasts are modified by α-benzyl-α-bromo-malodinitrile (BBMD).On addition of 50 μM BBMD to chloroplasts the following phenomena can be observed: (1) electron flow to an acceptor like 2,6-dichlorophenolindophenol is partly deflected to electron flow to oxygen; (2) the electron flow to oxygen is carbonyl cyanide m-chlorophenylhydrazone sensitive but 3-(3,4-dichlorophenyl)-1,1-dimethylurea insensitive; (3) variable fluorescence is abolished but basal fluorescence is not altered; (4) a strong photobleaching of carotenoids is induced. BBMD seems a very efficient acceptor for electrons from the primary electron acceptor of Photosystem II, resulting in a BBMD-mediated electron transport from this primary acceptor to oxygen.On pretreatment of chloroplasts with 50 μM BBMD the effects are different; (1) electron flow to 2,6-dichlorophenolindophenol, ferricyanide, or NADP is almost completely inhibited and is not restored by addition of artificial electron donors: (2) no electron flow to oxygen is observable unless BBMD again is added to reaction media; (3) no variable fluorescence is observable but basal fluorescence is not affected; (4) there is no photobleaching of carotenoids unless BBMD again is added; (5) no reduction of C-550 can be recorded. Pretreatment of chloroplasts with BBMD seems to induce an intense cycling of electrons around Photosystem II and only anew added BBMD can interrupt this cycling.