A high potential acceptor for photosystem II

The effects of ferricyanide on Photosystem II reactions have been investigated by measurements of microsecond and millisecond prompt fluorescence and microsecond-delayed fluorescence in dark-adapted chloroplasts: (1) Titrations using ferri-ferrocyanide mixtures on: (a) the fast phase of the increase in fluorescence yield observed during a xenon flash, and (b) the normalised area above the millisecond fluorescence induction curve for chloroplasts inhibited by DCMU, showed a pH dependent mid point potential of 400 mV at pH 7.0 which varied by approx. -60 mV/pH unit between pH 6 and 8.5. (2) A saturating laser flash induced a fluorescence increase (as monitored by a weak measuring beam) of only 50% of that reached following a second flash in chloroplasts preincubated with ferricyanide and inhibited by 3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU) prior to illumination. In the absence of ferricyanide, the fluorescence level reached after a single flash was initially close to that measured after a second flash (although the level subsequently declined). (3) The initial amplitude of the microsecond-delayed fluorescence excited by a single laser flash was diminished in chloroplasts dark-adapted with ferricyanide. In the presence of DCMU and ferricyanide, the amplitude was also diminished for the first flash of a series, but subsequently enhanced above the level obtained in chloroplasts in the presence of DCMU alone. (4) The above effects were not seen if DCMU was added to the chloroplasts before ferricyanide, or if the period of incubation with ferricyanide was much less than 4 min. (5) These results suggested the presence of a second acceptor Q2, with Em7 = 400 mV and n = 1, before the DCMU block in Photosystem II. There is 0.35--1 equivalent of the acceptor per reaction centre, and its reduction occurs within less than 5 mus. The role of the acceptor in double turnovers of the photochemistry during a single flash and its likely operating redox potential are discussed.