Effects of photoinhibition on the PS II acceptor side including the endogenous high spin Fe2+ in thylakoids,PS II-membrane fragments and PS II core complexes

Effects of photoinhibition at 0 °C on the PS II acceptor side have been analyzed by comparative studies in isolated thylakoids, PS II membrane fragments and PS II core complexes from spinach under conditions where degradation of polypeptide(s) D1(D2) is highly retarded. The following results were obtained by measurements of the transient fluorescence quantum and oxygen yield, respectively, induced by a train of short flashes in dark-adapted samples: (a) in the control the decay of the fluorescence quantum yield is very rapid after the first flash, if the dark incubation was performed in the presence of 300 M K₃[Fe(CN)₆]; whereas, a characteristic binary oscillation was observed in the presence of 100 M phenyl-p-benzoquinone with a very fast relaxation after the even flashes (2nd, 4th. . . ) of the sequence; (b) illumination of the samples in the presence of K₃[Fe(CN)₆] for only 5 min with white light (180 W m^-2) largely eliminates the very fast fluorescence decay after the first flash due to Q_A^- reoxidation by preoxidized endogenous non-heme Fe³⁺, while a smaller effect arises on the relaxation kinetics of the fluorescence transients induced by the subsequent flashes; (c) the extent of the normalized variable fluorescence due to the second (and subsequent) flash(es) declines in all sample types with a biphasic time dependence at longer illumination. The decay times of the fast (6–9 min) and the slow degradation component (60–75 min) are practically independent of the absence or presence of K₃[Fe(CN)₆] and of anaerobic and aerobic conditions during the photo-inhibitory treatment, while the relative extent of the fast decay component is higher under anaerobic conditions. (d) The relaxation kinetics of the variable fluorescence induced by the second (and subsequent) flash(es) become retarded due to photoinhibition, and (e) the oscillation pattern of the oxygen yield caused by a flash train is not drastically changed due to photoinhibition.Based on these findings, it is concluded that photoinhibition modifies the reaction pattern of the PS II acceptor side prior to protein degradation. The endogenous high spin Fe²⁺ located between Q_A and Q_B is shown to become highly susceptible to modification by photoinhibition in the presence of K₃[Fe(CN)₆] (and other exogenous acceptors), while the rate constant of Q_A^- reoxidation by Q_B(Q_B^-) and other acceptors (except the special reaction via Fe³⁺) is markedly less affected by a short photoinhibition. The equilibrium constant between Q_A^- and Q_B(Q_B^-) is not drastically changed as reflected by the damping parameters of the oscillation pattern of oxygen evolution.