Photoinhibition as a function of the ambient redox potential in Tris-washed PSII membrane fragments

The mode of photoinhibition as a function of the ambient redox potential (E -ambient) in suspensions of Tris-washed PS II membrane fragments has been a nalyzed by monitoring flash-induced absorption changes at 830 nm. It was fo und: (a) the detectable initial amplitude, DeltaA (total)(830), as a measur e of the capacity to form the `stable' radical pair, P680(+.) Q(A)(-.), dra stically decreases during a 10 min photoinhibition at E-ambient values belo w +350 mV; (b) conversely, the normalized extent of the 18 mus relaxation k inetics, DeltaA(830)(18 mus) as a measure of the electron transfer from Y-Z to P680(+.) becomes highly susceptible to light stress when E-ambient exce eds values of about +350 mV; (c) effects of the ambient redox potentials ar e highly pronounced during light exposure under anaerobic conditions, while much smaller differences arise under aerobic conditions; (d) the extent of damage does not correlate with the total concentration of K-3[Fe(CN)(6)] a nd K-4[Fe(CN)(6)] in the suspension during photoinhibition but rather depen ds on the E-m-values; (e) qualitatively similar features are observed when the redox buffer system K-3[Fe(CN)(6)]/Na2S2O4 is replaced by K-2[IrCl6]/Na 2S2O4; (f) the characteristic E-ambient-dependence of photoinhibition is ob served only under anaerobic conditions. The results are discussed with resp ect to different redox components that might be involved, including brief c omments on a possible role of Cyt b559.