Photoinhibition as a function of the ambient redox potential in Tris-washed PS II 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 analyzed by monitoring flash-induced absorption changes at 830 nm. It was found: (a) the detectable initial amplitude, DeltaA (830) (total) , as a measure of the capacity to form the 'stable' radical pair, P680(+c) Q (A) (-c) , drastically decreases during a 10 min photoinhibition at E(ambient) values below +350 mV; (b) conversely, the normalized extent of the 18 mus relaxation kinetics, DeltaA (830) (18 mus) as a measure of the electron transfer from Y(Z) to P680(+c) becomes highly susceptible to light stress when E(ambient) exceeds values of about +350 mV; (c) effects of the ambient redox potentials are 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)] and K(4)[Fe(CN)(6)] in the suspension during photoinhibition but rather depends on the E( m )-values; (e) qualitatively similar features are observed when the redox buffer system K(3)[Fe(CN)(6)]/Na(2)S(2)O(4) is replaced by K(2)[IrCl(6)]/Na(2)S(2)O(4); (f) the characteristic E(ambient)-dependence of photoinhibition is observed only under anaerobic conditions. The results are discussed with respect to different redox components that might be involved, including brief comments on a possible role of Cyt b559.