INHIBITIONS OF PHOTOSYSTEM-II ACTIVITY BY PROTON GRADIENT

Nigericin stimulates PS II turnover in thylakoids at low but not at hi gh light intensity in a low-frequency regime (6 Hz) at pH 8. When the external pH is decreased, stimulation by the uncoupler is also observe d at high light intensity. Neither phosphorylation of LHCII nor unstac king of the thylakoids changes the effect of nigericin at pH 8. The ob servations are interpreted to indicate that Delta pH regulates PS II a ctivity at two levels: (a) the transfer of excitation energy to the re action centre; (b) PS II turnover.