Further studies on the role of chloride in photosynthetic O2 evolution in higher plants

Abstract

(1) Thylakoid preparations from two salt-tolerant higher plant species, Avicennia eucalyptifolia and Salicornia quinqueflora, were shown to require very high salt concentrations for Photosystem II electron transport at alkaline pH. (2) High concentrations (250-500 mM) of tetramethylammonium chloride were found to be as effective as NaCl in stimulating maximal O2-evolution activity, supporting the evidence that the effect is mediated by chloride and not by cations. (3) Light-response curves of O2 evolution at high and low concentrations of NaCl were indistinguishable in the light-limited region, providing further evidence for the O2-evolving complex as the site of chloride action. (4) A considerable shift in pH optimum towards the alkaline region for O2 evolution in the presence of increasing concentrations of NaCl, and pH-jump experiments in the presence and absence of NaCl indicate possible competitiveness or at least some form of interdependence of Cl- and OH- binding to the O2-evolving complex. (5) The results are discussed in the light of recent evidence concerning the alkaline inhibition of O2 evolution in non-halophytic thylakoids, and a comparison is made between the two systems. © 1983.