(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 O2evolution 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 O2evolution 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 O2evolution in non-halophytic thylakoids, and a comparison is made between the two systems. © 1983.
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