T H E Role of Chloride in 02 Evolution by Thylakoids from Salt-tolerant Higher Plants

(1) Thylakoids isolated from leaves of two salt-tolerant higher plant species were found to require high (greater than 250 mM) concentrations of C! for maximal rates of photosynthetic 02 evolution and maximum variable chlorophyll a fluorescence yield. These activities were also tolerant to extremely high (2-3 M) salt concentrations. Their pH dependence was markedly different in the absence and presence of sufficient salt levels. (2) When CIwas provided as CaCl2, as opposed to MgCl2, KCI or NaCl, higher rates of 02 evolution were obtained, suggesting that Ca 2+ has an important role in Photosystem II reactions. (3) The site of CIaction was located on the electron donor side of Photosystem II. (4) 02 evolution in the presence of optimal CI concentrations showed a pH dependence closely matched by that of 3sCI-NMR line broadening, which is indicative of CIbinding. This pH-dependent 3sCI-NMR line-width broadening was not altered significantly by treatment of the thylakoids with EDTA; it was, however, abolished by heat treatment. (5) Only anions with similar ionic radii (Br , NO~) were effective in replacing CI . Small anions such as F and O H were inhibitory; larger ions had no effect. The inhibition by F is due, at least in part, to displacement of C l . The selectivity is attributed to a combination of steric and ionic field effects. (6) It is proposed that CI facilitates Photosystem II electron transport by reversible ionic binding to the O~-evolving complex itself or to the thylakoid membrane in close proximity to it.