SUPPRESSION OF OXYGEN-EVOLVING SYSTEM IN SPINACH CHLOROPLAST MEMBRANES DUE TO RELEASE OF MANGANESE ON EXPOSURE TO OSMOTIC-STRESS IN-VITRO IN PRESENCE OF MAGNESIUM

When spinach chloroplast membranes were exposed to osmotic stress in v itro, by incubation in. 1.0 M sorbitol + l0 mM MgCl2, their oxygen evo lving system was suppressed. The possible reasons for such inactivatio n of PS II mediated oxygen evolution were examined. There were conform ational changes in the chloroplast membranes, as indicated by their ab sorption spectra. The pattern of sensitivity to DCMU was not altered. The sensitivity of PS II to water stress remained, even after a pre-wa sh treatment with NaCL (which removed 18 and 24 kD proteins) but not w hen the thylakoids were pretreated with NH2OH or CaCl2 (removed mangan ese and 33 kD). The manganese content of thylakoid membranes was marke dly reduced under osmotic stress in presence of magnesium. We suggest that exposure of chloroplasts to 1.0 M sorbitol in presence of Mg2+ re leased manganese from thylakoid membranes, thereby leading to a suppre ssion in oxygen evolution.