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 vitro, by incubation in 1.0 M sorbitol + 10 mM MgCI₂, their oxygen evolving system was suppressed. The possible reasons for such inactivation of PS II mediated oxygen evolution were examined. There were conformational changes in the chloroplast membranes, as indicated by their absorption spectra. The pattern of sensitivity to DCMU was not altered. The sensitivity of PS II to water stress remained, even after a prewash treatment with NaCI (which removed 18 and 24 kD proteins) but not when the thylakoids were pretreated with NHjOH or CaCI₂ (removed manganese and 33 kD). The manganese content of thylakoid membranes was markedly reduced under osmotic stress in presence of magnesium. We suggest that exposure of chloroplasts to 1.0 M sorbitol in presence of Mg²⁺ released manganese from thylakoid membranes, thereby leading to a suppression in oxygen evolution.