Reversible inactivation of photosystem II reaction centers in cation-depleted chloroplast membranes

Isolated pea chloroplasts were washed once in 10 m m NaCl and were then suspended in “low-salt” medium. Approximately one-half of the photosystem II reaction centers of these salt-depleted membranes were found to be photochemically inactive. These units became active in the presence of low concentrations of divalent cations (5–10 m m Mg 2+ ) or high concentrations of monovalent cations (150–200 m m Na + ), as evidenced by a twofold increase in the steady-state flash yield of oxygen evolution under short (~10-μs) saturating repetitive flashes (two per second). The half-maximal increase in flash yield occurred at ~2 mM Mg 2+ or ~75 m m Na + . The flash yield of hydroxylamine oxidation in these low-salt chloroplasts increased twofold after Mg 2+ addition, indicating that the cation action was close to the reaction-center chlorophyll complex. The relation between flash yield and dark time between flashes was not changed significantly by Mg 2+ , indicating that the rate-limiting step of the overall electron transport (H 2 0 —→ ferricyanide) was not affected significantly. When the rate-limiting step was bypassed using silicomolybdate as the photosystem II electron acceptor (in the presence of diuron), the reduction rate doubled in the presence of Mg 2+ , even under continuous, saturating light. In glutaraldehyde-fixed chloroplasts, Mg 2+ did not increase the flash yield of O 2 evolution; this suggests that protein conformational changes in the chloroplast membranes were involved in Mg 2+ activation of photosystem II centers.