Control of electron flow in chloroplasts by cations

Divalent cations have previously been shown to affect chloroplast electron transport and phosphorylation. Here, a new effect of divalent cations on high-energy states of chloroplasts is reported. It was found that when washed chloroplasts were suspended in sucrose, high (uncoupled) rates of electron flow were observed unless low concentrations of salts of monovalent (approximately 1 m m ) or divalent (approximately 20 μ m ) cations were added. The inhibition of the high rate of electron flow induced by divalent cations was reversed by uncouplers. Higher rates of ATP formation were observed when electron flow was controlled by cations than when it was not so controlled. Cations increased the intensity of delayed light and the extent of the 515-nm absorption change; two parameters which reflect in some way the efficiency of energy conservation. In contrast, cations had little effect on the rate of uptake or decay of H + ions or on the total extent of the proton gradient, suggesting that they act on an earlier intermediate. These results indicate that the presence of cations profoundly affects the coupling of the energy available in redox reactions to the energy-conserving mechanism in the chloroplast membrane.