The role of monovalent cations in the photosynthesis of isolated intact spinach chloroplasts was investigated. When intact chloroplasts were assayed in a medium containing only low concentrations of mono- and divalent cations (about 3 mval l(-1)), CO2-fixation was strongly inhibited although the intactness of chloroplasts remained unchanged. Addition of K(+), Rb(+), or Na(+) (50-100 mM) fully restored photosynthesis. Both the degree of inhibition and restoration varied with the plant material and the storage time of the chloroplasts in "low-salt" medium. In most experiments the various monovalent cations showed a different effectiveness in restoring photosynthesis of low-salt chloroplasts (K(+)>Rb(+)>Na(+)). Of the divalent cations tested, Mg(2+) also restored photosynthesis, but to a lesser extent than the monovalent cations.In contrast to CO2-fixation, reduction of 3-phosphoglycerate was not ihibited under low-salt conditions. In the dark, CO2-fixation of lysed chloroplasts supplied with ATP, NADPH, and 3-phosphoglycerate strictly required the presence of Mg(2+) but was independent of monovalent cations. This finding excludes a direct inactivation of Calvin cycle enzymes as a possible basis for the inhibition of photosynthesis under low-salt conditions.Light-induced alkalization of the stroma and an increase in the concentration of freely exchangeable Mg(2+) in the stroma, which can be observed in normal chloroplasts, did not occur under low-salt conditions but were strongly enhanced after addition of monovalent cations (50-100 mM) or Mg(2+) (20-50 mM).The relevance of a light-triggered K(+)/H(+) exchange at the chloroplast envelope is discussed with regard to the light-induced increase in the pH and the Mg(2+) concentration in the stroma, which are thought to be obligatory for light activation of Calvincycle enzymes.