The effect of divalent cations on cation fluxes across the chloroplast envelope and on photo synthetic reactions of intact spinach chloroplasts was investigated.
In the absence of EDTA, divalent cations inhibited photosynthetic CO2-fixation and PGA-reduction at low PGA concentrations, but had almost no effect on the reduction of OAA, BQ, and on PGA-reduction at high PGA concentrations. The inhibitory effect of Ca2+ was greater than that of Mg2+. Inhibition of photosynthesis was greater when the divalent cations were added in the dark than when added in the light. In spite of its inhibitory effect, Mg2+ partially restored the Ca2+ inhibited photosynthesis, indicating the involvement of a Mg2+/Ca2+ antagonism in the inhibitory effect.
The inhibitory effect of divalent cations is stronger in a medium with low concentrations of K+ than in the presence of 20 - 50 mᴍ KCl. Mg2+ induced a release of plastidal K+ and an in crease of stromal H+ concentration.
The results indicate that external Mg2+ in the absence of EDTA does not influence neither photosynthetic electron transport nor photophosphorylation, but inhibits the light activation of some enzymes of the carbon reduction cycle. The latter is assumed to be due to an acidification of the stroma pH and the decrease of endogenous K+ level. Since the chloroplast envelope has only a very low permeability towards Mg2+, possible mechanisms are discussed by which Mg2+ changes the properties of the chloroplast envelope and thus secondarily induces the observed effects.