Regulation of chloroplast photosynthetic activity by exogenous magnesium.

Magnesium was most inhibitory to photosynthetic reactions by intact chloroplasts when the magnesium was added in the dark before illumination. Two millimolar MgCl(2), added in the dark, inhibited CO(2)-dependent O(2) evolution by Hordeum vulgare L. and Spinacia oleracea L. (C(3) plants) chloroplasts 70 to 100% and inhibited (pyruvate + oxaloacetate)-dependent O(2) evolution by Digitaria sanguinalis L. (C(4) plant) mesophyll chloroplasts from 80 to 100%. When Mg(2+) was added in the light, O(2) evolution was reduced only slightly. O(2) evolution in the presence of phosphoglycerate was less sensitive to Mg(2+) inhibition than was CO(2)-dependent O(2) evolution.Magnesium prevented the light activation of several photosynthetic enzymes. Two millimolar Mg(2+) blocked the light activation of NADP-malate dehydrogenase in D. sanguinalis mesophyll chloroplasts, and the light activation of phosphoribulokinase, NADP-linked glyceraldehyde-3-phosphate dehydrogenase, and fructose 1,6-diphosphatase in barley chloroplasts. The results suggest that Mg(2+) inhibits chloroplast photosynthesis by preventing the light activation of certain enzymes.