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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.

Regulation of Chloroplast Photosynthetic Activity by Exogenous Magnesium