MATERIALS AND METHODS Plant Growth and Chloroplast Isolation. Spinach (Spinacia oleracea L.) plants were grown in the greenhouse or growth chamber-grown (12-h photoperiod, 22°C day/170C night). Leaves were ground briefly in a Waring Blendor2 using an iced slurry containing 0.33 M sorbitol, 10 mm Na4P207 (pH 6.5), 5 mM MgC12, and 2 mm isoascorbate (12). The chloroplasts were pelleted by centrifugation at 2,000g for 5 s. Chloroplasts from greenhousegrown spinach were washed in a buffer containing 0.4 M sorbitol, 50 mM Hepes-NaOH (pH 7.6), 10 mm NaHCO3, and 5 mm EDTA. Chloroplasts were resuspended carefully in a medium containing 0.33 M sorbitol, 50 mm Hepes-NaOH (pH 7.6), 1 mm MnCl2, 1 mM MgCl2, and 2 mm EDTA. The final suspension of chloroplasts (70-80% intact) contained about 0.5 mg/ml of Chl.02 Evolution. Rates of 02 evolution were determined polarographically using a Clark-type platinum O2 electrode placed in a 2-ml. H20-jacketed cuvette at 250C. The reaction mixture contained 0.33 M sorbitol, 50 mM Hepes-NaOH (pH 7.6), 1 mm MnCl2, 1 mm MgCl2, 2 mm EDTA, 250 units catalase, 5 mM NaHCO3, 30 to 60 ,ig Chl, and 0.25 to 0.5 mm Pi. A 75 w flood lamp that emitted 60 nE/cm2.s provided illumination for photosynthesis.Stromal pH and KV. Stromal pH was determined by the equilibration of radioactive bicarbonate ions across the envelope following the method developed by Heldt and Sauer (7). Polyethylene tubes were set up containing 30 ll 2.5 N NaOH in the bottom of the tube with a 70-,ul layer of silicone oil (Wacker AR 200) 2Mention of a trademark or proprietary product does not constitute a guarantee or warranty of the product by the United States Department of Agriculture and does not imply its approval to the exclusion of other products that may also be suitable.

(8,9,13,17), barley (8,9,11), and lettuce (2). Results obtained with spinach and barley chloroplasts suggested that Mg2+ inhibits photosynthesis by preventing the light activation of NADP-glyceraldehyde-3-P dehydrogenase, phosphoribulokinase, and fructose-1,6-bisphosphatase (9). It was later postulated that Mg2+ inhibits 02 evolution and the light activation of photosynthetic enzymes by stimulating Pi exchange across the chloroplast envelope (8). The postulate was supported by several lines of evidence.First, Mg2' reduced the optimal Pi concentration required for 02 evolution (8) and inhibition by Mg2e of both 02 evolution and the light activation of photosynthetic enzymes was prevented by metabolites which compete with Pi for uptake on the phosphate translocator (11). Second, the activation of photosynthetic enzymes by light in a reconstituted system (stromal proteins plus thylakoid membranes) was inhibited by Pi but not by Mg2+ (10).