Effect of pH on chloroplast photosynthesis. Inhibition of O2 evolution by inorganic phosphate and magnesium.

1. The pH optimum of CO2-dependent O2 evolution by barley (Hordeum vulgare L.) chloroplasts was found to be between 7.8 and 8.2. The addition of 1 mM MgCl2 in the dark inhibited O2 evolution over the entire pH range tested and resulted in a much sharper pH profile centered around pH 8.2. 2. The pH optimum for O2 evolution, in the presence and absence of 1 mM MgCl2, was acid-shifted 0.3--0.4 pH units by 2 mM NH4Cl. The pH optimum of O2 evolution, with and without 1 mM MgCl2, was base-shifted by 2 mM sodium acetate, approx. 0.5 pH units relative to the controls. 3. O2 evolution in the presence of bicarbonate plus 3-phosphoglycerate or ribose-5-phosphate was considerably less sensitive to pH than CO2-dependent O2 evolution in the absence of substrate. With these substrates, both in the presence and absence of 1 mM MgCl2, the pH optimum was broad and was centered around pH 7.8. 4. Inhibition of CO2-dependent O2 evolution by inorganic phosphate and magnesium increased as the pH of the reaction mixture was decreased below the optimum. Decreasing the pH from 8.2 to 7.6, reduced over 3-fold the concentration of inorganic phosphate required to inhibit O2 evolution completely. For magnesium, a similar change in pH reduced the concentration required to inhibit O2 evolution 50% approx. 5-fold. At pH 8.2, magnesium inhibition required inorganic phosphate. Magnesium was not required for inhibition of O2 evolution by inorganic phosphate, but incresaed the relative inhibition observed. 5. Illumination of intact barley chloroplasts increased the activity of NADP-glyceraldehyde-3-P dehydrogenase, phosphoribulokinase and fructose-1,6-diphosphatase. MgCl2 and inorganic phosphate prevented this increase in enzyme activity at concentrations that completely inhibited CO2-dependent O2 evolution. 6. The results obtained suggest that magnesium inhibition of O2 evolution may be caused by enhanced phosphate exchange across the chloroplast envelope.