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

1. 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. 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. 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. 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 increased the relative inhibition observed. 5. 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. 6. The results obtained suggest that magnesium inhibition of O₂ evolution may be caused by enhanced phosphate exchange across the chloroplast envelope.