Correlation between photosynthesis and the transthylakoid proton gradient

In isolated intact chloroplasts, maximal rates of photosynthetic O₂ evolution (in saturating HCO^?₃) are associated with a critical transthylakoid proton gradient as a result of the stoichiometric consumption of 2 mol NADPH and 3 mol ATP/mol CO₂ fixed. Studies with the fluorescent probe 9-aminoacridine reveal that in the illuminated steady state the critical ΔpH is 3.9.CO₂-dependent O₂ evolution is inhibited by increases of 0.1–0.2 in ΔpH that occur when catalase is omitted from the medium, NO^?₂ is included as an electron acceptor, or when chloroplasts are illuminated under low partial pressures of O₂. Low concentrations of antimycin (0.33 μM) or NH₄Cl (0.33 mM) decrease ΔpH and relieve this inhibition of electron flow. The energy transfer inhibitor quercetin lowers the high ATP/ADP ratio associated with these conditions, but does not lower ΔpH or relieve the inhibition.A decrease of ΔpH below 3.9 by weaker illumination, millimolar levels of NH₄Cl or micromolar levels of antimycin, results in lower rates of photosynthesis owing to limitation by the phosphorylation rate.These findings show that in absence of rate limitation by the carbon cycle, the extent of thylakoid energization is related to the ratio of ATP to NADPH production and in turn, the rate of CO₂ assimilation.