Relationship between photosynthetic electron transport and pH gradient across the thylakoid membrane in intact leaves

Under conditions (0.2% CO2; 1% O-2) that allow high rates of photosynthesis, chlorophyll fluorescence was measured simultaneously with carbon assimilation at various light intensities in spinach (Spinacia oleracea) leaves. Using a stoichiometry of 3 ATP/CO2 and the known relationship between ATP synthesis rate and driving force (Delta pH), we calculated the light-dependent pH gradient (Delta pH) across the thylakoid membrane in intact leaves. These Delta pH values were correlated with the photochemical (q(P)) and nonphotochemical (q(N)) quenching of chlorophyll fluorescence and with the quantum yield of photosystem II (Phi(PSII)). At Delta pH > 2.1 all three parameters (q(P), q(N), and Phi(PSII)) changed very steeply with increasing Delta pH (decreasing pH in the thylakoid). The observed pH dependences followed hexacooperative titration curves with slightly different pK(a) values. The significance of the steep pH dependences with slightly different pK(a) values is discussed in relation to the regulation of photosynthetic electron transport in intact leaves.