pH effects on light dependence of the stoichiometry of proton influx and efflux to electron transport in spinach chloroplasts.

Initial and steady state rates of proton transport at low light intensity have been measured and compared with steady state rates of electron transport in the pH range of 6.0-7.6 in envelope-free spinach chloroplasts. At pH 6-7, the H(+)/e(-) values computed using the initial rate of proton transport varied with light intensity, from a value of 2 at low light to almost 5 at high light. In contrast, the H(+)/e(-) values computed using the steady state rate of proton transport did not show a dependence on light intensity, having a constant value of 1.7±0.2. Likewise, at pH 7.6, the H(+)/e(-) ratio, computed using either the initial or steady state rates of proton transport did not vary with light intensity but was constant at H(+)/e(-)=1.7±0.1. Analysis of the light dependence of electron and proton transport allowed determination of (a) the quantam requirements of transport, (b) the rates of transport at light saturation, and (c) H(+)/e(-) ratios for initial and steady state proton transport. Extrapolating the initial proton transport to zero light, we found that both H(+)/photon and H(+)/e(-) values were not strongly dependent on pH, approaching a near constant value of 2.0. Using the initial rate of proton transport extrapolated to saturating light intensity we found the H(+)/e(-) ratio to be strongly pH-dependent. We suggest that internal pH controls electron transport at high light intensities. The true stoichiometry is reflected only in measurements taken at low light using the initial proton transport data. Our findings and interpretation reconcile some conflicting data in the literature regarding the pH-dependence of the H(+)/e(-) ratio and support the concept that internal pH controls noncyclic electron transport.