Contribution of electric field (Delta psi) to steady-state transthylakoid proton motive force (pmf) in vitro and in vivo. Control of pmf parsing intoDelta psi and Delta pH by ionic strength

The observed levels of DeltaG(ATP) in chloroplasts, as well as the activati on behavior of the CF1CF0-ATP synthase, suggest a minimum transthylakoid pr oton motive force (pmf) equivalent to a Delta pH of similar to2.5 units. If as is commonly believed, all transthylakoid pmf is stored as Delta pH, thi s would indicate a lumen pH of less than similar to5. In contrast, we have presented evidence that the pH of the thylakoid lumen does not drop below p H similar to5.8 [Kramer, D. M., Sacksteder, C. A., and Cruz, J. A. (1999) P hotosynth. Res. 60, 151-163], leading us to propose that Delta psi can cont ribute to steady-state pmf: In this work, it is demonstrated, through assay s on isolated thylakoids and computer simulations, that thylakoids can stor e a substantial fraction of pmf as Delta psi, provided that the activities of ions permeable to the thylakoid membrane in the chloroplast stromal comp artment are relatively low and the buffering capacity (beta) for protons of the lumen is relatively high. Measurements of the right-induced electrochr omic shift (ECS) confirm the ionic strength behavior of steady-state Delta psi in isolated, partially uncoupled thylakoids. Measurements of the ECS in intact plants illuminated for 65 s were consistent with low concentrations of permeable ions and similar to 50% storage of pmf as Delta psi. We propo se that the plant cell, possibly at the level of the inner chloroplast enve lope, can control the parsing of pmf into Delta psi and Delta pH by regulat ing the ionic strength and balance of the chloroplast. In addition, this wo rk demonstrates that, under certain conditions, the kinetics of the light-i nduced ECS can be used to estimate the fractions of pmf stored as Delta psi and Delta pH both in vitro and in vivo.