The size of the lumenal proton pool in leaves during induction and steady-state photosynthesis

This report describes a new method to measure the chloroplastic lumenal proton pool in leaves (tobacco and sunflower). The method is based on measurement of CO₂ outbursts from leaves caused by the shift in the CO₂ + H₂O ↔ HCO₃ ⁻ + H⁺ equilibrium in the chloroplast stroma as protons return from the lumen after darkening. Protons did not accumulate in the lumen to a significant extent when photosynthesis was light-limited, but a large pool of >100 μmol H⁺ m⁻² accumulated in the lumen as photosynthesis became light-saturated. During thylakoid energization in the light, large amounts of protons are moved from binding sites in the stroma to binding sites in the lumen. The transthylakoidal difference in the chemical potential of free protons (ΔpH) is largely based on the difference in the chemical potential of bound protons in the lumenal and stromal compartments (pK). Over the course of the dark-light induction of photosynthesis protons accumulate in the lumen during reduction of 3-phosphoglycerate. The accumulation of electrons in reduced compounds of the stroma and cytosol is the natural cause for accumulation of a stoichiometric pool of lumenal protons during this transient event.