Further Studies of Proton Translocations in Chloroplasts After Single-Turnover Flashes. I. Proton Uptake

Abstract

Damped, binary oscillations were observed in proton uptake by class C pea chloroplasts given a train of light flashes. The oscillation at pH 7.8 is predictable if the species accepting protons is either the doubly reduced secondary acceptor B�- or a plastoquinone PQ- from the pool, if 0.29 of the secondary acceptor is B- in dark-adapted chloroplasts and if a miss factor α = 0.12 governs the amval of electrons at B or B- after a flash. The rate of proton uptake was measured with varied pH, ionic strength and temperature. The half-time was 95 ms at pH 7.8 and 21°C. Using double flashes separated by variable intervals showed that the species able to accept protons was generated (t½) about 0.8 ms after a flash. The results are consistent with protons from the external solution reacting relatively slowly with univalent anions, which have earlier promptly supplied protons to B2- or PQ2-. Under conditions of cyclic and of non-cyclic electron flow, H+/e- stoichiometries were 1.1 and 1.0 respectively, and so the results do not support a Q-cycle model for pea chloroplasts.