LIGHT-INDUCED PROTON TRANSPORT BY CHLOROPLASTS SUSPENDED IN FLUID MEDIA AT SUB-ZERO TEMPERATURES - KINETICS AND STOICHIOMETRY

Chloroplasts suspended in a medium containing ethanediol and water (1:1, v/v) at –16°C show light‐induced proton uptake and subsequent dark efflux. Proton uptake in continuous light showed biphasic kinetics. A 1 ms flash caused a single turnover of the photochemical centres at –16 °C. Under the same conditions 3 H+ were taken up from the external medium in the presence of methyl viologen as electron acceptor. The flash‐induced proton uptake was exponential and monophasic with t1/2= 3 s. The flash‐induced proton release into the thylakoid interior was biphasic, with half‐times of less than 0.1 s and 3 s. The fast phase represented approximately 30% of the total release and may be correlated with the oxidation of water. The half‐time of reduction of cytochrome f in the dark following illumination in the presence of 2 mM NH4Cl (2.5 s) is similar to the half‐time of the slow phase of proton release, suggesting a correlation between the kinetics of cytochrome f reduction and plastoquinol oxidation. Copyright © 1979, Wiley Blackwell. All rights reserved