Light‐Induced Proton Transport by Chloroplasts Suspended in Fluid Media at Sub‐Zero Temperatures: Kinetics and Stoichiometry

Abstract

Chloroplasts [Spinacia oleracea] suspended in a medium containing ethanediol and water (1:1, vol/vol) at -16.degree. 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 centers at -16.degree. C. Under the same conditions 3H+ 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 [half time] = 3 s. The flash-induced proton release into the thylakoid interior was biphasic, with t1/2 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 t1/2 of reduction of cytochrome f in the dark following illumination in the presence of 2 mM NH4Cl (2.5 s) is similar to the t1/2 of the slow phase of proton release, suggesting a correlation between the kinetics of cytochrome f reduction and plastoquinol oxidation.