Kinetics of plastoquinol oxidation by the Q-cycle in leaves

•Reduction of P700 and PC was determined from 810 and 950 nm transmittance in leaves.•Electrochromic shift (ECS) slow phase shows that the Q-cycle is operating in leaves.•The slow phase is observable when ATP synthase is inactivated (e.g. in the dark).•PQH2 is oxidized with multi-phase kinetics: τd = 1,τp = 5.6, τs = 16 ms (22°).•τd diffusion, τp primary PQH2 oxidation, τs Q-cycle + secondary PQH2 oxidation.

Electrochromic shift measurements confirmed that the Q-cycle operated in sunflower leaves. The slow temporarily increasing post-pulse phase was recorded, when ATP synthase was inactivated in the dark and plastoquinol (PQH2) oxidation was initiated by a short pulse of far-red light (FRL). During illumination by red light, the Q-cycle-supported proton arrival at the lumen and departure via ATP synthase were simultaneous, precluding extreme build-up of the membrane potential. To investigate the kinetics of the Q-cycle, less than one PQH2 per cytochrome b6f (Cyt b6f) were reduced by illuminating the leaf with strong light pulses or single-turnover Xe flashes. The post-pulse rate of oxidation of these PQH2 molecules was recorded via the rate of reduction of plastocyanin (PC+) and P700+, monitored at 810 and 950 nm. The PSII-reduced PQH2 molecules were oxidized with multi-phase overall kinetics, τd = 1, τp = 5.6 and τs = 16 ms (22 °C). We conclude that τd characterizes PSII processes and diffusion, τp is the bifurcated oxidation of the primary quinol and τs is the Q-cycle—involving reduction of the secondary quinol at the n-site, its transport to the p-site, and bifurcated oxidation there. The extraordinary slow kinetics of the Q-cycle may be related to the still unsolved mechanism of the “photosynthetic control.”