The xanthophyll cycle modulates the kinetics of nonphotochemical energy dissipation in isolated light-harvesting complexes, intact chloroplasts, and leaves of spinach

We analyzed the kinetics of nonphotochemical quenching of chlorophyll fluor escence (qN) in spinach (Spinacia oleracea) leaves, chloroplasts, and purif ied light-harvesting complexes. The characteristic biphasic pattern of fluo rescence quenching in dark-adapted leaves, which was removed by preillumina tion, was evidence of light activation of qN, a process correlated with the de-epoxidation state of the xanthophyll cycle carotenoids. Chloroplasts is olated from dark-adapted and light-activated leaves confirmed the nature of light activation: faster and greater quenching at a subsaturating. transth ylakoid pH gradient. The light-harvesting chlorophyll a/b-binding complexes of photosystem II were isolated from dark-adapted and light-activated leav es. When isolated from light-activated leaves, these complexes showed an in crease in the rate of quenching in vitro compared with samples prepared fro m dark-adapted leaves. In all cases, the quenching kinetics were fitted to a single component hyperbolic function. For leaves, chloroplasts, and light -harvesting complexes, the presence of zeaxanthin was associated with an in creased rate constant for the induction of quenching. We discuss the signif icance of these observations in terms of the mechanism and control of qN.