XANTHOPHYLL CYCLE-DEPENDENT QUENCHING OF PHOTOSYSTEM-II CHLOROPHYLL-AFLUORESCENCE - FORMATION OF A QUENCHING COMPLEX WITH A SHORT FLUORESCENCE LIFETIME

Excess light triggers protective nonradiative dissipation of excitatio n energy in photosystem II through the formation of a trans-thylakoid pH gradient that in turn stimulates formation of zeaxanthin and anther axanthin. These xanthophylls when combined with protonation of antenna pigment-protein complexes may increase nonradiative dissipation and, thus, quench chlorophyll a fluorescence. Here we measured, in parallel , the chlorophyll a fluorescence lifetime and intensity to understand the mechanism of this process. Increasing the xanthophyll concentratio n in the presence of a pH gradient (quenched conditions) decreases the fractional intensity of a fluorescence lifetime component centered at approximate to 2 ns and increases a component at approximate to 0.4 n s. Uncoupling the pH gradient (unquenched conditions) eliminates the 0 .4-ns component. Changes in the xanthophyll concentration do not signi ficantly affect the fluorescence lifetimes in either the quenched or u nquenched sample conditions. However, there are differences in fluores cence life-times between the quenched and unquenched states that are d ue to pH-related, but nonxanthophyll-related, processes. Quenching of the maximal fluorescence intensity correlates with both the xanthophyl l concentration and the fractional intensity of the 0.4-ns component. The unchanged fluorescence lifetimes and the proportional quenching of the maximal and dark-level fluorescence intensities indicate that the xanthophylls act on antenna, not reaction center processes. Further, the fluorescence quenching is interpreted as the combined effect of th e pH gradient and xanthophyll concentration, resulting in the formatio n of a quenching complex with a short (approximate to 0.4 ns) fluoresc ence lifetime.