| Abstract: | Abstract— The photosystem II (PSII) reaction center in higher plants is susceptible to photoinhibitory molecular damage of its component pigments and proteins upon prolonged exposure to excess light in air. Higher plants have a limited capacity to avoid such damage through dissipation, as heat, of excess absorbed light energy in the PSII light-harvesting antenna. The most important pho-toprotective heat dissipation mechanism, induced under excess light conditions, includes a concerted effect of the trans-thylakoid pH gradient (ΔpH) and the carotenoid pigment interconversions of the xanthophyll cycle. Co-incidentally, both the photoprotective mechanism and photoinhibitory PSII damage decrease the PSII chlorophyll a (Chi a) fluorescence yield. In this paper we present a comparative fluorescence lifetime analysis of the xanthophyll cycle- and photoinhibition-dependent changes in PSII Chi a fluorescence. We analyze multifrequency phase and modulation data using both multicomponent exponential and bimodal Lorentzian fluorescence lifetime distribution models; further, the lifetime data were obtained in parallel with the steady-state fluorescence intensity. The photoinhi-bition was characterized by a progressive decrease in the center of the main fluorescence lifetime distribution from ～2 ns to ～0.5 ns after 90 min of high light exposure. The damaging effects were consistent with an increased nonra-diative decay path for the charge-separated state of the PSII reaction center. In contrast, the ΔpH and xanthophyll cycle had concerted minor and major effects, respectively, on the PSII fluorescence lifetimes and intensity (Gilmore et ah, 1996, Photosynth. Res., in press). The minor change decreased both the width and lifetime center of the longest lifetime distribution; we suggest that this change is associated with the ΔpH-induced activation step, needed for binding of the deepoxidized xanthophyll cycle pigments. The major change increased the fractional intensity of a short lifetime distribution at the expense of a longer lifetime distribution; we suggest that this change is related to the concentration-dependent binding of the deepoxidized xanthophylls in the PSII inner antenna. Further, both the photoinhibition and xanthophyll cycle mechanisms had different effects on the relationship between the fluorescence lifetimes and intensity. The observed differences between the xanthophyll cycle and photoinhibition mechanisms confirm and extend our current basic model of PSII exciton dynamics, structure and function. |
