PHOTOSYSTEM-II CHLOROPHYLL-A FLUORESCENCE LIFETIMES AND INTENSITY AREINDEPENDENT OF THE ANTENNA SIZE DIFFERENCES BETWEEN BARLEY WILD-TYPE AND CHLORINA MUTANTS - PHOTOCHEMICAL QUENCHING AND XANTHOPHYLL CYCLE-DEPENDENT NONPHOTOCHEMICAL QUENCHING OF FLUORESCENCE

Photosystem II (PS II) chlorophyll (Chi) a fluorescence lifetimes were measured in thylakoids and leaves of barley wild-type and chlorina f1 04 and f2 mutants to determine the effects of the PS II Chi a+b antenn a size on the deexcitation of absorbed light energy. These barley chlo rina mutants have drastically reduced levels of PS II light-harvesting Chls and pigment-proteins when compared to wild-type plants. However, the mutant and wild-type PS II Chi a fluorescence lifetimes and inten sity parameters were remarkably similar and thus independent of the PS II light-harvesting antenna size for both maximal (at minimum Chl flu orescence level, F-o) and minimal rates of PS II photochemistry (at ma ximum Chi fluorescence level, F-m). Further, the fluorescence lifetime s and intensity parameters, as affected by the trans-thylakoid membran e pH gradient (Delta pH) and the carotenoid pigments of the xanthophyl l cycle, were also similar and independent of the antenna size differe nces. In the presence of a Delta pH, the xanthophyll cycle-dependent p rocesses increased the fractional intensity of a Chi a fluorescence li fetime distribution centered around 0.4-0.5 ns, at the expense of a 1. 6 ns lifetime distribution (see Gilmore et al. (1995) Proc Natl Acad S ci USA 92:2273-2277). When the zeaxanthin and antheraxanthin concentra tions were measured relative to the number of PS II reaction center un its, the ratios of fluorescence quenching to [xanthophyll] were simila r between the wild-type and chlorina f104. However, the chlorina f104, compared to the wild-type, required around 2.5 times higher concentra tions of these xanthophylls relative to Chi a+b to obtain the same lev els of xanthophyll cycle-dependent fluorescence quenching. We thus sug gest that, at a constant Delta pH, the fraction of the short lifetime distribution is determined by the concentration and thus binding frequ ency of the xanthophylls in the PS II inner antenna. The Delta pH also affected both the widths and centers of the lifetime distributions in dependent of the xanthophyll cycle. We suggest that the combined effec ts of the xanthophyll cycle and Delta pH cause major conformational ch anges in the pigment-protein complexes of the PS II inner or core ante nnae that switch a normal PS II unit to an increased rate constant of heat dissipation. We discuss a model of the PS II photochemical appara tus where PS II photochemistry and xanthophyll cycle-dependent energy dissipation are independent of the Peripheral antenna size.