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
Am. Gilmore et al., 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, Photosynthesis research, 48(1-2), 1996, pp. 171-187
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.