XANTHOPHYLL CYCLE-DEPENDENT QUENCHING OF PHOTOSYSTEM-II CHLOROPHYLL-AFLUORESCENCE - FORMATION OF A QUENCHING COMPLEX WITH A SHORT FLUORESCENCE LIFETIME
Am. Gilmore et Hazlett Tl",govindjee, XANTHOPHYLL CYCLE-DEPENDENT QUENCHING OF PHOTOSYSTEM-II CHLOROPHYLL-AFLUORESCENCE - FORMATION OF A QUENCHING COMPLEX WITH A SHORT FLUORESCENCE LIFETIME, Proceedings of the National Academy of Sciences of the United Statesof America, 92(6), 1995, pp. 2273-2277
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.