The xanthophyll cycle-dependent dissipation of excitation energy in higher plants is one of the most important regulatory and photoprotective mechanisms in photosynthesis. Using parallel timeresolved and pulse-amplitude modulation fluorometry, we studied the influence of the intrathylakoid pH and the xanthophyll cycle carotenoids on the PSII chlorophyll (Chl) a fluorescence yield in thylakoids of Arabidopsis, spinach, and barley. Increases in concentrations of dithiothreitol in thylakoids, which have a trans-thylakoid membrane pH gradient and are known to have decreased conversion of violaxanthin (V) to zeaxanthin (Z), lead to (1) decreases in the fractional intensity of the ∼0.5 ns Chl a fluorescence lifetime (τ) distribution component and simultaneous increases in a 1.6-1.8 ns fluorescence component and (2) increases in the maximal fluorescence intensity. These effects disappear when the pH gradient is eliminated by the addition of nigericin. To quantitatively explain these results, we present a new mathematical model that describes the simultaneous effects of the chloroplast trans-thylakoid membrane pH gradient and xanthophyll cycle pigments on the PSII Chl a fluorescence τ distributions and intensity. The model assumes that (1) there exists a specific binding site for Z (or antheraxanthin, A) among or in an inner antenna complex (primarily CP29), (2) this binding site is activated by a low intrathylakoid pH (pK ≈4.5) that increases the affinity for Z (or A), (3) about one Z or A molecule binds to the activated site, and (4) this binding effectively "switches" the fluorescence τ distribution of the PSII unit to a state with a decreased fluorescence τ and emission intensity (a "dimmer switch" concept). This binding is suggested to cause the formation of an exciton trap with a rapid intrinsic rate constant of heat dissipation. Statistical analysis of the data yields an equilibrium association constant, K a , that ranges from 0.7 to 3.4 per PSII for the protonated/activated binding site for Z (or A). The model explains (1) the relative fraction of the ∼0.5 ns fluorescence component as a function of both Z and A concentration and intrathylakoid pH, (2) Interest in the molecular mechanisms used by higher plants to adapt and acclimate to light levels in excess of that used in photosynthesis has recently increased. One possible photoprotective mechanism involves xanthophyll cycledependent thermal dissipation of excess absorbed light energy in the light-harvesting complexes of photosystem II (PSII) (1-8). Light harvesting in the PSII antenna and the xanthophyll cycle-dependent heat dissipation mechanism are related to the structure and organization of the PSII pigmentproteins. The PSII holochrome is composed of (a) the PSII core that includes chlorophyll proteins CP43 and CP47 and reaction center proteins D1, D2, and cytochrome b 559 , (b) the minor inner antenna, labeled as CP24, CP26, and CP29, and (c) the major peripheral antenna complex that includes trimeric assemblies of the light-harvesting complex ...

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