Excitation transfer and quenching in photosystem II, enlightened by carotenoid triplet state in leaves

Accumulation of carotenoid (Car) triplet states was investigated by singlet–triplet annihilation, measured as chlorophyll (Chl) fluorescence quenching in sunflower and lettuce leaves. The leaves were illuminated by Xe flashes of 4 μs length at half-height and 525–565 or 410–490 nm spectral band, maximum intensity 2 mol quanta m⁻² s⁻¹, flash photon dose up to 10 μmol m⁻² or 4–10 PSII excitations. Superimposed upon the non-photochemically unquenched F_md state, fluorescence was strongly quenched near the flash maximum (minimum yield F_e), but returned to the F_md level after 30–50 μs. The fraction of PSII containing a ³Car in equilibrium with singlet excitation was calculated as T_e = (F_md—F_e)/F_md. Light dependence of T_e was a rectangular hyperbola, whose initial slope and plateau were determined by the quantum yields of triplet formation and annihilation and by the triplet lifetime. The intrinsic lifetime was 9 μs, but it was strongly shortened by the presence of O₂. The triplet yield was 0.66 without nonphotochemical quenching (NPQ) but approached zero when NP-Quenched fluorescence approached 0.2 F_md. The results show that in the F_md state a light-adapted charge-separated PSII_L state is formed (Sipka et al., The Plant Cell 33:1286–1302, 2021) in which Pheo⁻P680⁺ radical pair formation is hindered, and excitation is terminated in the antenna by ³Car formation. The results confirm that there is no excitonic connectivity between PSII units. In the PSII_L state each PSII is individually turned into the NPQ state, where excess excitation is quenched in the antenna without ³Car formation.