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Recently, excitonic carotenoid-chlorophyll interactions have been proposed as a simple but effective model for the down-regulation of photosynthesis in plants. The model was proposed on the basis of quenching-correlated electronic carotenoid-chlorophyll interactions (Car S(1) → Chl) determined by Recently, excitonic carotenoid−chlorophyll interactions have been proposed as a simple but effective model for the down-regulation of photosynthesis in plants. The model was proposed on the basis of quenching-correlated electronic carotenoid−chlorophyll interactions (Car S1 → Chl) determined by Car S1 two-photon excitation and red-shifted absorption bands. However, if excitonic interactions are indeed responsible for this effect, a simultaneous correlation of quenching with increased energy transfer in the opposite direction, Chl Qy → Car S1, should be observed. Here we present a systematic study on the correlation of Car S1 → Chl and Chl → Car S1 energy transfer with the occurrence of red-shifted bands and quenching in isolated LHCII. We found a direct correlation between all four phenomena, supporting our conclusion that excitonic Car S1−Chl interactions provide low-lying states serving as energy traps and dissipative valves for excess excitation energy.
