On the regulation of photosynthesis by excitonic interactions between carotenoids and chlorophylls.

Selective 2-photon excitation (TPE) of carotenoid dark states, Car S1, shows that in the major light-harvesting complex of photosystem II (LHCII), the extent of electronic interactions between carotenoid dark states (Car S1) and chlorophyll (Chl) states, φCouplingCar S1−Chl, correlates linearly with chlorophyll fluorescence quenching under different experimental conditions. Simultaneously, a linear correlation between both Chl fluorescence quenching and φCouplingCar S1−Chl with the intensity of red-shifted bands in the Chl Qy and carotenoid absorption was also observed. These results suggest quenching excitonic Car S1−Chl states as origin for the observed effects. Furthermore, real time measurements of the light-dependent down- and up-regulation of the photosynthetic activity and φCouplingCar S1−Chl in wild-type and mutant (npq1, npq2, npq4, lut2 and WT+PsbS) Arabidopsis thaliana plants reveal that also in vivo the quenching parameter NPQ correlates always linearly with the extent of electronic Car S1–Chl interactions in any adaptation status. Our in vivo measurements with Arabidopsis variants show that during high light illumination, φCouplingCar S1−Chl depends on the presence of PsbS and zeaxanthin (Zea) in an almost identical way as NPQ. In summary, these results provide clear evidence for a very close link between electronic Car S1–Chl interactions and the regulation of photosynthesis. These findings support a photophysical mechanism in which short-living, low excitonic carotenoid–chlorophyll states serve as traps and dissipation valves for excess excitation energy