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

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Abstract
Selective 2-photon excitation (TPE) of carotenoid dark states, Car S 1 , shows that in the major light-harvesting complex of photosystem II (LHCII), the extent of electronic interactions between carotenoid dark states (Car S 1 ) and chlorophyll (Chl) states, φ Coupling Car S 1 −Chl , correlates linearly with chlorophyll fluorescence quenching under different experimental conditions. Simultaneously, a linear correlation between both Chl fluorescence quenching and φ Coupling Car S 1 −Chl with the intensity of red-shifted bands in the Chl Q y and carotenoid absorption was also observed. These results suggest quenching excitonic Car S 1 −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 φ Coupling Car S 1 −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 S 1 –Chl interactions in any adaptation status. Our in vivo measurements with Arabidopsis variants show that during high light illumination, φ Coupling Car S 1 −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 S 1 –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.

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