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

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. ɸ Car S 1 -Chl Coupling , correlates linearly with chlorophyll fluorescence quenching under different experimental conditions. Simultaneously, a linear correlation between both ChI fluorescence quenching and ɸ Car S 1 -Chl Coupling with the intensity of red-shifted bands in the Cht 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 downand up-regulation of the photosynthetic activity and ɸ Car S 1 -Chl Coupling 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, ɸ Car S 1 -Chl Coupling 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 carotenoidchlorophyll states serve as traps and dissipation valves for excess excitation energy. [ABSTRACT FROM AUTHOR]

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