INTENSITY AND TIME-DEPENDENCE OF THE CAROTENOID TRIPLET QUENCHING UNDER LIGHT FLASHES OF RECTANGULAR SHAPE IN CHLORELLA

Abstract— As is known the chlorophyll fluorescence of photosynthetizing plants is strongly quenched by carotenoid triplet states if the exciting light intensity is high enough (> 10 kW/m²). This light-induced quenching process was studied by measuring the relative yield of chlorophyll fluoresccncc excited with a pulsed argon laser at 20 C in light adapted algae as function of time (within lo μ s ) and exciting light intensity (<400 kW/m²). The experimental yield against time and yield against intensity curves have been adequately explained by a statistical model of Photosystem 2 (PS 2) units based upon the existence of freely moving excitons according to which the carotenoid triplet quenchers are randomly distributed and are perfect traps for excitons. Accepting the hypothesis that carotenoid triplet quenching occurs only in PS 2 units. it could be concluded that the height of the true zero level of PS 2 fluorescence is somewhat lower than the half value of the fluorescence level of the dark adapted state.