Two wavelength-dependent mechanisms of sensitisation of light-induced quenching in the isolated light-harvesting complex II.

The efficiency of visible light in inducing fluorescence quenching in the isolated light-harvesting complex II (LHCII) of higher plants is investigated by action spectroscopy in the visible portion of photosynthetic active radiation. The efficiency spectrum displays a relatively homogenous quenching yield across the most intense electronic transitions of the chlorophyll a and carotenoid pigments, indicating that quenching proceeds from the equilibrated state of the complex. Larger yields are observed in the 510-640-nm window, where weak transitions of LHCII-bound chromophores occur. This observation is interpreted in terms of an additional quenching sensitisation process mediated by these electronic transitions.