Chloroplast-located flavonoids can scavenge singlet oxygen

The hypothesis was tested that flavonoids may scavenge singlet oxygen (O-1(2)) in mesophyll cells of Phillyrea latifolia exposed to excess-light stress. In cross-sections taken from leaves developed at 10% (shade) or 100% (sun) solar irradiance, we evaluated the excess photosynthetically active radiation (PAR)-induced accumulation of O-1(2) in mesophyll cells by imaging the fluorescence quenching of the specific O-1(2) probe N-[2-(diethylamino)ethyl]-N-[(2,5-dihydro-2,2,5,5-tetramethyl-1H-pyrrol-3-yl)methyl]-5-(dimethylamino)-1-naphthalenesulfonamide (DanePy). The intracellular location of flavonoids was also analyzed using three-dimensional deconvolution microscopy. Photo-induced quenching of DanePy fluorescence was markedly greater in the mesophyll of shade leaves than in that of sun leaves, the former showing a negligible accumulation of mesophyll flavonoids. The photo-induced generation of O-1(2) was inversely related to the content of flavonoids in the mesophyll cells of sun leaves. Flavonoids were located in the chloroplasts, and were likely associated with the chloroplast envelope. Here we provide relevant evidence for the potential scavenger activity of chloroplast-located flavonoids against O-1(2) and new insights into the photo-protective role of flavonoids in higher plants.