Quercetin is one of the most common flavonoids in plants, and has been already known to have several physiological functions in plants such as a protecting against sunlight damage and a resistance to microbial pathogens, and a naturally occurring antioxidant in food and living cells. However, there are few reports about an antioxidative function in living cells of plants. In this study, we have investigated the constituents of onion containing an amount of quercetin and its derivatives. The five components of onion were isolated by chromatographic methods and their structures were elucidated by IR, UV, NMR and MS spectra. Compound 2 was 2,5,7,3',4'-pentahydroxyflavan-3,4-dione which has been already reported as one of the oxidaiton products of quercetin. Compound 4 was quercetin and 3 was its glucoside derivative. Compound 6a and 6b were each structural isomers of bisflavonols which were the oxidation dimers of quercetin. One of the isomers, 6a was produced from quercetin by the lipid peroxidation and the radical-scavenging reaction. It can be presumed that quercetin in the outer scales of the bulbs of onion act to protect plants against the oxidative damage by inhibiting or quenching free radicals and reactive oxygen species.
