Oxidation of Flavonols by Hydrogen Peroxide in Epidermal and Guard Cells of <italic>Vicia faba</italic> L.

Abstract

Epidermal strips of Vicia faba were found to contain kaempferol and quercetin glycosides. These flavonols were oxidized by H2O2 and oxidation was inhibited by KCN (3.5 nM). Quercetin glycosides were more sensitive to H2O2 than kaempferol glycosides. Oxidation could be detected in epidermal strips even at 30 μM H2O2. Flavonol oxidation by H2O2 was observed in both guard and epidermal cells. In guard cells, oxidation appeared as the bleaching of absorption bands of flavonols. Epidermal cells could be roughly divided into two types on the basis of their absorption characteristics in the UV-light region. In one type, only flavonol oxidation was observed; in the other, both flavonol and 3,4-dihydroxyphenylalanine (DOPA) oxidation were observed. Oxidation of flavonols and DOPA by H2O2 was also observed in cell-free extracts of the epidermal strips, even at 10μ H2O2. Oxidation was inhibited by 1 mM KCN, suggesting the participation of peroxidase in the reactions. The data obtained in this study indicate the cellular specific distribution of phenolic compounds in the epidermis and the possibility of their oxidation by H2O2 generated in epidermal and guard cells.