Observations on the Germ Aleurone of Barley. Phenol Oxidase and Peroxidase Activity

Strips of tissue containing the germ aleurone layer were removed from dry, harvest-ripe grains of barley (Hordeum vulgare L.) and incubated in buffered solutions of phenolic compounds, with and without the addition of hydrogen peroxide. Peroxidase and o-diphenol oxidase activity were found in the material released into the incubation medium, and in the cytoplasm of the germ aleurone cells. Peroxidase activity was located in the cell walls and appeared to be high in the region where the germ aleurone covering the embryonic axis merges into that which adheres to the scutellum i.e. the region in which a row of germ aleurone cells becomes lignified following germination. Monophenol oxidase activity was not detected in the released enzymes or in the intact tissue. Although hydroquinone was oxidized in the cytoplasm of the germ aleurone tissue, unequivocal evidence of the presence of laccase was not obtained. The oxidation of endogenous phenolic substances by phenol oxidases and peroxidases is discussed in relation to anti-microbial defence mechanisms which appear to operate in the germ aleurone during germination.