Effect of Gamma Radiation on the Synthesis of Scopoletin and Scopolin in Grapefruit Peel in Relation to Radiation Damage

Two unknown compounds, featuring blue fluorescence under ultraviolet light on thin-layer chromatography plates, were found to accumulate in the peel of gamma-irradiated grapefruit. The two compounds were isolated and identified as scopoletin (6-methoxy-7-hydroxycoumarin) and scopolin (7-glucoside of scopoletin). Both the content of the two compounds and radiation damage (peel pitting) were positively correlated with radiation doses up to 300 krads. Irradiation with 300 krads also increased the incorporation of <tex-math>$\text{phenylalanine-}{}^{14}{\rm C}$</tex-math> into the scopoletin molecule. Rate of incorporation and content of free and hydrolyzed scopoletin increased with time following the irradiation. It is suggested that phenolic compounds which accumulate in the peel after irradiation contribute to the formation of radiation damage in citrus fruits.