Onion bulbs contain quercetin, quercetin 4'-glucoside (Q4'G) and quercetin 3,4'-diglucoside (Q3,4'G), almost all of which are localized in the abaxial epidermis of scales. There is a question of whether these compounds are synthesized in the abaxial epidermis or transported to the epidermis. To elucidate this, activities of enzymes that participate in the formation and degradation of quercetin glucosides were measured in the abaxial epidermis, adaxial epidermis and mesophyll. Activities of glucosyltransferases, which transform quercetin to Q4'G and Q4'G to Q3,4'G, were highest in the abaxial epidermis. Activity of phenylalanine ammonia-lyase was also higher in the abaxial epidermis than in other tissues. These results suggest that the flavonol glucosides are synthesized in the abaxial epidermis. Activities of glucosidases, which transform Q3,4'G to Q4'G and Q4'G to quercetin, were highest in the abaxial epidermis, suggesting that quercetin, which is present in the abaxial epidermis, is formed by deglucosidation of quercetin glucosides in the epidermal cells.
