Regulation of skin color in apples

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Abstract
The literature of the regulation of color in apple skin is reviewed and compared with current knowledge of the regulation of flower color. Color in apple skin is a blend of various amounts of chlorophyll, carotenoids, and anthocyanins/flavonols. A variety of red colors are produced by cyanidin glycosides copigmented with flavonols and other compounds. The concentration and identification of flavonols, proanthocyanidins, and cyanidin glycosides in apple skin are tabulated. From a knowledge of the pigments present in apples, and the biosynthetic pathway present in flowers, a possible biosynthetic pathway to cyanidin glycosides, proanthocyanidins (e.g., catechin), and flavonols (e.g., quercetin glycoside) can be derived. Anthocyanin biosynthesis is developmentally regulated; flavonols and proanthocyanidins are high in juvenile fruit, decrease in concentration during fruit growth, and then increase during fruit ripening. In some cultivars, anthocyanins also increase during ripening. Light is a key regulatory factor in apple skin reddening. Since nored apples, such as Granny Smith and Golden Delicious, accumulate quercetin glycosides and catechin/epicatechin, the color production in red apples is likely to involve the induction of enzymes between leucocyanidin and cyanidin glycosides. Potential research areas in molecular regulation of apple color are outlined.