External control of anthocyanin formation in apple

Abstract Based on an extensive review of world literature, it is concluded that external factors may be involved in anthocyanin formation in apple either by providing the substrate for metabolic processes that are under endogenous control, or by interfering with endogenous control itself. The capability of anthocyanin formation in apple basically appears to consist of a light-dependent, phytochrome-mediated process that probably requires photosynthetic activity for its expression after the initial peak of anthocyanin formation concurring with the phase of cell division. In immature fruit, anthocyanin formation appears to be repressed, probably by endogenous gibberellins (GA). The practice of fruit bagging seems to promote anthocyanin formation by temporarily raising the level of phytochrome, whereas most external factors and cultural practices apparently modify the responsiveness (sensitivity) to light by either reducing or increasing, independently of each other, the temporary repression of anthocyanin formation. Anthocyanin formation may be promoted either directly by reducing GA activity, or indirectly by increasing the level of ethylene and/or ABA, which are GA antagonists.