Turning the Shaded Side of Apple Fruit to Sunlight Exposure Increases Favonoids and Total Phenolics in Both the Original Shaded Peel and the Original Sun-exposed Peel of 'Fortune' and 'Mutsu'

The shaded peel of ‘Fortune’ (a red cultivar) and ‘Mutsu’ (a yellow/green cultivar) apple was exposed to full sun by turning fruit 180º at about one week before harvest to determine the expression of key genes involved in anthocyanin synthesis in response to sunlight exposure and their relationships with the levels of anthocyanins and other phenolics. For the unturned (control) fruit, the shaded peel had lower expression levels of MdMYB10 (a transcriptional factor) and seven structural genes in anthocyanin biosynthesis (MdPAL, MdCHS, MdCHI, MdF3H, MdDFR1, MdLDOX, and MdUFGT), and lower levels of anthocyanins and favonols than the sun-exposed peel in both cultivars. Exposure of the shaded peel to full sun caused marked up-regulation of the expression of MYB10 and all seven structural genes, which peaked between 6 h and 30 h after fruit turning, consequently leading to higher levels of anthocyanins, flavonols, and total phenolics than in the shaded peel and even in the sun-exposed peel of control fruit. Interestingly, the levels of flavonols were higher in the shaded peel of turned fruit (the original sun-exposed peel) than in the sun-exposed peel of both control and turned fruit in both cultivars. These results indicate that sunlight exposure promotes the synthesis of anthocyanins and other phenolic compounds by up-regulating the expression of MYB10 and the structural genes involved in anthocyanin biosynthesis. Turning shaded fruit peel to sun exposure increases not only the levels of anthocyanins, flavonols and total phenolics in the original shaded peel, but also the levels of flavonols and total phenolics in the original sun-exposed peel of both red and yellow/green apple cultivars.