**4. Anthocyanin accumulation during fruit maturation**

Anthocyanin production depends mainly on carbohydrates (especially galactose), which are formed during photosynthesis and glucose metabolism in apple leaves and are later transported to the fruit [18]. Therefore, anthocyanin synthesis is closely associated with plant metabolism and depends on the stage of fruit development. For instance, during fruit growth, anthocyanin concentrations in "Gala" apple peel are low and fluctuating, and they then increase markedly on the blush side near maturation [19]. Similarly, Chalmers et al. [20] found that anthocyanin accumulation in red apple cultivars occurs rapidly during the transition from the immature to mature stages and precedes the normal harvest date by about 2–3 weeks. Honda et al. [21] also reported that anthocyanin production proceeded rapidly in the fruit peel of potted trees of red-skinned cultivar "Misuzu Tsugaru" during the last week before harvest, not only under control conditions (25°C 12 h/15°C 12 h) but also under hotter temperature conditions (29°C 12 h/19°C 12 h) for 5 weeks. A continuous increase in anthocyanin concentration, especially in the last 2 weeks before harvest, on both the blushed and shaded sides of the apple fruit was also found in a superior redness sport of "Shotwell Delicious" called "Topred Delicious" [18]; later, the authors reported that the largest increase in anthocyanin concentration and color development in the eight "Gala" apple strains occurred from 2 weeks before the commercial harvest date, and the increase continued for up to 1 week after the harvest date [17]. These findings show the importance of the ripening period for anthocyanin accumulation in apple fruit peel.
