Peel colour in apple (Malus × domestica Borkh.): An economic quality parameter in fruit market

Apple is one of the most important and popular fruits in the world. Among the various fruit quality traits, skin color is the preferred one which determines a cultivar's market acceptance. Fruit color in apple is dogged primarily by the ground color of the skin and secondarily by the imposed anthocyanin pigmentation, if present. The prime anthocyanin pigment in apple is Cyanidin 3- galactoside commonly named idaein. However, Cyanidin 3- pentoside was also identified in two species. Cyanidin arabinosylgalactoside has also been mentioned from the flesh of Komsomeletes apples but has not been reported elsewhere and is either specific to this cultivar having arisen from a mutation or is a mis-identification. Various factors like light, temperature, mineral nutrition, growth regulators, and carbohydrate availability etc. effect anthocyanin accumulation. The intensity of light and low temperature is crucial for the accumulation of anthocyanins in the skin of apple fruit. The light intensity especially UV-B, could activate anthocyanin synthesis pathway through photoreceptor and finally promote the accumulation of anthocyanins. In apple, low temperature could promote anthocyanin synthesis and high temperature could inhibit accumulation of the same. MdMYB1, the light responsive regulatory factor was found to control the transcription of apple flavonoid genes. Transcription of MdMYBA, MdDFR, MdCHS, MdLDOX and MdUFGluT in apple peel has also been found to be regulated by light particularly UV-B radiation. Optimum temperature for anthocyanin accumulation was found to be 25 °C. Application of Urea increases the Chlorophyll and Carotenoid concentration in fruit skin and reduces anthocyanins concentration in the blush side of fruit at maturity. Ethylene is a key factor in the regulation of anthocyanins biosynthesis and color development in apples. It has been found that the accumulation of anthocyanins is clearly stimulated by ethephon (an ethylene releasing agent) and delayed by ABG-3168 (a likely inhibitor of ethylene production). Anthocyanin reddening in apple fruit skin has been a topic of genetic studies since the 1930's. Several studies have indicated a monogenic approach of inheritance and a single dominant gene Rf was proposed to control anthocyanin reddening on apple fruit skin. Recent molecular genetic studies strongly support the hypothesis of a major gene inheritance as has been reported in the earlier literature. However, there has been much controversy among researchers and many of the studies have been questionable. It therefore requires a thorough analysis and understanding of the development of over color in apple so as to increase the apple trade in national and international market. In this view, an updated summary of published reports on the peel color in apple has been attempted but the experimental groundwork still needs to be done to overcome the misconception.