Dynamics of ascorbic acid content in apple (Malus x domestica) during fruit development and storage

Vitamin C is a crucial antioxidant and cofactor for both plants and humans. Apple fruits generally contain low levels of vitamin C, making vitamin C content an interesting trait for apple crop improvement. With the aim of breeding high vitamin C apple cultivars it is important to get an insight in the natural biodiversity of vitamin C content in apple fruits. In this study, quantification of ascorbic acid (AsA), dehydroascorbic acid (DHA), and total AsA (AsA + DHA) in apple pulp of 79 apple accessions at harvest revealed significant variation, indicating a large genetic biodiversity. High density genotyping using an 8 K SNP array identified 21 elite and 58 local cultivars in this germplasm, with local accessions showing similar levels of total AsA but higher amounts of DHA compared to elite varieties. Out of the 79 apple cultivars screened, ten genotypes with either the highest or the lowest concentration of total AsA at harvest were used for monitoring vitamin C dynamics during fruit development and storage. For all these cultivars, the AsA/DHA ratio in both apple pulp and peel increased throughout fruit development, whereas the AsA/DHA balance always shifted towards the oxidized form during storage and shelf life, putatively reflecting an abiotic stress response. Importantly, at any point during apple fruit development and storage, the apple peel contained a higher level of vitamin C compared to the pulp, most likely because of its direct exposure to abiotic and biotic stresses.