Comparison of ascorbic acid metabolism during the development of two jujube varieties

Jujube (Ziziphus jujuba Mill.) is a fruit crop containing a high level of ascorbic acid (AsA). However, the regulation of AsA accumulation differs significantly in various jujube germplasm resources and tissues, and the factors contributing to this discrepancy remain unknown. To explore the mechanisms underlying AsA metabolism, we investigated the dynamics of AsA content in the fruits, leaves and petals during the development of two jujube varieties, Z. jujuba Mill. cv. 'Zhanshanmizao 1' (ZM1) and Z. jujuba Mill. cv. 'Zhanshanpingguozao' (ZP). The diversity of AsA content in different jujube tissues demonstrated that the fruits had the highest levels of AsA, whereas the petals had the lowest. The pattern of AsA accumulation in the fruits and leaves was similar during the development of both jujube varieties. The activity of GalDH was correlated with the AsA levels in the ZM1 fruits. The activity of GalLDH was positively correlated with the AsA levels in ZP leaves. In contrast, the activities of GalDH and GalLDH were not correlated with AsA content during leaf growth in ZM1. We also analysed the expression patterns of genes involved in AsA metabolism. The results showed that AsA biosynthesis in jujube fruits was mainly related to the L-galactose pathway. Moreover, expression of genes involved in AsA biosynthesis (GME1, GMP1, GGP and GalLDH) was associated with the AsA concentrations in ZM1 and ZP fruits. GME, GMP and GalDH may be the key genes for AsA synthesis in ZM1 and ZP leaves.