Developmental dynamics of ascorbic acid metabolism in wild jujube fruit: An integrated physiological and transcriptomic analysis

Wild jujube (Ziziphus jujuba var. spinosa) fruit is rich in ascorbic acid (AsA), but developmental coordination among AsA accumulation, enzyme activities and candidate-gene expression remains unclear. We compared wild jujube with Ziziphus jujuba Mill. cv. 'Lingwuchangzao' grown in the same orchard across five developmental stages. We measured fruit traits, sugars, acids, AsA, total ascorbic acid (T-AsA), dehydroascorbic acid (DHA) and five AsA-related enzyme activities, and analyzed repeated sampling with linear mixed-effects models. Differential expression, weighted gene co-expression network analysis (WGCNA), predicted protein-interaction networks and RT-qPCR were applied to 15 wild-jujube RNA-seq samples; ZjVTC2 was transiently overexpressed in Nicotiana benthamiana. At the young-fruit stage, wild jujube contained 778.75 ± 19.14mg·100g⁻¹ FW AsA and 861.77 ± 17.44mg·100g⁻¹ FW T-AsA. At maturity, AsA content remained higher in wild jujube than in 'Lingwuchangzao' (612.34 ± 13.53 versus 321.22 ± 4.31mg·100g⁻¹ FW). Significant material × developmental-stage interactions occurred for AsA, T-AsA and DHA (P < 0.01). Material differences in DHAR, MDHAR, GalLDH and APX varied by stage, whereas the MIOX interaction was not significant. Transcriptomic and network analyses identified VTC2, GLDH, HXK2, PGMP, and the bHLH gene LOC107430586 as candidates. Transient ZjVTC2 overexpression increased leaf AsA by 72.65% (P < 0.01). Thus, AsA accumulation differed between materials in a stage-dependent manner, and ZjVTC2 was functionally supported, whereas the other candidate genes require validation.