A Vitamin C Fermentation By-Product Enhances Chilling Tolerance in Pak Choi by Modulating Ascorbate Redox Status and Augmenting Antioxidant Defense Capacity

Early-spring chilling disrupts redox balance and limits leafy-vegetable growth. This study aimed to determine whether residue after evaporation (RAE), a vitamin C fermentation by-product, could improve chilling tolerance in pak choi, and whether its effects varied with application route and co-application of KH2PO4. Treatments were control (CK), foliar 0.2% KH2PO4 (KP), foliar or root-zone RAE (Rf and Rr), and their RAE+KP combinations (Rf-KP and Rr-KP). Plants were assessed 24 h after treatment before chilling (T1), after 48 h at 4 °C (T2), and after 72 h of post-chilling growth (T3). At T1, RAE-containing treatments increased ascorbate (AsA) by 13.91–26.16% and the AsA/dehydroascorbate (DHA) ratio by 17.08–32.90%, before detectable growth or oxidative-injury differences. At T2, RAE-containing treatments increased AsA and AsA/DHA by 25.58–49.38% and 38.52–69.13%, enhanced antioxidant defenses, and reduced chilling injury, leaf damage, and hydrogen peroxide accumulation. Root activity increased by 25.40% (Rr) and 39.37% (Rr-KP). At T3, all treatments increased aboveground fresh weight by 22.71–109.05%, and the damaged-leaf rate decreased by 42.40–71.24%. Rf-KP and Rr-KP had the highest comprehensive chilling-tolerance scores at T2 and T3. The RAE × KH2PO4 interaction for this score was significant at T2 but not at T3. Both RAE routes improved chilling tolerance. Early ascorbate redox changes preceded later stress responses, while RAE × KH2PO4 interactions varied among traits and stages. These findings support further evaluation of RAE as a potential agricultural input for improving chilling tolerance in leafy vegetables.