Role of ascorbic acid in enhancing hypoxia tolerance in roots of sensitive and tolerant apple rootstocks

• Hypoxia stress brought about oxidative damage and decrease in root AsA content.
• AsA partially alleviated hypoxia-induced growth inhibition and oxidative stress.
• AsA-triggered the up-regulation of antioxidant enzymes related the AsA–GSH cycle.
• Additional AsA enhanced hypoxia tolerance of hypoxia-sensitive apple rootstocks.

Ascorbic acid (AsA) has an important role in plant growth and development, protecting cells against oxidative challenges when exposed to environmental stresses. We investigated the mechanism by which exogenous AsA enhances hypoxia tolerance and alleviates oxidative damage in sensitive and tolerant apple rootstocks (Malus sieversii and M. hupehensis). Although such stress inhibited seedling growth in both species, the extent of this effect differed greatly between the two genotypes. Under stress, dry weights were significantly lower from the hypoxia-sensitive M. sieversii than from the controls, but they increased significantly with the addition of AsA. The contents of hydrogen peroxide (H2O2) and superoxide radicals (O2−) were significantly increased in roots of both species exposed to hypoxia. This led to oxidative damage, which was indicated by the accumulation of malondialdehyde (MDA). However, that reaction in M. sieversii was significantly suppressed when AsA was supplemented. Exogenously applied AsA enhanced the activities of ascorbate peroxidase (APX), dehydroascorbate reductase (DHAR), monodehydroascorbate reductase (MDHAR), glutathione reductase (GR), superoxide dismutase (SOD), peroxidase (POD) and catalase (CAT) in both species, albeit to a greater extent in M. sieversii than in M. hupehensis under hypoxia stress. Exogenously applied AsA raised its endogenous levels of both Malus species exposed to hypoxia, but to a greater extent in M. sieversii. AsA applications also slightly enhanced root glutathione (GSH) contents in stressed plants, but differences were not statistically significant between the treated and untreated plants. AsA possibly enhance hypoxia tolerance by elevating its endogenous level and improving the capacity of the ascorbate–glutathione cycle, thereby decreasing the occurrence of oxidative damage in the membranes.