Changes in sugar metabolism caused by exogenous oxalic acid related to chilling tolerance of apricot fruit

• Treatment with oxalic acid (OA) significantly enhanced chilling tolerance of apricot fruit.
• The content of sucrose and sorbitol decresed, whereas content of glucose and fructose increased in OA-treated apricot.
• Higher content of glucose, fructose in OA-treated fruit could be result of breakdown of sucrose and sorbitol.
• High levels of fructose and glucose could account for the OA-enhanced chilling tolerance in the apricot fruit.

Enhancement of chilling tolerance has been observed in fruits treated with oxalic acid (OA). To learn how OA may play a role in modulating chilling injury in apricot, the fruit were treated with OA (5 mM) and stored at 2 ± 1 °C for five weeks. OA treatment significantly inhibited chilling injury, electrolyte leakage and accumulation of hydrogen peroxide and malondialdehyde in apricot fruit. Sorbitol, sucrose, fructose and glucose were the major soluble sugars in the fruit. Contents of glucose and fructose were enhanced, meanwhile, the levels of sucrose and sorbitole were decreased by OA treatment during storage. OA treatment enhanced activities of the enzymes related to increasing glucose and fructose, and suppressed sucrose synthase synthesis activity which could explain the lower content of sucrose. The activities of sucrose synthase cleavage function, acid invertase, NAD+-sorbitol dehydrogenase and sorbitol oxidase were significantly enhanced by OA. Meanwhile, OA-treated apricot fruit showed lower sucrose synthase synthesis activity. The degradation of sorbitol in apricot fruit was accelerated by OA treatment. Our results indicate that higher chilling tolerance of OA treated fruit was associated with higher content of reducing sugars (glucose and fructose).