Linolenic Acid Treatment Promoted Wound Healing in Apple Fruit and the Underlying Mechanisms

To study the effect of linolenic acid (LA) on wound healing in postharvest apple fruit and to explore the underlying biochemical mechanisms, artificially wounded apple fruit (cv. Fuji) were treated with 1 mmol/L LA or sterile water containing a small amount of ethanol as a control. The wound healing effect of LA was evaluated by measuring disease index and mass loss rate of apple. The key enzyme activities related to phenylpropane metabolism and its metabolite contents, H₂O₂ content and peroxidase (POD) activity at the wounds were measured. Compared to the control group, LA treatment decreased significantly the mass loss of wounded fruit and the disease index of inoculated fruit during wound healing. LA treatment increased the activities of four key enzymes involved in phenylpropane metabolism (phenylalanine ammonia lyase (PAL), cinnamic acid-4-hydroxylase (C4H), 4-coumaryl coenzyme A ligase (4CL) and cinnamyl alcohol dehydrogenase (CAD)), and elevated the levels of four phenolic acids (erucic acid, ferulic acid, cinnamic acid and caffeic acid) and three lignin alcohol monomers (coniferyl alcohol, cinnamyl alcohol and sinapyl alcohol), as well as total phenols, flavonoids, and lignin at the wounds. LA treatment also enhanced H₂O₂ content and POD activity. In summary, LA could promote wound healing of apple fruit by activating phenylpropanoid metabolism, and increasing H₂O₂ content and POD activity.