Phenylalanine Alleviates Postharvest Chilling Injury of Plum Fruit by Modulating Antioxidant System and Enhancing the Accumulation of Phenolic Compounds

Research background. Low temperature storage causes chilling injury in plum (Prunus domestica L.) fruits. Consequently, any treatments with beneficial effects against these symptoms would achieve attention. For this purpose, phenylalanine treatments were applied on 'Stanley' plum fruits. The main purpose of the present study is to investigate the influence of the exogenous application of phenylalanine on fruit quality, chilling tolerance, and antioxidant capacity of 'Stanley' plums during cold storage. Experimental approach. Phenylalanine at different concentrations was applied on 'Stanley' plums. Following phenylalanine application, plums were cold stored. Chilling injury, antioxidant capacity, electrolyte leakage, malondialdehyde, proline and internal contents of anthocyanin, flavonoids, phenols, ascorbic acid and some antioxidant enzymes were assessed. Results and conclusions. Phenylalanine treatment significantly alleviated chilling injury in plum fruits by enhancing antioxidant capacity and increasing the activity of phenylalanine ammonia lyase enzyme (PAL). Phenylalanine-treated fruits had higher mass fractions of ascorbic acid, anthocyanins, flavonoids and phenols, as well as a higher total antioxidantactivity than the control fruits during low temperature storage. Phenylalanine at 7.5 mM was the most effective treatment in enhancing the activity of PAL, the accumulation of phenolic compounds and in reducing the severity of chilling injury. Treatments delayed mass loss and maintained fruit firmness. In addition, the application of 7.5 mM phenylalanine improved the activities of antioxidant enzymes (superoxide dismutase, catalase and ascorbate peroxidase), decreased the accumulation of hydrogen peroxide, and increased the endogenous content of proline. Moreover, phenylalanine maintained membrane integrity, manifested by a reduced electrolyte leakage and malondialdehyde accumulation. Novelty and scientific contribution. In the current study, chilling injury had a positive correlation with the activities of PAL and antioxidant enzymes. However, negative correlations were observed between the chilling injury and ascorbic acid mass fraction, and antioxidant capacity. Considering the results, phenylalanine treatment could be an encouraging approach to alleviate the severity of chilling injury and thus preserve nutritional quality of plums during low temperature storage.