Relationships between melatonin and salicylic acid treatments in delaying the senescence of postharvest pear fruit

Fruit senescence is an important stage in the post-ripening fruit life process and has a direct impact on the maintenance of postharvest fruit quality. Pears are among the most widely cultivated fruit trees in China and worldwide. However, there are problems such as poor postharvest preservation, high rotting rates, and serious economic losses in the pear industry. Previous studies have shown that melatonin and salicylic acid can delay fruit senescence; however, the relationship between them during this process has not been elucidated. Therefore, to clarify the relationships of melatonin and salicylic acid in regulating pear fruit senescence, this study applied melatonin, salicylic acid and their combined treatments at different concentrations to 'Xin li No.7′ pear fruit and analyzed the physiological, biochemical, and transcriptional effects of melatonin and salicylic acid on pear fruit senescence. The results showed that melatonin, salicylic acid and their combined treatments delayed the percentage of decayed pear fruit during postharvest storage. Melatonin, salicylic acid and their combined delayed the increase in weight loss of pear fruit and decrease in the internal quality (firmness, soluble solid content, and total soluble sugar) of pear fruit. This reduced the peak value of the endogenous ethylene release rate and the activity of cell wall-degrading enzymes (polygalacturonase, pectin methylesterase, and lipoxygenase). It also increased the activity of antioxidant enzymes (superoxide dismutase, peroxidase, and catalase, and ascorbate peroxidase) and the levels of endogenous melatonin and salicylic acid in pears. However, melatonin and salicylic acid combined treatments did not present additive effects. In addition, the transcriptome data of pear peel and pulp treated with different concentrations of melatonin and salicylic acid at different concentrations during senescence storage were analysed using GO and KEGG enrichment to reveal the molecular relationships between them at the transcriptional level. In addition, we found that exogenous melatonin and salicylic acid upregulated the expression of PR1-like genes and downregulated the expression of NPR-like genes. They can also promote the expression of ETR1 and EBF-1 and inhibit the expression of EIN3-like1 and CTR1 in the ethylene signalling pathway in postharvest pear fruit. Our results preliminarily revealed the biochemical and molecular relationships between melatonin and salicylic acid in delaying pear fruit senescence and provided a theoretical basis to further elucidate the relationship between melatonin and salicylic acid in delaying the senescence of postharvest pear fruit.