Abstract:

【Objective】Cold storage is a very effective way to improve berry storage ability and to extend shelf life. However, cold storage decreases fruit quality, including sugars, acids and aroma. The paper was aimed to determine the effects of melatonin treatment on berry quality of Shine Muscat under different storage temperatures and to investigate the melatonin-induced changes of metabolites involved in quality formation. 【Method】The berries were treated via soaking with 5 and 50 μmol∙L ^-1 melatonin and stored at room temperature or 1℃. The berries at different days after storage were collected for quality analysis. Melatonin and aroma were determined by using HPLC-MS and GC-MS, respectively. Capillary electrophoresis was used to detect soluble sugars and organic acids. The wide-target metabolomics was employed to analyze the differentially accumulated metabolites. 【Result】The exogenous melatonin treatment significantly increased melatonin content of the berries, and 50 µmol∙L ^-1melatonin was more effective than 5 µmol∙L ^-1 melatonin. Additionally, the melatonin treatment increased accumulation of melatonin more effectively at low temperature than it did at room temperature, e.g., the melatonin content of the berries treated with 50 µmol∙L ^-1 melatonin at low temperature was 2.6 folds higher than that at room temperature. The water loss rate of berries decreased at low temperature. The melatonin treatment did not significantly affect water loss rate, skin and pulp firmness. At room temperature, 5 µmol∙L ^-1 melatonin significantly increased content of glucose and fructose; however, 50 µmol∙L ^-1 melatonin led to the contrary results. The cold storage produced negative effects on soluble sugars; in contrast, these melatonin treatments increased content of soluble sugars at low temperature and the increments exceeded 19.2% at 40 days after cold storage. The cold storage increased titratable acidity and particularly malic acid compared to storage at room temperature. In contrast, the melatonin significantly decreased titratable acidity and particularly malic acid of the berries with more than 53.5% decrement compared with the non-treated control under cold storage. Notably, the berry aroma was largely reduced by cold storage, and melatonin and particularly 5 μmol∙L ^-1 melatonin largely increased amount of total aroma and aroma components. The berries treated with 5 µmol∙L ^-1 melatonin accumulated 2.12- and 1.6-fold higher aroma than the control berries at 30 and 40 days after cold storage, respectively. Additionally, the melatonin increased the amount of characteristic aromas, including (E)-2-hexenal, linalool, and 2, 4-Di-tert-butylphenol. Based on the wide-target metabolomics analysis, 232 differentially accumulated metabolites in the berries treated with 5 µmol∙L ^-1 melatonin were identified compared with the control berries under cold storage, which were primarily related to biosynthesis of amino acids, aminoacyl-tRNA biosynthesis, arginine biosynthesis, alanine, aspartate and glutamate metabolism, and phenylalanine metabolism. 【Conclusion】Compared with storage at room temperature, the cold storage decreased content of part of the soluble sugars and most of aroma components. The berries treated with melatonin accumulated significantly higher soluble sugars and aroma components and lower organic acids, thereby exhibited improved berry quality, while 5 µmol∙L ^-1 melatonin was more effective. Additionally, the increased aroma amount might be dominantly associated with the changed amino acid metabolism in the berries treated with melatonin under cold storage.

Key words: grape, melatonin, cold storage, berry quality, widely targeted metabolomics