Effect of exogenous melatonin treatment on the storage quality and flavor of Akebia trifoliata fruit

【Objective】Akebia trifoliata (Thunb.) Koidz is an underutilized fruit crop attracting growing interest due to its considerable nutritional and potential economic value. However, its commercial development is severely hampered by rapid postharvest deterioration, primarily caused by pronounced climacteric respiration and the characteristic ventral suture dehiscence, coupled with its usual maturity in hot and rainy seasons. The convergence of these biotic and abiotic factors typically limits its marketable shelf-life to a mere 3-5 days under ambient conditions. Although existing preservation technologies can mitigate this issue to some extent, their practical application is often constrained by safety concerns and cost- effectiveness considerations. Given that melatonin (Mt), a potent natural signaling molecule, has been proven effective in regulating postharvest physiological processes in various fruits, and considering that its application in A. trifoliata remains unexplored, this study aimed to establish a scientifically grounded postharvest protocol. We systematically evaluated the impact of exogenous Mt on critical quality parameters, including physiological stability, nutritional retention, and volatile flavor compound profiles throughout storage.【Methods】A sufficient quantity of A. trifoliata fruits were harvested from the Chongzhou Germplasm Nursery at Sichuan Agricultural University at a physiological maturity stage, indicated by a ventral suture color transition from gray to white. Fruits exhibiting uniform size, freedom from mechanical injury or pathological symptoms, and consistent maturity were selected. Following immediate transport to the laboratory, the fruits were randomized into two groups: an untreated control group (CK) and a treatment group subjected to immersion in a 0.5 mmol·L-1 aqueous Mt solution for 30 min. Subsequently, all fruits underwent air- drying under ambient laboratory conditions to eliminate surface moisture. Both groups were then stored under identical, rigorously maintained environmental conditions: (25±1) ℃ and 90%-95% relative humidity. Destructive sampling for comprehensive analysis was performed at 0 (day of treatment), 3, 7, 10, and 15 days of storage. Physicochemical parameters were measured as follows: weight loss was determined gravimetrically by measuring the reduction in fruit mass at each sampling time point relative to the initial mass. pH and total soluble solids (TSS) were measured using a pH meter and sugar meter, respectively. Reducing sugars were quantified by Fehling’s reagent titration with the blue color disappearance as an endpoint. Total protein was assayed using a kit: fruit tissue was homogenized in 0.9% NaCl, centrifuged at 2500 r·min- 1 for 10 min, supernatant absorbance was read at 595 nm after Coomassie Brilliant Blue staining. Free amino acids were separated by high- performance liquid chromatography (HPLC) using a Hitachi L- 8900 system equipped with a Hitachi 855-4507 column and detected at 570/440 nm. Vitamin C content was determined by 2,6-dichlorophenolindophenol titration: 5 g tissue was extracted in 2% HCl, filtered, diluted to 20 mL, and titrated to faint pink endpoint that persisted for at least 30 seconds. Total flavonoids were quantified at 510 nm using rutin as a standard. Volatile flavor compounds were analyzed using HS-GCIMS, and compounds were identified by comparing their retention indices and drift times to standards in the NIST/IMS database. All statistical analysis were performed using SPSS 25.0 software. Data were subjected to one-way analysis of variance (ANOVA).【Results】Exogenous Mt treatment significantly enhanced the postharvest quality of A. trifoliata fruits by modulating key physiological, nutritional, and sensory attributes. Physiologically, Mt application significantly attenuated the rate of water loss, resulting in a 33% lower cumulative weight loss compared to the CK group by the end of storage (day 15). Furthermore, Mt-treated fruits exhibited a significantly attenuated decline in both pH and TSS content than the control group at the end of the storage period. Regarding nutritional quality, the Mt treatment enhanced the retention of total flavonoid, which were 109.68% higher than CK on day 15, and maintained elevated vitamin C levels throughout the storage period. Analysis of free amino acid profiles revealed that exogenous Mt regulated the accumulation of aromatic amino acids. The combined concentrations of phenylalanine (Phe) and tyrosine (Tyr) accounted for 13.73% of total amino acids in the Mt group by day 15, representing a 14.32% increase over the CK group at the same time point. Volatile organic compound analysis identified 71 compounds, where the Mt treatment inhibited the generation of harmful volatile substances during the later stages of storage and delays flavor deterioration, in contrast to the CK group that showed significant divergence in volatile composition by day 15.【Conclusion】 Our results demonstrate that postharvest application of 0.5 mmol·L-1 exogenous Mt effectively inhibited the increase in weight loss rate of A. trifoliata fruits during storage, maintained the stability of pH and TSS, and enhanced nutritional quality by promoting the accumulation of aromatic amino acids, vitamin C, and total flavonoids. Additionally, Mt treatment inhibits the generation of harmful volatile substances in the later stages of storage and delays the process of flavor deterioration. This systematic investigation conclusively demonstrates that postharvest application of 0.5 mmol· L- 1 exogenous melatonin constitutes a highly effective strategy for extending the shelf-life and preserving the quality of A. trifoliata fruit, which provides a theoretical reference for the application of Mt in post-harvest storage of A. trifoliata fruits.