Influences of post-harvest melatonin treatment on preservation quality and shelf life of fresh-cut cauliflower

Fresh-cut cauliflower has special economic significance in post-harvest management, as a vegetable with a high perishable rate. Among them, melatonin is a kind of indoleamine that plays an important role against abiotic stress. This study aims to evaluate the effect of 0.05, 0.10, and 0.50 mmol/L of melatonin on the preservation quality of nutritionals in the fresh-cut cauliflower during 16-day storage at 4°C. Six samples of each replicate were randomly taken at 0, 4, 8, 12, and 16 d after treatment to measure weight loss, firmness, color, polygalacturonase (PG), and lipoxygenase (LOX) activities, and PG and LOX expression. The rest of the fruits were immediately frozen in liquid nitrogen and stored at −80 °C before determining the endogenous melatonin levels and DPPH radical scavenging activity, total phenolic, ascorbic acid, and total glucosinolates contents. The results showed that the nutritional quality was clearly delayed (P<0.05) by the 0.10 mmol/L melatonin treatment, as disclosed by the weight loss, firmness, color, total glucosinolates content, and endogenous melatonin content of fresh-cut cauliflower samples. Furthermore, 0.10 mmol/L melatonin treatment was optimal for the enhancement (P<0.05) of total phenolic content and antioxidant capacity, indicating that it delayed (P<0.05) the reduction in ascorbic acid. Moreover, 0.10 mmol/L melatonin treatment enhanced (P<0.05) PG expression, where decreased first and then increased (P<0.05) LOX gene expression during storage, and delayed (P<0.05) the activities of PG and LOX. After storage for 16 d, the changes of cellular structure in the 0.10 mmol/L treatment group and control group were observed under a Transmission Electron Microscope (TEM). The results showed the 0.10 mmol/L treatment sample cells presented obvious slight plasmolysis after storage for 16 d, where a small amount of autophagy was observed, while the slight plasmodesmata were also found between the cells. However, some variations of cells were also observed as follows: the mitochondria of cells in the ultrastructure of cauliflower florets in the absence of melatonin treatment (control) were moderately swollen, the endoplasmic reticulum was moderately expanded, autophagy appeared, and the cytoplasm and cell wall were separated. Consequently, the finding demonstrated that the melatonin treatment may be expected to serve as a useful technique to extend the postharvest life and improve quality in fresh-cut cauliflower.