Exogenous melatonin ameliorates yellowing of postharvest pak choi (Brassica rapa subsp. chinensis) by modulating chlorophyll catabolism and antioxidant system during storage at 20 °C

The objective of this study was to investigate the regulatory mechanism of melatonin (MT) treatment on leaf yellowing of pak choi during storage at 20 °C. Results showed that 500 μmoL L−1 MT alleviated the yellowing process of pak choi as proved by the maintenance of high level of leaf colour, chlorophyll content and chlorophyll fluorescence parameters (Fo, Fm and Fv/Fm), and the market life of pak choi was extended by 2 d compared to the control. The weight loss and respiration rate were effectively alleviated by MT treatment. MT treatment restrained chlorophyll breakdown by down-regulating the activity of chlorophyll-degrading enzymes (chlorophyllase CLH, pheophytinase PPH and Mg-dechelatase MDS) and expression of their related genes (BcCLH, BcPPH1/2, BcSGR, BcHCAR, BcNYC1 and BcCAO). Meanwhile, MT suppressed the over-accumulation of reactive oxygen species (ROS) (superoxide anion O2·− and hydrogen peroxide H2O2), and lipid membrane oxidation and associated gene (BcLOX) expression during senescence, thus helping to keep green colour of pak choi. This relationship was also connected to the maintenance of higher level in antioxidant ability of antioxidase (superoxide dismutase SOD, ascorbate peroxidase APX, peroxidase POD and catalase CAT) and antioxidants (ascorbate-glutathione (AsA-GSH)), as well as high expression of their related genes (BcSOD, BcPOD, BcCAT, BcGPX and BcAPX) in pak choi treated with MT. In conclusion, these findings suggested that MT could delay the yellowing of pak choi, involving the regulation of chlorophyll metabolism, ROS metabolism, membrane lipid degradation, and antioxidant capacity. The results provide a possible target for delaying the yellowing of green vegetables.