Exogenous Melatonin Alleviated Leaf Yellowing via Inhibiting Respiration and Ethylene Biosynthesis during Shelf Life in Pakchoi

Postharvest yellowing of leafy plant is a manifestation of senescence, and melatonin (MT) is known to delay leaf senescence in some higher plants. Herein, we investigated the effect of exogenous MT treatment on postharvest pakchoi by monitoring the ethylene biosynthesis and respiratory metabolism. Results showed that exogenous MT effectively extended the shelf life, delayed leaf yellowing, minimized the alteration in Fv/Fm ratio and maintained higher integrity of chloroplast in pakchoi. There was a significant correlation between yellowing index, respiration rate and ethylene production. MT treatments greatly delayed the yellowing process of pakchoi that was associated with the reduced activity of glycolysis pathway and tricarboxylic acid cycle (TCA), increased proportion of pentose phosphate pathway (PPP) in respiratory metabolism, as manifested by the lower activity of phosphohexose isomerase (PHI), succinate dehydrogenase (SDH) and cytochrome C oxidase (COX), downregulated the expression of their corresponding genes, but enhanced the activity and expression level of 6 phosphogluconate dehydrogenase (6PGDH). MT also markedly maintain chlorophyll content by inhibiting ethylene production and action during shelf life, likely a consequence of reduced activities of 1-aminocyclopropane-1-carboxylate (ACC) synthase (ACS) and ACC oxidase (ACO), as well as the expression levels of their related genes. These results collectively indicate that melatonin alleviated leaf yellowing of postharvest pakchoi might be attributed to the suppression of the ethylene biosynthesis and respiratory metabolism, and our findings contribute to provide a good candidate measure for extending shelf life and reducing postharvest loss of pakchoi.