Abstract: Abstract: Broccoli (Brassica oleracea L. var. italica Planch) florets are one of the nutrient-rich vegetables, which contain carbohydrates, protein, carotenoids, dietary fiber, vitamins, antioxidant substances, and anti-carcinogenic compounds. Fresh broccoli is therefore highly popular. Color is an extremely important quality attribute of fresh broccoli. "Yellowing" is the term used to describe the color quality. Under ambient temperature, the florets yellow rapidly accompanied by chlorophyll degradation, and the shelf life of broccoli is only 2-3 d. Therefore, it is important to keep the green color as long as possible for quality maintenance. As a safe and beneficial indoleamine molecule, melatonin exists in different organs of plant, such as roots, stems, leaves, flowers and fruits. Apart from acting as endogenous elicitor and signaling role, melatonin has a direct antioxidant activity for attenuation of biotic and abiotic stress, including salt, drought, cold, and pathogens. The senescence of fruits and vegetables is usually accompanied by increased level of reactive oxygen species. However, whether melatonin regulates the senescence and yellowing of postharvest broccoli remains unknown. In this study, to explore an effective preservation technology for postharvest broccoli, the effects of 100 μmol/L melatonin treatment on the yellowing index, chromatic aberration, endogenous melatonin content, total chlorophyll content, respiratory pathway, energy level, and antioxidant activity of fresh-cut broccoli were investigated based on the results of our preliminary experiments. The results showed that, the yellowing index in no-treated and distill water-treated broccolis were 1.93, 2.08 on the sixth day, respectively. However, the yellowing index of melatonin-treated group was only 1.71 on the seventh day. Meanwhile, melatonin treatment inhibited the increase of the values of L*, a*, and b*. The contents of total chlorophyll in postharvest broccoli decreased with the extent of storage time, while the content of total chlorophyll in melatonin-treated group were 18.03%, 24.15% highter than that in no-treated and distill water-treated groups on the sixth day, respectively. Additionally, at the substrate oxidation level, melatonin treatment reduced the total respiration rate of postharvest broccoli florets by reducing the operating proportion of embden-meyerhof-parnas (EMP), tricarboxyfic acid cycle (TCA) and phosphopentose pathway (PPP). At the level of electron transport chain, melatonin treatment decreased the operating proportion of cytochrome pathway (CP) and increased the operating proportion of alternate pathway (AP) (P<0.05). Meanwhile, melatonin treatment maintained higher contents of adenosine triphosphate (ATP) and adenosine diphosphateadenosine (ADP), and a lower content of adenosine monophosphate (AMP), thereby showing a higher energy status in the melatonin-treated samples during the whole storage period (P<0.05). Furthermore, melatonin treatment ameliorated the accumulation of O2?? and H2O2 compared to the control broccolis (P<0.05), which might be associated with the increased activities of superoxide dismutase (SOD), catalase (CAT) and peroxidase (POD), and the deduction was supported by an increased activities of SOD, CAT and POD isozymes. Overall, these results suggest that melatonin can delay the senescence process of postharvest broccoli florets through regulating the respiratory metabolism and antioxidant activity, which will provide a new theoretical basis for the application of melatonin and the preservation of broccoli.