Effects of Melatonin Treatment on Lignification and Browning in Bamboo Shoots (Acidosasa edulis) Without Sheaths During Cold Storage

The effects of melatonin treatment on inhibiting lignification and browning, including firmness, lignin content, enzyme activities, and the expression level of genes encoding corresponding enzymes in bamboo (Acidosasa edulis) shoots without sheaths during cold storage were investigated. Bamboo shoots without physical injuries or infections and similar in shape and size were removed from their sheaths and cleaned with tap water and then disinfected with 150 μL/L sodium hypochlorite for 5 min, rinsed with tap water, and air-dried. The bamboo shoots were soaked in 0.2 mmol/L melatonin solution and water (control) (both containing 0.05% Tween-80 and 3 mL anhydrous ethanol) at 22 ℃ for 30 minutes, air-dried at room temperature for approximately 60 min and subsequently stored at 6±1 °C accompanied by 75%~80% RH for 15 days. Quality parameters and activities of key enzymes and gene expression involved in lignification and browning of bamboo shoots were periodically determined during cold storage to investigate the related mechanism. The results showed that melatonin treatment significantly (P<0.05) decreased the activities of phenylalanine ammonia lyase (PAL), peroxidase (POD) and cinnamyl alcohol dehydrogenase (CAD), and the expression of these lignin synthesis-related enzymes, which contributed to retarding lignification process in bamboo shoots. Meanwhile, melatonin treatment decreased hydrogen peroxide (H₂O₂) content and the production rate of superoxide anion radical (O₂⁻·), delayed the increase of MDA content and relative leakage rate by significantly (P<0.05) increasing the activities of superoxide dismutase (SOD), catalase (CAT) and the expression of their encoding genes. On the other hand, melatonin treatment reduced the activities of polyphenol oxidase (PPO) and the expression of the encoding genes in bamboo shoots, consequently delaying the enzymatic browning of bamboo shoots. Thus, exogenous melatonin treatment could effectively inhibit browning and lignification of cold shelled yellow sweet bamboo shoots.