Abstract: Hydroponic Chinese chive often tastes less pungent due to the changes in cultivation methods，which threatens the sustainable development of the hydroponic Chinese chive sector. The decline of the flavor of hydroponic chive intensity is primarily associated with the production of total S-alk(en)yl cysteine sulfoxides（CSOs）in its leafy tissues. In order to improve the flavor of hydroponic Chinese chive，the effects of different concentrations of melatonin（MT）on growth，development，leaf quality，and flavor were investigated. The results showed that the plant height，leaf width/length，root activity，dry（fresh）weight per plant，and theoretical yield of Chinese chive reach maximums under 100 μmol • L-1 MT treatment. MT treatment significantly increased the activities of SOD and POD，whereas the content of MDA and H2O2 significantly decreased. 75 or 100 μmol • L-1 MT effectively promoted the accumulation of soluble sugar，soluble protein，vitamin C，total flavonoids，as well as total phenols in Chinese chives. Intriguingly，the results revealed that the leaf pungency of hydroponic Chinese chive was boosted by MT treatments，even comparable to that of soil-grown counterparts. Accordingly，the expression of three AtuFMOs，encoding key enzymes of CSO biosynthesis，was also significantly increased under MT treatments. Taken together，exogenous MT treatment could improve the yield and quality of hydroponic Chinese chive，and 75 μmol • L-1 MT is recommended，which provides a trade-off between the crop yield and flavor quality of hydroponic Chinese chive.

Key words: hydroponic Chinese chive, melatonin, S-alk(en)yl cysteine sulfoxide, quality, flavor