Abstract: The melon cultivar ‘Yangjiaosu’ was subjected to the treatment of 18 ℃/12 ℃ (day/night) in an artificial climate chamber for 6 days, and the activities of nitrogen metabolism related enzymes \[nitrate reductase (NR), glutamine synthetase (GS), glutamate synthase (GOGAT) and glutamate dehydrogenase (GDH)\], the contents of total N, NO₃^--N and NH₄⁺-N as well as the osmotic adjustment substances of melon leaf were then determined. The results showed that, compared with the control, sublow temperature treatment reduced the contents of total N, NO₃^--N and the NR activity, but increased the content of NH₄⁺-N, thereby leading to the growth inhibition of melon. Exogenous MT treatment significantly improved the activities of nitrogen metabolism related enzymes, especially the activities of GS and GOGAT, effectively reducing the content of NH₄⁺-N. Moreover, MT treatment increased the contents of proline, soluble protein and soluble sugar, and alleviated the damage of sublow temperature on the cell membrane by reducing the relative electrical conductivity and MDA content of melon leaves. In short, this work suggested that exogenous MT would enhance the sublow temperature adaptability of melon by decreasing the leaf content of NH₄⁺-N, increasing the contents of osmotic adjustment substances and reducing the membrane lipid peroxidation levels.