Objective The effects of melatonin on the physiological properties of soybean (Glycine max (Linn.) Merr.) under copper (Cu) stress were investigated to provide some reference for the mitigation of heavy metal stress.

Method In a pot experiment, the Cu-sensitive variety ‘GuiZao 1’ and the Cu-tolerant variety ‘Brazil 13’ were selected and applied with 100 μmol·L⁻¹ melatonin under 0.5 mmol·L⁻¹ Cu stress to explore the effects of melatonin on soybean physiological indicators (root length, plant height, fresh weight, relative chlorophyll content and Cu content), superoxide dismutase (SOD), catalase (CAT), peroxidase (POD) activities, malondialdehyde (MDA), soluble sugar and soluble protein contents.

Result Application of 100 μmol·L⁻¹ melatonin under Cu stress increased the root length, plant height, fresh weight and relative chlorophyll content of both soybean varieties. The Cu contents in shoots and roots of both varieties were significantly reduced by melatonin treatment under Cu stress by 25.01% and 18.40% (‘GuiZao 1’), 26.84% and 20.28% (‘Brazil 13’), respectively. By applying melatonin under Cu stress, ‘GuiZao 1’ had a significant increase of POD activity in shoots by 48.35%, and ‘Brazil 13’ had a significant increase of CAT activity in roots by 56.84%; ‘GuiZao 1’ and ‘Brazil 13’ had significantly higher SOD activities in shoots by 19.07% and 7.30%, respectively; MDA content in shoots was significantly reduced by 8.05% and 26.56%. In the single application or mixed application under Cu stress, melatonin had no significant effect on the soluble protein contents of the two varieties. The amounts of soluble sugar in shoots of ‘GuiZao 1’ and ‘Brazil 13’ increased by 149.70% and 58.75% in the mixed application treatment under Cu stress.

Conclusion The appropriate concentrations of melatonin can mitigate the effects with Cu stress on soybean growth and development by improving antioxidant enzyme activity, inhibiting membrane lipid oxidation, reducing Cu uptake and increasing osmoregulatory substances.