Effects of melatonin on the growth and cadmium characteristics of Cyphomandra betacea seedlings

To determine whether the melatonin (MT) could reduce cadmium (Cd) accumulation in Cyphomandra betacea seedlings, different concentrations of MT were added to the nutrient solution and soil to study its effects on the growth and Cd characteristics of C. betacea seedlings. Fifty micromoles per liter (μmol/kg) of MT increased the biomass of C. betacea seedlings, but 100–200 μmol/L (μmol/kg) MT decreased seedling biomass in both the nutrient solution and soil cultivation experiments. The photosynthetic pigment contents showed no significant changes compared with the control when the MT dose was 150 μmol/L (μmol/kg) or less, while 200 μmol/L (μmol/kg) MT decreased the photosynthetic pigment contents. Low levels of MT improved the antioxidant enzyme activities of C. betacea seedlings, whereas high MT levels reduced them. MT increased the Cd contents in different organs of C. betacea seedlings in the nutrient solution cultivation experiment, but only 50 μmol/kg MT increased the Cd contents in stems, leaves, and shoots of C. betacea seedlings in the soil cultivation experiment. Therefore, only low levels of MT can promote the growth of C. betacea seedlings, and MT is not suitable for reducing Cd accumulation in C. betacea seedlings.