Melatonin and Nitrogen Applications Modulate Early Growth and Related Physio-biochemical Attributes in Maize Under Cd Stress

Melatonin is effective in modulating plant metabolism under stress conditions whereas nitrogen (N) is essential for plant growth. To explore the effects of melatonin and N applications on early growth, associated physiological and biochemical traits, and cadmium (Cd) uptake in maize under Cd stress, a pot experiment was conducted. Two melatonin levels, i.e., 0 and 200 μM, and two N levels, i.e., 1/4 N and full N dose of Kimura nutrient solution, were employed on two widely cultivated maize varieties, i.e., Yuecainuo 2 and Yuebaitiannuo 7, under Cd stress (150 μM). Results showed that melatonin application improved plant biomass and seedling growth, especially in low N treatment which further activated antioxidant defense system in terms of superoxide dismutase (SOD), peroxidase (POD), and catalase (CAT) while melatonin application variably affected activities of antioxidants under Cd stress. In addition, melatonin application and low N treatment activated enzymes involved in N metabolism in Yuecainuo 2 as well as resulted in higher Cd concentration in shoot and root of maize seedlings in both maize varieties. Overall, short-term low N treatment showed marginal decrease in early growth of maize with higher Cd accumulation, while melatonin application could accelerate the growth of maize seedlings and further improve the Cd absorption in maize seedlings.