Melatonin Alleviates Cadmium-Induced Toxicity by Modulating Antioxidant Defence Mechanisms, Growth and Photosynthesis in Brassica juncea

Melatonin (MEL) is an important signal molecule and regulates plant growth and development under stress conditions. Cadmium (Cd) has been reported to regulate MEL content in rice; however, its role in Cd-stress is unknown and needs to be discussed. Thus, the present study was conducted to evaluate the response of exogenously applied melatonin on morphology, biochemistry, stress tolerance, and metal accumulation attributes of Cd-stressed Brassica juncea . Cd (100 mg/kg) was applied to the soil at the 7th-day stage, and at the 25-day stage, and the plants were sprayed with different concentration of MEL (50, 70, 75, and 100 M) or without (control) for consecutive 5 days. Various determents were assessed after 30 and 45 DAS. The results demonstrated that excess Cd accumulated in plant tissues, mostly in the roots, thus hampering plant growth, as evident by decreased photosynthesis rate, transpiration, and elevated stress biomarkers. However, the follow-up treatment of MEL significantly reduced Cd-induced toxicity and improved growth, photosynthesis, biochemical attributes, and antioxidant levels. Moreover, lower metal accumulation was observed in aerial tissues of plants, suggesting a potential role of MEL in limiting Cd translocation from root to shoot. The results demonstrated that 75 M MEL was more effective in growth optimisation as well as improving the stress resilience in plants under Cd stress. These findings can be applied to the development of new strategies for improving crop productivity on a sustainable basis, particularly in metal-polluted soils.