Enhancing wheat resilience to arsenic toxicity by boosting antioxidant defense and secondary metabolite accumulation through melatonin.

Arsenic (As) contamination is a major environmental constraint that impairs wheat growth and productivity by disrupting cellular redox homeostasis, photosynthesis, and metabolic processes. Melatonin (MT), a multifunctional plant signaling molecule, has emerged as a promising regulator of plant tolerance to abiotic stresses; however, its role in mitigating As toxicity in wheat remains insufficiently understood. Wheat seedlings of three cultivars (Anaj-17, Galaxy-13, and Nayab-11) were grown in pots containing dried sand and exposed to As stress (20 mg L⁻¹) with or without MT supplementation (75 and 150 µM). Arsenic stress markedly increased As accumulation in grains, malondialdehyde (MDA), hydrogen peroxide (H₂O₂), and electrolyte leakage (EL%), while significantly reducing biomass, chlorophyll content, and gas exchange parameters. In contrast, MT application, particularly at 150 µM, effectively alleviated As-induced oxidative damage by enhancing the activities of catalase (CAT), peroxidase (POD), and superoxide dismutase (SOD). Melatonin also promoted the accumulation of glycine betaine, proline, total soluble sugars, total soluble proteins, and phenolic compounds, contributing to improved physiological performance and stress tolerance. Among the tested cultivars, Anaj-17 exhibited the greatest resilience to As stress and the strongest response to MT treatment, followed by Galaxy-13 and Nayab-11, indicating considerable genotypic variation in As tolerance. These findings demonstrate that melatonin enhances wheat tolerance to arsenic toxicity by strengthening antioxidant defenses, improving osmotic adjustment, and promoting the accumulation of protective metabolites, highlighting its potential as an effective strategy for enhancing heavy-metal stress resilience in wheat and other crop species.