Complementary Antioxidant and Photosynthetic Protection by Melatonin and Chitosan Biostimulants Alleviates Lead Toxicity in Quinoa

Abstract Melatonin and chitosan have emerged as potential biostimulants for mitigating heavy metal toxicity in plants. This study investigated the ameliorative effects of melatonin (MEL) and chitosan (CHI) foliar application on quinoa grown under lead (Pb) stress using a 3×3×3 factorial experiment with three Pb levels (0, 50, and 100 mg kg − 1 soil) and three concentrations of each biostimulant (MEL: 0, 50, 100 µM; CHI: 0, 1, 3 g L − 1 ), measuring 27 physiological, biochemical, and yield traits in root and shoot. Lead significantly affected 26 of 27 traits (p < 0.05), reducing growth, photosynthetic efficiency, pigment content, and yield while increasing oxidative stress markers. Both MEL and CHI significantly affected 20 and 21 traits, respectively (p < 0.05). Notably, CHI uniquely improved vegetative growth parameters including leaf number (F = 18.75), leaf area (F = 18.88), plant height (F = 10.00), and stem diameter (F = 21.30) (all p < 0.001). Significant Pb×MEL×CHI three-way interactions were detected for six traits, including root volume, stem diameter, malondialdehyde content, activity of catalase and peroxidase, and chlorophyll b and carotenoids (all p < 0.001). Principal component analysis revealed that PC1 explained 75.5% of total variance, separating treatments primarily by Pb stress level. Combined MEL and CHI treatments produced additive protective effects, shifting plant physiological status toward unstressed controls. These findings demonstrate that melatonin and chitosan provide complementary amelioration of Pb toxicity in quinoa through coordinated enhancement of antioxidant defense and protection of photosynthetic pigments.