Synergistic Application of Taurine and Spermidine Enhances Wheat Tolerance to Neodymium Stress Through Redox Homeostasis, Metabolic Adjustment, and Nutrient Regulation

Neodymium (Nd) contamination is an emerging environmental challenge that threatens crop productivity and food quality in agroecosystems. Although taurine (Tau) and spermidine (Spd) are recognized as important regulators of plant stress responses, their interactive role in mitigating rare-earth-element toxicity remains largely unexplored. The present study evaluated the potential of foliar-applied (Tau 10 mM) and (Spd 1 mM), applied individually and in combination, to alleviate Nd toxicity (150 mg kg-1 NdCl3·6H2O) in wheat (Triticum aestivum L.) cv. Faisalabad-2008. Nd exposure significantly (p ≤ 0.05) impaired soil physico-chemical properties, depressed microbial biomass carbon and dehydrogenase, urease, alkaline phosphatase and β-glucosidase activities, restricted root architecture and shoot biomass, suppressed gas exchange and pigment biosynthesis and triggered oxidative damage, as evidenced by elevated malondialdehyde (MDA) and hydrogen peroxide (H2O2). The combined Tau + Spd foliar spray restored the soil quality and microbial functionality, enhanced the activities of superoxide dismutase (SOD), catalase (CAT), peroxidase (POX), and ascorbate peroxidase (APX), increased the proline, glutathione and total soluble sugars and improved the photosynthetic pigments and stomatal conductance relative to the Nd-only treatment. The dual application also reduced Nd accumulation in roots, shoots, and grains, restored the K+/Na+ ratio and macronutrient acquisition and improved the grain yield, protein, starch, and micronutrient density. Pearson correlation, radar, and principal component analyses confirmed coordinated recovery across soil, physiological, and grain-quality traits. These findings demonstrate that taurine and spermidine act synergistically to enhance wheat tolerance to Nd toxicity through the modulation of antioxidant defenses, osmotic adjustment, nutrient homeostasis, and metal exclusion mechanisms, highlighting their potential as sustainable biostimulants for crop production in rare-earth-element-contaminated soils.