Sulfur and polymer coating on urea enhances antioxidant defense, nutrient balance, secondary metabolites, and nitrogen use efficiency in salt‑stressed wheat

Soil salinity is a growing threat to wheat productivity. This study evaluated whether different coated urea formulations– especially polymer-sulfur-coated urea (PSCU) – could help wheat cope with different salt stress levels: non-saline control (1.3 dS m − 1 , showing the natural background salinity of the tested soil), moderate (6 dS m − 1 ), and high (12 dS m − 1 ), and four nitrogen treatments: normal urea, polymer-coated urea, polymer-zinc-coated urea, and PSCU were given (287.50 mg N/pot) in three equal splits at sowing, 25, and 50 days after sowing. High salinity alone decreased grain yield and crop biomass approximately by 35% and 23%, respectively, while shoot dry weight decreased by more than half. Salinity also increased Na⁺ uptake, electrolyte leakage, and reactive oxygen species that reduced photosynthetic pigments, RWC, and K⁺ uptake. All coated urea performed better compared to normal urea; however, PSCU was most promising. PSCU decreased oxidative damage, enhanced NPK uptake, and preserved maximum leaf water status and chlorophyll. Using PSCU, secondary metabolites (i.e., flavonoids, total phenols, and tocopherol) were enhanced by several‑fold in severe salt stress. Furthermore, indices of nitrogen use efficiency (NUE) (i.e., partial factor productivity, nitrogen balance intensity, and uptake efficiency etc.) also maximized under PSCU application. While PSCU could not fully restore control-level wheat performance in high salinity, it consistently alleviated salinity damage better than the other coatings. The present study suggests that applying polymer-sulfur-coated urea could be a promising strategy to improve wheat resilience and nitrogen efficiency in salt-affected soils.