Nitrogen alleviates salt stress in maize: Genotypic variations in ion balance, antioxidant response and nitrogen...

Soil salinization severely restricts maize growth and productivity. Nitrogen application mitigates salt stress, but the underlying physiological mechanisms and genotypic variations remain unclear. This study investigated how varying N levels (0.1, 2, 10 mM) alleviate salt stress (150 mM NaCl) in two maize inbred lines with contrasting salt tolerance: Mo17 (tolerant) and B73 (sensitive). Salt stress significantly reduced plant biomass, K⁺/Na⁺ ratio, and nitrogen accumulation in both genotypes. However, increasing nitrogen supply enhanced salt tolerance by optimizing root K⁺/Na⁺ homeostasis, enhancing nitrogen metabolism enzyme activities, and improving antioxidant defenses. It is worth noting that genotypic differences dictated nitrogen-dependent response: Mo17 achieved a higher biomass at low nitrogen (2 mM) level, whereas B73 required high nitrogen (10 mM) level to achieve comparable biomass. Nitrogen application also differentially regulated antioxidant enzymes enhancing leaf peroxidase (POD) and catalase (CAT) activities under salt stress, with Mo17 showing stronger low-nitrogen adaptability. Meanwhile, nitrogen supply also promotes the activity of nitrogen metabolism enzymes and nitrogen accumulation. Correlation analysis indicated that root K⁺ contents and leaf CAT activity as key biomarkers positively linked to plant biomass. Our results demonstrate that nitrogen alleviates salt stress in maize by coordinately regulating ion balance, nitrogen metabolism, and antioxidant defense, but efficacy is genotype-dependent. These findings provide physiological insights for optimizing nitrogen management in saline soils and increasing maize yield.