Physiological and Molecular Mechanisms Underlying the Differential Responses of Wheat Seedlings to Different Types of Salt Stress

Soil salinization, driven by rapid climate change, poses a serious threat to wheat (<i>Triticum aestivum</i> L.) production worldwide. The studies on the effect of sodium chloride stress on wheat have detailed reports, while the effects of Na₂SO₄, NaHCO₃, and Na₂CO₃ stresses remain to be investigated. Here, we investigated the differential growth and physiological responses of wheat seedlings to equimolar concentrations of NaCl, Na₂SO₄, NaHCO₃, and Na₂CO₃. Alkaline salts (NaHCO₃ and Na₂CO₃) induced significantly more severe growth inhibition, chlorophyll degradation, and oxidative damage compared to neutral salts (NaCl and Na₂SO₄). This was evidenced by heightened lipid peroxidation, reactive oxygen species accumulation, and membrane injury, particularly under Na₂CO₃ stress. The antioxidant defenses were precisely tailored, which alkaline stress strongly activated ascorbate while neutral salts preferentially enhanced catalase activity. Osmotic adjustment was also stress-specific, with alkaline conditions triggering extreme proline accumulation up to 7.5-fold in roots. Ion homeostasis was profoundly disrupted under alkaline stress, marked by excessive Na⁺ uptake, severe K⁺ depletion, and significant reductions in nitrogen and phosphorus. Notably, gene expression analysis revealed stress-specific regulation of key genes involved in ion transport (e.g., <i>SOS1</i>) and antioxidant defense. Our findings revealed distinct stress-specific regulatory mechanisms in wheat, with alkaline causing more severe oxidative stress and membrane damage than salt. In addition, we examined the tissue expression and evolution of <i>SOD</i> genes, which showed the expansion and duplication of the <i>SOD</i> gene family in terrestrial plants. Our study unveils the divergent physiological pathways activated by different salts, providing novel insights into wheat stress adaptation and a theoretical basis for breeding salt-tolerant cultivars.