Foliar-applied selenium nanoparticles improve antioxidant defense and photosynthetic efficiency to enhance salt stress tolerance in cowpea (Vigna unguiculata L.)

• Foliar Se-NPs improved cowpea growth and yield under salinity stress. • Se-NPs enhanced photosynthesis, water status, and antioxidant defenses. • Optimal 1.0 mM Se-NPs increased NPK uptake and reduced Na + accumulation. • Se-NPs boosted osmoprotectants and phenolics, enhancing ROS scavenging. • Leaf anatomy improved with Se-NPs, supporting photosynthesis and resilience. Soil salinity is a significant constraint to cowpea ( Vigna unguiculata L.) productivity, impairing growth, physiology, antioxidant defense, and yield. The present study evaluated the potential of foliar-applied selenium nanoparticles (Se-NPs) to improve salt tolerance in cowpea grown under saline field conditions [electric conductivity (EC) = 7.55–7.61 dS m -1 ; equivalent to moderate salinity levels] during two consecutive summer seasons (2023 and 2024). Plants were treated with 0.25, 0.5, 1.0, or 1.5 mM Se-NPs at 25, 40, and 55 days after sowing. Foliar application of Se-NPs significantly ( P <0.05) enhanced growth, physiology, and yield compared with untreated controls. The 1.0 mM treatment produced the most significant improvements, increasing plant height (49%), biomass (49%), pod number (125%), 100-seed weight (27%), and seed yield (50%). Se-NPs also enhanced photosynthetic pigments, efficiency, and relative water content, while stimulating antioxidant defense through higher enzymatic (catalase, peroxidase, superoxide dismutase, ascorbate peroxidase, and glutathione reductase) and non-enzymatic (ascorbate, glutathione, α-tocopherol) antioxidants, as well as osmoprotectants (proline and soluble sugars). Moreover, Se-NPs enhanced leaf nutrient uptake [nitrogen, phosphorus, and potassium (K)], reduced sodium (Na⁺) accumulation, and improved the K⁺/Na⁺ ratio, thereby minimizing oxidative damage (malondialdehyde, hydrogen peroxide, superoxide, and electrolyte leakage). Secondary metabolites, including phenolics, flavonoids, and flavonols, also increased, which supports their radical scavenging activity. Anatomical modifications in Se-NP–treated plants, such as thicker mesophyll tissues, larger vascular bundles, and wider xylem vessels, further contributed to improved photosynthesis and stress resilience. Overall, foliar Se-NPs, particularly at 1.0 mM, effectively mitigated salinity stress and enhanced productivity, representing a promising biostimulant for sustainable cowpea cultivation in salt-affected soils.