Co-application of SiO₂ nanoparticles and epibrassinolide enhances salt stress tolerance in Cucumis sativus

Salinity stress is a major abiotic constraint that severely limits plant growth, physiological performance, and agricultural productivity worldwide. This study investigated the effectiveness of the co-application of silicon dioxide nanoparticles (SiO₂-NPs; 50 ppm) and 24-epibrassinolide (EBL; 0.01 µM) in enhancing salt stress tolerance in Cucumis sativus exposed to elevated levels (50, 100, and 200 mM) of NaCl. Salinity stress significantly reduced shoot and root growth, biomass accumulation, leaf area, relative water content, chlorophyll content, gas exchange characteristics, and photosynthetic efficiency. However, co-application of SiO₂-NPs and EBL significantly alleviated these adverse effects by improving growth attributes, SPAD chlorophyll content, stomatal conductance, photosynthetic rate, Fv/Fm ratio, and performance index under stress conditions. The combined treatment of SiO₂-NPs and EBL also enhanced antioxidant defense mechanisms by increasing the activities of superoxide dismutase (SOD), catalase (CAT), peroxidase (POX), and glutathione reductase (GR), thereby reducing hydrogen peroxide (H₂O₂) accumulation, lipid peroxidation, and electrolyte leakage. In addition, improved osmotic adjustment was evidenced by enhanced proline accumulation and regulation of proline metabolism-related enzymes, including P5CS and ProDH. Overall, the findings demonstrate that co-application of SiO₂-NPs and EBL effectively mitigates salinity-induced oxidative and osmotic stress, thereby improving physiological and biochemical resilience in Cucumis sativus. This integrated approach may provide a promising strategy for enhancing crop productivity under saline conditions.