Proline functionalized ZnO nanoparticles promote salinity resilience in the vegetative phase of two cultivars of mung bean by alleviating oxidative stress and restoring Na⁺/K⁺ balance

Salinity stress severely limits growth and productivity of Vigna radiata L. by disturbing physiological performance, redox balance, and ionic homeostasis. This study investigated the effectiveness of zinc oxide nanoparticles (ZnO NPs, 20 ppm) and proline-functionalized ZnO nanoparticles (P-ZnO NPs, 20 ppm) in mitigating NaCl-induced (40 mM) salinity stress in two cultivars, GUJ-4 and GUJ-6, at the vegetative stage. Salinity significantly reduced leaf area, shoot growth, biomass accumulation, photosynthetic pigments, and membrane stability, while increasing hydrogen peroxide, electrolyte leakage, lipid peroxidation, proline accumulation, and Na⁺ content. Foliar application of NPs markedly improved plant performance under saline conditions. Zinc oxide NPs restored leaf area by 45.9–51.1% and fresh biomass by 62.7–63.8% relative to salt-stressed plants, whereas P-ZnO NPs showed superior recovery, enhancing leaf area by 66.2–72.4% and fresh biomass by 76.3–89.1%. Notably, P-ZnO treatment elevated growth parameters beyond control levels in both cultivars. Nanoparticle application effectively reduced Na⁺ accumulation, improved K⁺ retention, and lowered the Na⁺/K⁺ ratio, thereby re-establishing ionic balance. Salinity-induced oxidative stress, reflected by increased H ₂ O ₂ and MDA levels and enhanced antioxidant enzyme activities (SOD, CAT, POD, APX, and GR), was substantially alleviated, with P-ZnO NPs promoting better normalization of antioxidant responses. Cultivar-specific differences were observed, with GUJ-6 exhibiting greater salt sensitivity, reflected by higher stress marker accumulation, but also showing a stronger response to NP treatments, particularly P-ZnO NPs. Overall, P-ZnO NPs demonstrated greater efficacy than unmodified ZnO in enhancing salinity tolerance in both cultivars, highlighting the promise of nano-enabled strategies for improving mung bean resilience under saline conditions.