Employing Titanium Dioxide Nanoparticles as Biostimulant against Salinity: Improving Antioxidative Defense and Reactive Oxygen Species Balancing in Eggplant Seedlings

Salinity is a major abiotic stress that affects the agricultural sector and poses a significant threat to sustainable crop production. Nanoparticles (NPs) act as biostimulants and significantly mitigate abiotic stress. In this context, this experiment was designed to assess the effects of foliar application of titanium dioxide (TiO₂) nanoparticles at 200 and 400 ppm on the growth of eggplant (<i>Solanum melongena</i>) seedlings under moderate (75 mM) and high (150 mM) salinity stress. The TiO₂-NPs employed were characterized by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), thermal gravimetric analysis (TGA), and scanning electron microscopy (SEM) analysis. The seedlings were assessed physiologically, growth-wise, and biochemically. The seedlings were significantly affected by their physiological attributes (Fv'/Fm', Fv/Fm, NPQ), growth (root length, shoot length, number of leaves, fresh biomass, dry biomass, leaf greenness), antioxidative enzymes (SOD, POD, CAT, APx, GR), stress indicators (H₂O₂, MDA), and toxic ion (Na⁺) concentrations. The maximum decrease in physiological and growth attributes in eggplant seedling leaves was observed with no TiO₂-NP application at 150 mM NaCl. Applying TiO₂-NPs at 200 ppm showed significantly less decrease in Fv'/Fm', root length, shoot length, number of leaves, fresh biomass, dry biomass, and leaf greenness. In contrast, there were larger increases in SOD, POD, CAT, APx, GR, and TSP. This led to less accumulation of H₂O₂, MDA, and Na⁺. No significant difference was observed in higher concentrations of TiO₂-NPs compared to the control. Therefore, TiO₂-NPs at 200 ppm might be used to grow eggplant seedlings at moderate and high salinity.