Green-synthesized zinc and iron nanoparticles enhance basil drought tolerance via antioxidant regulation

Abstract Background Drought is a major abiotic constraint that disrupts growth, physiology, and metabolic balance in medicinal and aromatic plants. This study examined how green-synthesized zinc (Zn) oxide and iron (Fe) oxide nanoparticles (NPs) influence drought tolerance in basil ( Ocimum basilicum L.). A factorial greenhouse experiment evaluated two irrigation regimes (100% and 50% field capacity, FC) combined with four foliar treatments: control, Zn (100 mg L⁻ 1 ), Fe (100 mg L⁻ 1 ), and Zn + Fe (50 + 50 mg L⁻ 1 ), which represent agronomically realistic and field-applicable concentrations. Nanoparticles biosynthesized using sage ( Salvia officinalis ) extract were characterized by scanning electron microscopy/energy dispersive X-ray spectroscopy (SEM/EDS). Results Under drought conditions, NP supplementation mitigated growth reductions by increasing plant height by up to 26.7%, leaf number by 30.6%, and biomass by 22.6% compared to untreated drought controls. The combined Zn + Fe treatment elicited trait-specific responses, particularly increasing phenolic and flavonoid contents by 53% and 48%, respectively, and notably enhancing catalase (CAT) and ascorbate peroxidase (APX) activities by 204% and 86% relative to drought controls. Consequently, oxidative stress markers (malondialdehyde [MDA], hydrogen peroxide [H₂O₂]) were reduced by nearly 50%. Zn primarily enhanced non-enzymatic antioxidant capacity by increasing cupric reducing antioxidant capacity [CUPRAC], ferric reducing antioxidant power [FRAP], and 2,2-diphenyl-1-picrylhydrazyl [DPPH] by 135%, 48%, and 17%, respectively, whereas Fe supported enzymatic detoxification through a 27% increase in superoxide dismutase (SOD) activation compared to drought controls. Correlation analysis showed strong associations among antioxidant capacity, pigment levels, and growth traits, and principal component analysis (PCA) distinguished Zn + Fe-treated plants within antioxidant-rich and high-biomass clusters, indicating coordinated redox regulation. Conclusions Foliar nano-micronutrition with green-synthesized Zn NPs and Fe NPs strengthened antioxidant defenses, stabilized pigments, and improved growth resilience under drought, providing physiological and biochemical evidence of nanoparticle-mediated modulation of stress responses.