Iron oxide nanoparticles mitigate salinity stress in Ziziphora clinopodioides by enhancing growth, ion homeostasis, and essential oil content

Salinity is a major environmental constraint limiting the growth, physiological performance, and productivity of medicinal plants. This study investigated the effects of iron oxide nanoparticles (Fe₂O₃-NPs) and bulk ferric oxide on growth, ionic balance, oxidative stress, photosynthetic pigments, bioactive compounds, and essential oil production in Ziziphora clinopodioides under saline conditions using a factorial experiment based on a randomized complete block design with three replications, including four Fe₂O₃-NP concentrations (0, 50, 100, and 150 mg L⁻¹), three bulk ferric oxide levels (0, 100, and 150 mg L⁻¹), and three salinity levels (0, 80, and 160 mM NaCl). Salinity significantly reduced plant growth, RWC, photosynthetic pigments, K⁺ concentration, and K⁺/Na⁺ ratio while increasing Na⁺ accumulation and oxidative stress markers; under severe salinity (160 mM NaCl), shoot dry weight, plant height, RWC, and total chlorophyll declined by 61.1%, 52.7%, 27.9%, and 37.3%, respectively, while Na⁺, H₂O₂, and MDA increased by 181.6%, 129.6%, and 747.1%, respectively. Application of Fe₂O₃-NPs at 100 mg L⁻¹ markedly alleviated salt-induced damage, increasing shoot dry weight by 150% (from 3.36 to 8.40 g), plant height by 113% (from 5.32 to 11.34 cm), RWC by 29.6% (from 59.87% to 77.58%), total chlorophyll by 37.8% (from 0.37 to 0.51 mg g⁻¹ FW), K⁺ content by 36.4% (from 0.55 to 0.75 mg g⁻¹ DW), and K⁺/Na⁺ ratio by 173% (from 0.22 to 0.60) relative to untreated salt-stressed plants, while reducing Na⁺ accumulation by 49.4% (from 2.45 to 1.24 mg g⁻¹ DW), H₂O₂ by 48.4% (from 0.62 to 0.32 µmol g⁻¹ FW), and MDA by 71.8% (from 4.32 to 1.22 nmol g⁻¹ FW). Moderate concentrations of Fe₂O₃-NPs and bulk ferric oxide also enhanced total phenolics, flavonoids, PAL activity, and essential oil content, whereas the highest nanoparticle concentration (150 mg L⁻¹) exhibited phytotoxic effects; principal component analysis explained 91.72% of total variance and revealed strong positive associations among growth traits, photosynthetic pigments, secondary metabolites, K⁺ nutrition, and ionic homeostasis, while Na⁺, H₂O₂, and MDA were negatively associated with plant performance. Overall, Fe₂O₃-NPs at 100 mg L⁻¹ were more effective than bulk ferric oxide in mitigating salinity stress by improving water status, photosynthetic capacity, ion homeostasis, and antioxidant-related responses, highlighting the potential of nano-enabled iron fertilization as a sustainable strategy for enhancing the productivity and phytochemical quality of Z. clinopodioides under saline conditions.