Enhancing salt tolerance in Mentha × gracilis through foliar applications of titanium and nano-titanium.

Salinity is an abiotic stress that negatively affects plant growth and the synthesis of secondary metabolites. This study aimed to evaluate the effects of foliar applications of titanium (Ti) and nano-titanium (nano-Ti) at concentrations of 0, 50, and 100 mg/L under salinity levels of 0, 50, and 100 mM NaCl in controlled greenhouse conditions. A factorial experiment based on a completely randomized design with four replications was conducted. A comprehensive set of morphological (plant height, fresh and dry biomass), physiological (photosynthetic pigments, soluble carbohydrates, proline, and protein content), and biochemical parameters (antioxidant enzyme activities including superoxide dismutase, guaiacol peroxidase, and ascorbate peroxidase), as well as essential oil (EO) content and composition, were assessed. Salinity stress markedly reduced plant growth, chlorophyll content, and EO yield, while increasing oxidative stress markers such as malondialdehyde (MDA) and hydrogen peroxide (H₂O₂). The application of 100 mg/L nano-titanium under non-stress conditions significantly increased plant height (47.01 cm), fresh weight (87.33 g), and essential oil yield (0.639 g/pot). Moreover, essential oil content reached a maximum of 1.84% under 50 mM salinity with 100 mg/L nano-titanium, representing a 212% increase compared to the control. Nano-titanium application under salinity stress increased APX and SOD activities by 176% and 237%, respectively, compared to the control. GC-MS analysis revealed linalool, trans-caryophyllene, 1,8-cineole, and germacrene D as the major EO constituents, whose concentrations were notably influenced by both salinity level and Ti treatments. These findings suggest that nano-Ti has the potential to be used as a sustainable biostimulant to enhance growth and secondary metabolite production in M. × gracilis under saline environments.