Elucidating the phytotoxic endpoints of sub-chronic exposure to titanium dioxide nanoparticles in Endemic Persian Dracocephalum species

This study was designed to investigate the dichotomous effects of titanium dioxide nanoparticles (TiO₂NPs) at varying concentrations (0, 50, 100, 1000, and 2500 ppm) on the physiological, biochemical, and antioxidative defense responses of Persian dragonhead plants cultivated in hydroponic conditions. Over 21 days of treatment, an increase in fresh shoot biomass by 26.2% and plant height by 18.2% was observed at exposure to 50 ppm TiO₂NPs. Exposure to 100 ppm NPs negatively affected the biosynthesis of carotenoids, chlorophyll pigments (a, b, and total), and protein content. Field Emission Scanning Electron Microscopy (FE-SEM) and Transmission Electron Microscopy (TEM) analysis revealed TiO₂NPs deposition within intercellular spaces and cell walls of root tissues. The physiological stress was prominent in response to 2500 ppm NPs as evidenced by a significant increase in proline and sugar content compared to the control. The enzymatic antioxidative defense was significantly upregulated by the enhanced activity of catalase (CAT) across exposure ranges 100–2500 ppm NPs, ascorbate peroxidase (APX) at 100 and 2500 ppm NPs, and peroxidase (POD) at 100 ppm NPs in plant roots. The antioxidant proficiency was further corroborated by increases in total flavonoids by 30.43% at 2500 ppm, saponins by 253.7%, and iridoids by 22.3% at 100 ppm NPs, relative to control. The results suggest that TiO₂NPs fostered growth promotion at sub-lethal doses, and induced adverse biochemical changes at elevated concentrations, prompting the activation of intrinsic defense mechanisms to enhance plant resilience against NPs stresses. The optimal nano-stimulation performance was observed at 50 ppm TiO₂NPs, which was suggested for the high yield targets, signifying a potential boon for agricultural productivity.