Aim: Nanoparticles are superior to conventional elemental forms due to the novel physicochemical properties that enable them to act as plant growth promoters. Recent research on nanomaterials has been shown both positive and negative effects. Titanium dioxide nanoparticles (TiO2 NPs) are assayed worldwide in large quantities for various purposes. This study investigated the effect of TiO2 NPs on some physiological and biochemical traits and the induction of the antioxidant systems in Origanum vulgare L. and Origanum majorana L., in a completely randomized design with three repetitions. Materials and Method: Surface-sterilized seeds were sown with 2 cm depth in each plastic pots filled with a 1: 1 mix of loamy soil and sand. Until reaching the appropriate physiological age, they were exposed to greenhouse conditions (16 h light/8 h dark photoperiod). Two-month-old plants were foliar sprayed with 10, 60, and 120 mg/L TiO2 NPs and harvested 14 days after the last treatment. Plants sprayed with distilled water were used as controls. Changes in the photosynthetic pigments (the content of chlorophyll a, chlorophyll b, total chlorophyll, and carotenoid), biochemical properties (the levels of membrane stability index, malondialdehyde, and proline contents), and the activity of antioxidant enzymes (Guaiacol peroxidase, ascorbate peroxidase, polyphenol oxidase, superoxide dismutase, catalase, and glutathione s-transferase) were measured in the collected samples. Results: The comparison of data means showed that the treatment of 60 mg/L TiO2 NPs caused the maximum amount of chlorophyll a, total chlorophyll, and carotenoids in both Origanum species. However, the highest content of chlorophyll b in O. majorana was observed in the treatment of 120 mg/L TiO2 NPs. It is worth mentioning that the negative effect of the concentration of 120 mg/L TiO2 NPs on the membrane system of two species is statistically significant and remarkable due to the leakage of electrolytes. Although the increase in the concentration of TiO2 NPs led to fewer levels of malondialdehyde in O. vulgare in comparison with O. majorana; this concentration was found to be effective to the high levels of proline in O. vulgare. Most of the antioxidant enzymes (Guaiacol peroxidase, ascorbate peroxidase, polyphenol oxidase, superoxide dismutase, and glutathione s-transferase) of O. vulgare showed their maximum activity at the concentration of 60 mg/L TiO2 NPs. However, the lower activity of catalase in O. vulgare, compared to O. majorana was remarkable. Conclusion: The results of this research show that the treatment of 60 mg/L TiO2 NPs improves the physiological characteristics of the medicinal plant Origanum, including the increase of photosynthetic pigments, followed by the rate of photosynthesis and the increase of biomass. It is noteworthy that higher levels than the optimal concentrations of TiO2 NPs can lead to an increase in the levels of ROS and oxidative burst, which leads to a decrease in plant performance. Therefore, the plant response to nanoparticles depends significantly on the concentration and time of application, as well as the size, shape, and surface functionalization of the particles. Finally, O. vulgare is introduced as a more successful species due to the higher activity of antioxidant enzymes when treating TiO2 NPs.

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