Uptake, translocation, phytotoxicity, and hormetic effects of titanium dioxide nanoparticles (TiONPs) in Nigella arvensis L.

The extensive application of titanium dioxide nanoparticles (TiONPs) in agro-industrial practices leads to their high accumulation in the environment or agricultural soils. However, their threshold and ecotoxicological impacts on plants are still poorly understood. In this study, the hormetic effects of TiONPs at a concentration range of 0-2500 mg/L on the growth, and biochemical and physiological behaviors of Nigella arvensis in a hydroponic system were examined for three weeks. The translocation of TiONPs in plant tissues was characterized through scanning and transmission electron microscopy (SEM and TEM). The bioaccumulation of total titanium (Ti) was quantified by inductively coupled plasma atomic emission spectroscopy (ICP-AES). Briefly, the elongation of roots and shoots and the total biomass growth were significantly promoted at 100 mg/L TiONPs. As the results indicated, TiONPs had a hormesis effect on the proline content, i.e., a stimulating effect at the low concentrations of 50 and 100 mg/L and an inhibiting effect in the highest concentration of 2500 mg/L. A biphasic dose-response was observed against TiONPs in shoot soluble sugar and protein contents. The inhibitory effects were detected at ≥1000 mg/L TiONPs, where the synthesis of chlorophylls and carotenoid was reduced. At 1000 mg/ L, TiONPs significantly promoted the cellular HO generation, and increased the activities of antioxidant enzymes such as superoxide dismutase (SOD), ascorbate peroxidase (APX), and catalase (CAT). Furthermore, it enhanced the total antioxidant content (TAC), total iridoid content (TIC), and 2,2-diphenyl-1-picrylhydrazyl (DPPH) scavenging activity. Overall, the study revealed the physiological and biochemical alterations in a medicinal plant affected by TiONPs, which can help to use these NPs beneficially by eliminating their harmful effects.