Assessing the ecotoxicity of NiO nanomaterial and acetaminophen to barley and the beneficial effects of SiO2 nanomaterial co-application

Given the rising industrialization, nano-based products and pharmaceuticals are becoming relevant environmental contaminants, with potential risks to ecosystems and its primary producers. In this context, this study aimed to assess the phytotoxicity of nickel oxide nanomaterial (nano-NiO) and acetaminophen (AC), one of the most used pharmaceuticals all over the world, to Hordeum vulgare L. (barley). Combining standard procedures and biochemical parameters, a holistic approach was used to assess the biological effects of these two contaminants. The exposure of barley to increased concentrations (0, 87.8, 131.7, 197.5, 296.5, 444.4, 666.7, 1000 mg kg ) of both contaminants, during 14 days, resulted in a significant decrease in biomass and biometric parameters, followed by lipid peroxidation (LP), superoxide anion (O2) increase and cell death. Overall, nano-NiO was more toxic than AC and the evaluation of oxidative stress markers increased the sensitivity of the phytotoxic evaluation of these two contaminants for barley plants. Several studies report that silicon application is able to increase plant tolerance to different types of stress. Thus, the other main goal of this work was to assess the potential role of silicon dioxide nanomaterial (nano-SiO2) in enhancing the tolerance of barley to both contaminants. For this purpose, plants were grown for 14 days under both nano-NiO (120 mg kg) or AC (400 mg kg) exposure mixed with nano-SiO2 (3 mg kg). The exposure of barley to nano-NiO and AC caused a significant decrease in growth-related parameters, without compromising nitrogen assimilation enzymes. Upon nano-SiO2 co-exposure, the inhibitory effects of nano-NiO were partially reduced, though the same was not observed for plants under AC stress. Nano-NiO induced a negative response on the photosynthetic apparatus that was reverted by nano-SiO2 co-application. Plants growing under nano-NiO stress exhibited an overproduction of O2, which favored the occurrence of oxidative stress, but the co-treatment with nano-SiO2 reverted this tendency, generally lowering or maintaining the levels of LP and stimulating the anti-redox pathway of thiols. In opposition, AC did not caused significant oxidative damage, regardless of the presence of nano-SiO2. The evaluation of components of the plant antioxidant (AOX) system revealed that nano-NiO induced the accumulation of proline, along with a decrease in ascorbate, in leaves. Furthermore, SOD activity was significantly enhanced and catalase (CAT) and ascorbate peroxidase (APX) seemed to have a pivotal role in H2O2 detoxification in leaves and roots, respectively. The response of the AOX system was even more notorious upon nano-SiO2 co-exposure, reinforcing the protective role of this nanomaterial. Regarding AC, proline content was positively affected in leaves and some significant changes were tracked in SOD, CAT and APX activities; however, the co-application with nano-SiO2 did not majorly alter this pattern. Overall, the obtained results suggest the potential of nano-SiO2 in mitigating the phytotoxicity of both contaminants, although this protective role was more notorious upon nano-NiO coexposure.