Simultaneous application of titanium dioxide (TiO2) and zinc oxide (ZnO) nanoparticles ameliorates lead (Pb) stress effects in medicinal plant Echinacea purpurea (L.) Moench

In view of previously recorded effects of zinc oxide and titanium dioxide nanoparticles (ZnO and TiO2 NPs) in a wide range of plant processes including mitigation of stress effects, a greenhouse experiment was carried out to evaluate their impacts individually and in combination under lead (Pb) stress conditions. For this purpose, TiO2 NPs (50 mg L-1), ZnO NPs (50 mg L-1) and their combination TiO2 NPs + ZnO NPs (50 mg L-1) were applied in Echinacea purpurea (L.) Moench plants and key agronomic, physiological and biochemical traits were assayed under different Pb stress conditions (0, 50 and 100 mg kg-1). Results revealed that Pb stress conditions negatively affected some agronomic parameters (e.g. plant height, leaf area and number, fresh and dry weight), photosynthesis pigments, Fv/Fm, Zn and Fe content and enhanced MDA, H2O2, sugar content, proline, total phenols, SOD and APX enzymatic activities, and essential oil content. Applied TiO2 NPs + ZnO NPs treatments were successful at alleviating Pb stress conditions by improving agronomic traits, Chl a, b, carotenoids, Zn and decreasing MDA, H2O2, sugar, proline, phenolic compounds, SOD and APX enzymatic activities, Fe and Pb under both Pb stress conditions. All applied treatments enhanced essential oil content under control conditions; combination treatment (TiO2 NPs + ZnO NPs) reduced the content under moderate (50 mg kg-1 Pb) stress condition, but it increased under severe (100 mg kg-1 Pb) stress. GC/MS analysis revealed germacrene α-pinene, 1-pentadecene and D, myrcene, as the dominant constituents of the essential oil, respectively. Overall, combination of TiO2 NPs with ZnO NPs could be considered as the most effective treatment towards the alleviation of Pb stress impacts in E. purpurea plants, thus rendering this as an efficient approach for the protection of plants under heavy metal stress conditions.