B. Kim, K.L. Hayes, J. Mui, E.S. Sani, S.W. Eisenman, J.B. Sheffield
Keywords: aluminum oxide nanoparticle, fluorescence microscopy, confocal laser scanning microscopy, uptake, translocation, phytotoxicity
Summary:The toxic effects of aluminum (Al) ions on plant roots were identified over a century ago. However, only a handful studies have investigated the effects of aluminum oxide nanoparticles (Al2O3 NPs) on plant growth and development, even though Al2O3 NPs are one of the highest demanded metal oxides for commercial and industrial applications. Therefore, in this study, plant interactions with Al2O3 NPs were evaluated from seed germination and root growth to full maturity by measuring growth responses and Al uptake of lettuce plants (Lactuca sativa). The plants were grown for 6 weeks in hydroponic solutions and were then exposed to NPs or salts for additional 2 weeks at target concentrations (Al2O3 NPs of 0, 0.4, 1, and 2 mg/mL or AlCl3 of 0, 0.04, 0.4, and 1 mg/mL). While Al2O3 NPs treatment had no impacts on seed germination rates and root elongation by the tested concentration ranges, AlCl3 treatment at 0.4 and 1 mg/mL resulted in a significant reduction in emerging root lengths. Upon harvest, lettuce plants exposed to 1 and 2 mg/mL of Al2O3 NPs showed 10.4% and 30.4% decreases in biomass, whereas exposure to AlCl3 at 0.4 and 1 mg/mL reduced plant biomass to 22.3% and 9.96%, respectively, compared to the control. Both Al treatments increased Al concentration in roots in a dose-dependent manner, but only Al2O3 NPs showed higher Al uptake and translocation from roots to stems and leaves. In addition, significant adsorption and/or aggregation of Al2O3 NPs on the root surfaces as well as their translocation from root to stems and leaves were visually confirmed using fluorescently-labeled Al2O3 NPs with fluorescence and confocal laser scanning microscopic techniques. Further, Al2O3 NPs accumulation on the root surfaces appears to affect plant uptake of essential elements, lowering their concentration in plant tissues. These findings demonstrate that Al2O3 NPs behave differently from Al ions when interacting plants under hydroponic conditions. The higher uptake and translocation of NPs and lowered levels of nutrients in plant tissues are of special concern regarding the environmental safety of Al2O3 NPs.