Effects, uptake, and translocation of aluminum oxide nanoparticles in lettuce: A comparison study to phytotoxic aluminum ions

The widespread use of aluminum oxide nanoparticles (Al2O3 NPs) unavoidably causes the release of NPs into the environment, potentially having unforeseen consequences for biological processes. Due to the well-known issue of Al phytoxicity, plant interactions with Al2O3 NPs are cause for concern, but these interactions remain poorly understood. This study investigated the effects of Al2O3 NPs on lettuce (Lactuca sativa L.) to elucidate the similarities and differences in plant growth responses when compared to those of Al ions. Seed germination, root length, biomass production, and uptake of Al and nutrients were measured from hydroponically-grown lettuce with varying concentrations of Al2O3 NPs (0, 0.4, 1, and 2 mg/mL) or AlCl3 (0, 0.04, 0.4, and 1 mg/mL). The Al2O3 NPs treatments had a positive influence on root elongation, whereas AlCl3 significantly reduced emerging root lengths. While 0.4 mg/mL Al2O3 NPs promoted biomass, 1 and 2 mg/mL showed a 10.4% and 17.9% decrease in biomass, respectively, when compared to the control. Similarly, 0.4 and 1 mg/mL AlCl3 reduced biomass to 22.3% and 9.96%, respectively. Both treatments increased Al uptake by roots linearly; however, translocation of Al2O3 NPs into shoots was limited, whereas translocation of AlCl3 increased with increasing treatment concentration. Further, Al2O3 NPs adsorbed on the roots serve as adsorbents for macronutrients, promoting their absorption and uptake in plants, but not micronutrients. Calcium uptake was the most inhibited by AlCl3. A new in vivo imaging technique, with elemental analysis, confirmed that Al2O3 NPs were assimilated as particles, not ions, suggesting that the observed phytotoxicity is not due to Al ions being released from the NPs. Thus, it is concluded that Al2O3 NPs pose less phytoxicity than AlCl3, primarily due to NPs role on stimulated root growth, significant adsorption/aggregation on roots, limited lateral translocation to shoots, and increased uptake of macronutrients. [Display omitted] •The Al2O3 NPs aggregated onto root serve as adsorbents for plant macronutrients.•Al2O3 NPs help enhance uptake of plant macronutrients, but reduce micronutrients.•In vivo images of Al2O3 NPs in tissue indicate NPs are assimilated as particles.•Al2O3 NPs possess a lower potential than Al ions to translocate in shoots.•Al2O3 NPs do not undergo biotransformation after being taken up by roots.