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

The widespread use of aluminum oxide nanoparticles (AlO 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 AlO NPs are cause for concern, but these interactions remain poorly understood. This study investigated the effects of AlO 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 AlO NPs (0, 0.4, 1, and 2 mg/mL) or AlCl (0, 0.04, 0.4, and 1 mg/mL). The AlO NPs treatments had a positive influence on root elongation, whereas AlCl significantly reduced emerging root lengths. While 0.4 mg/mL AlO 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 AlCl reduced biomass to 22.3% and 9.96%, respectively. Both treatments increased Al uptake by roots linearly; however, translocation of AlO NPs into shoots was limited, whereas translocation of AlCl increased with increasing treatment concentration. Further, AlO 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 AlCl. A new in vivo imaging technique, with elemental analysis, confirmed that AlO 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 AlO NPs pose less phytoxicity than AlCl, primarily due to NPs role on stimulated root growth, significant adsorption/aggregation on roots, limited lateral translocation to shoots, and increased uptake of macronutrients.