This study investigates the impact of neodymium (Nd) nanoparticle (NdNP) toxicity on the physiological and biochemical responses of sorghum ( Sorghum bicolor) and oat ( Avena sativa) plants and evaluates the potential mitigating effects of arbuscular mycorrhizal fungi (AMF). Sorghum and oat plants were grown under controlled conditions with and without AMF inoculation, and subjected to NdNPs (500 mg Nd kg- 1 soil). Results revealed that Nd nanoparticles significantly reduced biomass in both species, with a 50% decrease in sorghum and a 59% decrease in oats. However, AMF treatment ameliorated these effects, increasing biomass by 69% in oats under Nd nanoparticles toxicity compared to untreated contaminated plants. Soluble sugar metabolism was notably affected; AMF treatment led to significant increases in fructose and sucrose contents in both sorghum (+31% and +23%, respectively) and oat (+25% and +37%, respectively) plants under NdNPs toxicity. Improved sugar metabolism via enhanced activities of sucrose phosphate synthase (+29-54%) and invertase (+39-54%) enzymes resulted in higher proline (+21-81%) and polyamines (+49-52%) levels in AMF-treated plants under NdNPs toxicity, along with alterations in the biosynthesis pathways of amino acids and fatty acids, resulting in better osmoprotection and stress tolerance. Moreover, citrate (+29-55%) and oxalate (+177-312%) levels increased in both plants in response to NdNPs toxicity, which was accompanied by a positive response of isobutyric acid to AMF treatment in stressed plants, which potentially might serve as mechanisms for plants to mitigate NdNPs toxicity. These findings suggest that AMF can significantly mitigate Nd-induced damage and improve plant resilience through enhanced metabolic adjustments, highlighting a potential strategy for managing rare earth element (REE) nanoparticle toxicity in agricultural soils.

The damage excessive neodymium (Nd) causes to animals and plants should not be underestimated. However, there is little research on the impact of pH and associated ions on the toxicity of Nd. Here, a biotic ligand model (BLM) was expanded to predict the effects of pH and chief anions on the toxic impact of Nd on wheat root elongation in a simulated soil solution. The results suggested that Nd3+ and NdOH2+ were the major ions causing phytotoxicity to wheat roots at pH values of 4.5-7.0. The Nd toxicity decreased as the activities of H+, Ca2+, and Mg2+ increased but not when the activities of K+ and Na+ increased. The results indicated that H+, Ca2+, and Mg2+ competed with Nd for binding sites. An extended BLM was developed to consider the effects of pH, H+, Ca2+, and Mg2+, and the following stability constants were obtained: logKNdBL = 2.51, logKNdOHBL = 3.90, logKHBL = 4.01, logKCaBL = 2.43, and logKMgBL = 2.70. The results demonstrated that the BLM could predict the Nd toxicity well while considering the competition of H+, Ca2+, Mg2+ and the toxic species Nd3+ and NdOH2+ for binding sites.