## 3 Results and Discussion
### Phytotoxicity Assays  
Phytotoxicity assays with quinoa seeds were conducted on untreated soils and soils treated with Cd and/or HANPs to assess Cd toxicity and the effects of HANPs on seed germination and early plant growth. Germination percentages (G%), shoot and root lengths, as well as the calculated GIndex and ApIndex values, are presented in Table 4 To assess the potential toxicity of Cd and the effect of HANPs on germination, root and shoot growth were measured for each seedling, and the GIndex and ApIndex values were calculated. The observed germination percentages for untreated soils without Cd were in accordance with the typical values reported for quinoa under non-stress conditions [39]. A reduction in germination rates was not evident as Cd concentrations increased, highlighting the lack of impact of Cd on seed germination. The results for shoot length and ApIndex showed no significant differences in quinoa shoot growth between soils without Cd (0 mg kg\({}^{-1}\)) and those treated with 5 mg kg\({}^{-1}\) of Cd, regardless of the presence of HANPs. However, there was a notable reduction in shoot development and ApIndex values in seedlings grown in soils with Cd concentrations of 10, 25, and 50 mg kg\({}^{-1}\) without HANPs. For soils treated with HANPs, the results showed that the HANPs mitigated the toxic effects of Cd at concentrations of 10 and 25 mg kg\({}^{-1}\), as shown by the higher shoot lengths and ApIndex values compared to untreated soils. With the application of 50 mg kg\({}^{-1}\) of Cd, the HANPs did not have a significant effect on shoot growth or ApIndex values, indicating a limitation in their effectiveness at the highest Cd concentration applied. The improvement in shoot growth and PIndex values in HANP-treated soils can be attributed to the ability of HANPs to reduce Cd toxicity. HANPs likely immobilize Cd through mechanisms such as adsorption, ion exchange, or precipitation, which decreases Cd availability in the soil and water solution [49; 50]. Additionally, HANPs may provide calcium and phosphate ions to the plants, which could support cellular functions and partially mitigate the stress induced by Cd [51]. However, the lack of significant improvement at 50 mg kg\({}^{-1}\) Cd suggests that the mitigation capacity of 1% HANPs is limited under high metal stress, which highlights the importance of optimizing nanoparticle dosage based on the level of contamination. The root length and G\({}_{\text{Index}}\) results indicated that adding HANPs to soils without Cd generally did not promote early root growth compared to the control. The G\({}_{\text{Index}}\) values below 90% indicate an inhibitory effect on early root growth when HANPs were added to soils, with Cd concentrations between 10 and 50 mg kg\({}^{-1}\). The potential phytotoxicity of HANPs may be due to several factors, including their interaction with trace elements and soil components, which may limit the availability of nutrients to the plant, or cause structural alterations in the plants [52]. Root length and G\({}_{\text{Index}}\) are susceptible indicators of heavy metal toxicity, as root tissues are directly exposed to contaminated soil. The observed inhibition of root growth in untreated soils at Cd concentrations \(\geq\)10 mg kg\({}^{-1}\) is consistent with the known toxic effects of Cd, such as alterations in cell division and elongation [28]. The improvement in G\({}_{\text{Index}}\) with HANP treatments at moderate Cd concentrations (\(\leq\)10 mg kg\({}^{-1}\)) indicates the partial mitigation of these effects. However, the inhibitory trend at higher Cd concentrations (\(>\)10 mg kg\({}^{-1}\)) suggests that HANPs may not completely shield root systems under extreme stress. It is noteworthy that, despite the variability of results in soils treated with HANPs, there were no significant differences in root growth, regardless of Cd concentration, compared to the control soil without Cd. This suggests an improvement in root development for quinoa seedlings in Cd-contaminated soils when HANPs were applied.