Foliar polystyrene nanoplastics modulate cadmium uptake and root-to-shoot translocation in hydroponic Spinacia oleracea L.: Time- and concentration-dependent responses.

The accumulation of plastics and heavy metals (HMs) in the environment has become a significant global concern due to their persistence, ubiquity, and potential ecological impacts. Atmospheric micro- and nanoplastics (MP, NP) are capable of undergoing long-range transport and can be deposited onto plant shoots through precipitation, thereby exposing aerial plant tissues to these particulate contaminants. In this study, the effects of varying concentrations of polystyrene nanoplastics (PSNPs) (0, 10, 50, 100, 200, and 400 mg L-1), applied via foliar spraying, and cadmium (Cd) (0 and 5 mg L-1) were evaluated on the growth parameters and chemical composition of spinach (Spinacia oleracea L.) grown under hydroponic conditions. The experiment was carried out over three exposure durations-2, 4, and 6 weeks-designated as t1, t2, and t3, respectively. Fifteen-day-old seedlings were transplanted into hydroponic containers and sampled at the end of each exposure period. The results demonstrated that PSNPs had a significant impact on plant growth and biomass accumulation. At t3, exposure to 200 mg L-1 of PSNPs resulted in an 83.33% and 87.34% increase in the dry weight of shoots and roots, respectively, compared to the control. However, this positive trend reversed at higher concentrations; at 400 mg L-1, shoot and root dry weights decreased by 49.87% and 27.87%, respectively, relative to the 200 mg L-1 treatment. Cd concentration in the shoot at t3 increased by 72.28% at 50 mg L-1 PSNPs compared to the control but declined by 31.32% at 400 mg L-1. The Cd translocation factor (TF) from root to shoot rose sharply with increasing PSNPs concentrations, reaching a peak increase of 194% at 400 mg L-1 at t3. Additionally, TF values exhibited a positive correlation with plant age, suggesting enhanced TF in older plants. These results highlight the potential risk of PSNPs in Cd-contaminated environments, where they may modify Cd uptake, distribution and root-to-shoot translocation in spinach plants. By facilitating Cd transfer to shoots, particularly under prolonged exposure, PSNPs could increase the likelihood of Cd accumulation in edible tissues. This interaction raises concerns regarding the safety of leafy vegetables grown in agricultural systems co-contaminated with MPs/NPs and HMs.