Effects of the combination of polystyrene nanoplastics and lead on seed germination and seedling growth of spinach (Spinacia oleracea)

Microplastics (MPs) are a new environmental pollutant that has attracted widespread attention because of their negative effects on organisms and the environment. However, studies on the impact of co-contamination of MPs and heavy metals on vegetables are limited. To explore the effects of polystyrene nanoplastics (PSNPs), lead (Pb), and their co-contamination on seed germination and seedling growth of spinach, we investigated the germination rate, germination vigor and germination index of seeds; root length, shoot length, superoxide dismutase (SOD) and peroxidase (POD) activities, and soluble protein content of seedlings of spinach (Spinacia oleracea), which were exposed to the control, PSNPs (200, 400, 800, and 1600 mg·L⁻¹) and Pb (5, 25, 50, and 100 mg·L⁻¹) and their combination (Pb 5 mg·L⁻¹ + PSNPs 200 mg·L⁻¹, Pb 5 mg·L⁻¹ + PSNPs 800 mg·L⁻¹, Pb 50 mg·L⁻¹ + PSNPs 200 mg·L⁻¹, and Pb 50 mg·L⁻¹ + PSNPs 800 mg·L⁻¹). Single effects of PSNPs (≥400 mg·L⁻¹) significantly decreased the germination rate, vigor, and index; however, there was no significant difference between 200 mg·L⁻¹ PSNPs and the control for those indicators. PSNPs at low concentrations (200 mg·L⁻¹) promoted the length of roots and shoots, but other PSNPs concentrations (≥400 mg·L⁻¹) had no significant impacts on roots and shoots. SOD activity was inhibited at a high concentration (≥800 mg·L⁻¹) of PSNPs, and the POD activity was induced when PSNPs ≤800 mg·L⁻¹, whereas POD was inhibited at the high PSNPs concentration (1600 mg·L⁻¹). The soluble protein content in spinach seedlings under different concentrations of PSNPs increased, but the content was significantly higher than the control at 800 mg·L⁻¹ PSNPs. Under Pb exposure alone, germination rate, vigor, and index decreased. Further, treatments with low Pb concentration (5 mg·L⁻¹) increased root and shoot length, whereas high concentrations (≥25 mg·L⁻¹) reduced them. Moreover, SOD inhibition and POD induction were observed following Pb treatment. With increased Pb concentration, the soluble protein content of spinach seedlings decreased firstly at low concentration (5 mg·L⁻¹) and then increased. Compared with single Pb treatment, combined effects of PSNPs and Pb were generally antagonistic to seed germination. For example, PSNPs weakened the promotion effects of low Pb concentration (5 mg∙L⁻¹), inhibited the growth of root and shoot of spinach seedlings, and alleviated the inhibitory effects of high Pb concentrations (50 mg∙L⁻¹) on seedling root and shoot growth. Low concentration (200 mg·L⁻¹) of PSNPs and Pb showed synergistic effects in spinach seedlings, such as enhanced induction effects of Pb on POD activity. The co-contamination of PSNPs at high concentrations (800 mg·L⁻¹) and Pb caused greater damage to seedlings, and the activities of SOD and POD decreased significantly. These results showed that PSNPs alleviated the inhibitory effects of Pb on spinach seed germination. Low concentrations of PSNPs (200 mg∙L⁻¹) and Pb mainly showed synergistic effects, whereas high concentrations of PSNPs (800 mg∙L⁻¹) and Pb mainly showed antagonistic effects. This study demonstrates that co-contamination of PSNPs and Pb has significant toxicity on seed germination and seedling growth, affecting the antioxidant system and soluble proteins of spinach. In conclusion, coexisting PSNPs can alter the bioavailability of Pb and plant performance. Our findings can help evaluate the individual and comprehensive toxicity of microplastics and heavy metals in vegetable crops.