Effect of foliar and root exposure to polymethyl methacrylate microplastics on biochemistry, ultrastructure, and arsenic accumulation in Brassica campestris L.

Despite the serious risk of microplastic pollution in the roots and leaves of crops, the phytotoxicity of microplastics (introduced via different exposure routes) in leafy vegetables remain insufficiently understood. Here, the effects of the root and foliar exposure of polymethyl methacrylate microplastic (PMMAMPs) on phytotoxicity, As accumulation, and subcellular distribution were investigated in rapeseed (Brassica campestris L). The relative chlorophyll content under PMMAMPs treatment decreased with time, and the 0.05 g L-1 root exposure decreased it significantly (by 9.97-20.48%, P < 0.05). In addition, superoxide dismutase (SOD), catalase (CAT), glutathione reductase (GR), and ascorbate peroxidase (APX) activities in rapeseed were more sensitive to PMMAMPs introduced through root exposure than through foliar exposure. There was dose-dependent ultrastructural damage, and root exposure had a greater impact than foliar exposure on root tip cells and chloroplasts. PMMAMPs entered the shoots and roots of rapeseed through root exposure. Under foliar exposure, PMMAMPs promoted As accumulation in rapeseed by up to 75.6% in shoots and 68.2% in roots compared to that under control (CK). As content in cell wall under PMMAMP treatments was 3.6-5.3 times higher than that of CK, as indicated by subcellular component results. In general, root exposure to PMMAMPs resulted in a stronger physiological impact and foliar exposure led to increased As accumulation in rapeseed.

Derivation and validation of soil cadmium thresholds for the safe farmland production of vegetables in high geological background area.

Excessive dietary intake of cadmium (Cd) poses toxicity risks to human health, and it is therefore essential to establish accurate and regionally appropriate soil Cd thresholds that ensure the safety of agricultural products grown in different areas. This study investigated the differences in the Cd accumulation in 32 vegetable varieties and found that the Cd content ranged from 0.01 to 0.24 mg·kg-1, and decreased in the order of stem and bulb vegetables > leafy vegetables > solanaceous crops > bean cultivars. A correlation analysis and structural equation model showed that pH, soil organic matter, and the cation exchange capacity had significant effects on Cd accumulation in the vegetables and explained 72.1 % of the variance. In addition, species sensitivity distribution (SSD) curves showed that stem and bulb vegetables were more sensitive to Cd than other types of vegetables. Using the Burr Type III function for curve fitting, we derived Cd thresholds of 6.66, 4.15, and 1.57 mg·kg-1 for vegetable soils. These thresholds will ensure that 20 %, 50 %, and 95 % of these vegetable varieties were risk-free, respectively. The predicted threshold of soil Cd was more than twice that of China's current National Soil Quality Standard (GB 15618-2018) for Cd values. Therefore, soil scenarios and cultivars should be considered comprehensively when determining farmland soil thresholds. The present results provide a new model for setting soil Cd criteria in high geological background areas.

[Effects of Nano-copper Oxide on Physiobiochemical Properties of Brassica chinensis L. and Its Heavy Metal Accumulation Under Cadmium Stress].

To investigate the effects of nano-copper oxide (CuO NPs) on plant growth, physio-biochemical characteristics, and heavy metal content under cadmium stress, a hydroponics experiment was conducted on the effects of single and combined treatments of CuO NPs (0, 10, 20, and 50 mg·L-1) and Cd (0, 1, and 5 µmol·L-1) on the fresh weight, photosynthetic pigment content, MDA content, antioxidant enzyme activity (CAT, POD, SOD, and GR), and Cu and Cd contents in Brassica chinensis L. The results showed that under the single addition of CuO NPs, the fresh weight and activities of CAT, POD, and GR were inhibited as a whole. Photosynthetic pigment content and SOD activity increased first and then decreased with the increase in CuO NPs concentration, whereas MDA content in leaves and roots, and Cu content in subcells of B. chinensis L. increased with the increasing of CuO NPs. As compared with that in the control, CuO NPs promoted the growth of B. chinensis L., and the fresh weight increased by 8.70%-44.87% at 1 µmol·L-1 Cd. When the content of Cd was up to 5 µmol·L-1, a low content (10 mg·L-1) of CuO NPs promoted the growth of B. chinensis L., whereas a high concentration (50 mg·L-1) showed an inhibitory effect. The addition of CuO NPs could increase photosynthetic pigment and MDA contents under different Cd stress, and MDA content in leaves and roots of B. chinensis L. increased by 4.34%-36.27% and 13.43%-131.04%, respectively, than that in the control groups. Under the same concentration of 1 µmol·L-1 Cd, the addition of CuO NPs decreased the activities of CAT and GR, whereas the activity of POD increased. When the content of Cd was up to 5 µmol·L-1, CuO NPs increased the POD activity and inhibited the activity of SOD and GR. The activities of CAT and CAT in the leaves of B. chinensis L. initially showed an increasing and then decreasing trend. CuO NPs and Cd showed antagonistic effects, the maximum reduction of Cd content in leaves and roots of Brassica chinensis L. under 1 µmol·L-1 Cd treatment was 45.64% and 33.39%, and that under 5 µmol·L-1 Cd treatment was 18.25% and 25.35%, respectively. The content of Cu and Cd in subcellular organs of the plants decreased, but the proportion of soluble components increased. These results indicated that CuO NPs at low concentrations promoted plant growth under Cd stress and further inhibited the absorption of Cd but increased the oxidative damage to B. chinensis L.

Single and joint toxicity of polymethyl methacrylate microplastics and As (V) on rapeseed (Brassia campestris L.).

Most microplastics and arsenic (As) have been released into farmland via industrial and agricultural activities, posing a potential threat to crop growth and food safety. Thus far, few studies have focused on the phytoxicity of microplastics and As to leafy vegetable. In this study, we evaluated the single and combined toxicological effects of polymethyl methacrylate (PMMA) and As(V) on rapeseed (Brassia campestris L.). Single treatments of two sizes of PMMA particles, namely PMMA nano-plastics (PMMANPs) and PMMA micro-plastics (PMMAMPs) and As(V) significantly (P < 0.05) inhibited the germination index (GI) of rapeseed. The IC50 indicates that PMMANPs were more toxic than PMMAMPs. Combine-pollution experiments demonstrated that the GI, biomass, root length, and sprout length of the rapeseed under the combined treatment were lower than those subjected to As(V) or PMMANPs single treatment. Analysis of variance showed that the interaction effects of PMMANPs and As(V) for GI and root length were significant, and there was synergistic interaction between PMMANPs and As(V) on rapeseed germination. PMMANPs promoted the accumulation of As in sprouts under high As(V) concentrations (40 and 60 mg/L). The activities of lipase in rapeseed generally increased under single and combined treatments of As(V) and PMMANPs, and while α-amylase activities first increased and then decreased with the increase of PMMANPs. It appears that the combined stress of microplastics and As(V) exhibited synergistic interaction on the growth of rapeseed.