Toxic Effects Of Combined Pollution Of Microplastics And Lead On Dandelion

In this paper,the effects of lead（Pb）on physiological indexes,antioxidant defense system,bioactive components,root microstructure and Pb uptake of dandelion seedlings in the presence of polystyrene（PS）carboxyl polystyrene（PS-COOH）and amino polystyrene（PS-NH₂）microplastics（MPs）were studied by hydroponic technique.The main results are as follows:（1）Effects of MPs and Pb combined pollution on dandelion photosynthesis and biomassThe single Pb treatment had no significant effect on chlorophyll content of dandelion seedlings,but significantly inhibited the activity of ribulose bisphosphate carboxylase（Ru Bis Co）,thereby induced biomass reduction.The inhibition of chlorophyll content and Ru Bis Co activity by the single PS-NH₂treatment was significantly higher than that of PS-COOH and PS treatment,indicating that positively charged surface of PS-NH₂has caused more toxicity.Compared to single Pb treatment,PS-COOH and PS-NH₂significantly increased the effect of Pb on plant photosynthesis,resulting in a decrease in dandelion dry weight by 1.56-6.23%,2.91-9.64%and 4.55-12.23%,respectively.（2）Effects of MPs and Pb combined pollution on dandelion antioxidant defense system.Compared with the control,single Pb and PS-NH₂treatments significantly increased the content of superoxide anion and hydrogen peroxide in the roots and leaves of dandelion,thus increased activity/content of superoxide dismutase,catalase,ascorbic acid and glutathione to reduce oxidative damage.However,the excessive reactive oxygen species led to an increase of malondialdehyde content,indicating an increase in the degree of membrane lipid peroxidation,may cause damage to the membrane structure.Compared with the single Pb treatment,the presence of MPs significantly increased the Pb oxidative damage to dandelion.（3）Effects of MPs and Pb combined pollution on bioactive components of dandelionCompared with the control,single Pb and PS-NH₂treatment significantly reduced the contents of flavonoids,polyphenols,polysaccharides,soluble protein and soluble sugarin dandelion seedlings.Compared with single Pb treatment,the presence of three MPs significantly increased the inhibitory effect of Pb on the above indexes,indicating that the medicinal and edible value of dandelion reduced.（4）Effects of MPs and Pb combined pollution on root microstructure of dandelionLaser confocal analysis showed that three MPs of different charges could penetrate into the root system of dandelion with fluorescence intensities of PS-NH₂>PS>PS-COOH,indicating that positively charged microplastics are more likely to enter the plants and exhibit more serious oxidative damage.In addition,the presence of Pb increased the light intensity of PS-COOH group.Transmission electron microscopy-energy dispersive X-ray spectra showed that Pb was mainly deposited in cell wall,vacuole or cytoplasm.（5）Pb accumulation in dandelion in the presence or absence of MPsCompared with single Pb pollution,PS-COOH and PS-NH₂reduced Pb content in roots and leaves of dandelion by 9.56-31.05%（3.34-23.18%）,18.51-39.48%（6.35-25.88%）,6.17-13.80%（3.49-10.98%）,respectively,indicating that the presence of different charged MPs inhibited the uptake of Pb by dandelion.This may be due to the influence of MPs on the adsorption sites of Pb on the root surface,inhibiting the root surface activity,thus affecting the uptake of Pb by the root.