The phytotoxic effects of soil contaminated by microplastics and heavy metal are one of the environmental issues of widespread concern.In order to clarify the effects of composite exposure of polyethylene(PE)microplastics and heavy metals(Pb),the common pollutants in soils,on seed germination,the germination experiments were conducted on wheat seeds and soybean seeds;the effects of single PE microplastics(0,100,300,500,1 000 and 1 500 mg/L)and the effects of single Pb(0,50,100,200,and 400 mg/L),as well as the combined effects of PE microplastics and Pb(300 mg/L-1+100 mg/L,300 mg/L+200 mg/L-1,500 mg/L+100 mg/L,500 mg/L+200 mg/L)on the germination and root elongation of wheat seeds and soybean seeds were studied.The experimental results show that the germination rates of wheat seeds and soybean seeds respectively were 84.83％-100％and 76.67％-100％,and the root lengths respectively were 7.32-9.18 cm and 10.90-12.30 cm when PE microplastics were from 100 mg/L to 1 500 mg/L.The germination and root elongation of wheat seeds and soybean seeds overall were promoted at low concentration of PE microplastics,and inhibited at high concentration of PE microplastics.Single Pb contamination inhibited the germination of wheat seeds and soybean seeds,and as the concentration of exposed Pb increases,the germination rates decreased,while the root elongation increased first and then decreased.Among them,PE microplastics and Pb pollution had more significant effects on soybean seed germination and root elongation.The inhibition strength of PE microplastics and Pb combined exposure on seed germination and root elongation of wheat and soybean increased with the increase of PE microplastics and Pb concentrations.PE microplastics can promote seed germination and root elongation,and antagonizing the effect of Pb pollution on seed germination and root elongation.However,the intensity of the antagonistic effect weakens with the increase of PE microplastics concentration and Pb concentration.Microplastics are hydrophobic,small in particle size and large in specific surface area,which can easily be adsorbed by the seed surface,thereby affecting the water absorption and respiration of the seed.which can easily be adsorbed by the seed surface,thereby affecting the water absorption and respiration of the seed.This may be a possible mechanism for inhibiting seed germination and root elongation.Microplastics have negative charge and strong electrostatic adsorption with Pb2+,and the decrease of the contact between Pb2+and seeds through competitive adsorption with seeds is the possible reason for PE microplastics to antagonize the toxicity of Pb to wheat seeds and soybean seeds.
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