Abstract:

A pot experiment was conducted under controlled conditions to comprehensively understand the toxic effects and ecological risks of microplastics on the grassland agro-ecosystem. Two types of microplastics， low density polyethylene （mLDPE） and polylactic acid （mPLA）， were added to soil at five different doses （0.1%， 0.5%， 1%， 5%， 10%， w/w， dry soil weight）. The growth and physiological characteristics of alfalfa （Medicago sativa） plants growing in these treatments were evaluated and compared with those of plants in the control （no microplastics）. The results showed that a high concentration （10%） mPLA reduced the biomass， total chlorophyll content， and net photosynthetic rate of alfalfa plants， while mLDPE significantly decreased the aboveground biomass and net photosynthetic rate of alfalfa plants. The malondialdehyde （MDA） content and the activities of superoxide dismutase （SOD）， peroxidase （POD）， and catalase （CAT） increased as the dose of MPs increased. Compared with the control， the 10% mPLA treatment group showed a 67.30% higher MDA content and 56.68% and 85.63% higher activities of SOD and CAT， respectively. Among the mLDPE treatments， only the 10% dose significantly affected SOD and POD activities. The results of this study show that the toxic effect of microplastics on alfalfa is related to both the type and concentration of the microplastics. High concentrations of mPLA strongly affected alfalfa plants by damaging the photosynthetic system， decreasing plant photosynthesis， and causing oxidative damage， resulting in growth inhibition， which had a stronger influence than mLDPE.

Key words: microplastic, Medicago sativa, photosynthesis, activity of antioxidant enzyme