Study On The Physiological Response Of Polyethylene Microplastics To Maize Seedings(Zea Mays L.)

Microplastics are plastic fragments and particles with particle size less than 5 mm.In recent years,microplastics have attracted much attention as pollutants.However,most of the research has focused on the alteration of water ecosystems and soil physical and chemical properties,and less on the effects of microplastics on higher plants.Maize is one of the three major crops in China,the toxicity and mechanisms of microplastics to maize are very scarce.In order to investigate the effect of microplastics on maize growth,polyethylene microplastics with high content in soil were selected as the research object to analyze the effect of maize seed germination and seedling growth by hydroponic and soil cultivation experiments.The micron-grade（5,180,800μm）and millimeter-grade（1,3,5 mm）polyethylene microplastics at different concentrations（0.05%,0.1%,0.2%and 0.3%）were adopted in this study.The physiological response mechanisms of polyethylene microplastics on maize seedlings were studied in terms of maize germination,photosynthetic properties,oxidative stress and nutrient elements.The main results are as follows:（1）The effect of polyethylene microplastics on the germination of maize seed and the growth of sprouts was analyzed.The particle size of polyethylene microplastics has no significant effect on the germination rate and germination potential of maize at the same concentration.The high concentration（3000 mg/L）of microplastics significantly inhibited the germination potential and germination rate of maize at the same particle size.In addition,microplastics inhibited the growth of maize and had effects on growth indicators such as coleoptile length,mesocotyl length,specific root length,specific root surface area and biomass of maize seedlings.The influence degree was correlated with the concentration and particle size of microplastics.（2）The effects of polyethylene microplastics on the growth and photosynthetic characteristics of maize seedlings were analyzed based on soil potting experiments.Compared with the control group,polyethylene microplastics had little effect on the growth of maize leaf length;the root biomass and root crown ratio of maize seedlings showed an increasing trend under microplastic exposure.The stem diameter only tended to increase under the exposure polyethylene microplastics with particle sizes of 3 and 5 mm,while the rest of the particle sizes had no significant effect.When polyethylene microplastics of the same particle size were exposed,the 3 and 5 mm microplastics inhibited the growth of maize plant height in the low concentration treatment group（500,1000 mg/kg）and significantly promoted the growth of maize seedlings in the high concentration treatment group（3000 mg/kg）;micron-grade microplastics（5,180 and800μm）had no significant effect on the height of maize seedlings.Microplastics showed different degrees of inhibition on specific root length and specific root surface area of maize seedlings,with inhibition rates ranged from 0～124.36%and 6.25～148.75%,respectively.Photosynthetic rate,transpiration rate,stomatal conductance and chlorophyll a of maize seedlings were significantly enhanced under 3 mm and 5 mm polyethylene microplastic,and it was no significant effect regular in used concentration.However,the 5μm,180μm,800μm and 1 mm polyethylene microplastics have no significant effect on the photosynthetic rate,transpiration rate,stomatal conductance and chlorophyll of maize.Correlation analysis showed that photosynthetic rate was positively correlated with transpiration rate,stomatal conductance and chlorophyll a,while it was not correlated with intercellular CO₂ concentration and chlorophyll b.（3）The effect of polyethylene microplastics on the antioxidant system of maize seedlings was analyzed based on soil pot experiments.The superoxide dismutase and peroxidase activities of maize seedlings exposed to 800μm,1 mm,3 mm and 5 mm polyethylene microplastics showed an increasing trend with increasing concentration.And the malondialdehyde content of maize seedlings decreased with increasing polyethylene microplastic concentration,which promoted the operation of the antioxidant system of maize seedlings.The superoxide dismutase and peroxidase activities showed a tendency to increase and then decrease with increasing concentrations of 5 and 180μm polyethylene microplastics,and the malondialdehyde content of maize seedlings showed a tendency to decrease and then increase with increasing concentrations of polyethylene microplastics.In particular,the antioxidant system of maize seedlings was damaged under the influence of high concentrations（3000 mg/kg）of polyethylene microplastics with particle sizes of 5 and 180μm.The overall results indicate that polyethylene microplastics stimulate the antioxidant system by altering the superoxide dismutase and peroxidase activities of maize seedlings and thereby causing the antioxidant system to respond to stress.（4）The effect of polyethylene microplastics on nutrient elements of maize seedlings was analyzed based on soil potting experiments.Nitrogen,phosphorus and potassium contents of shoot and root of maize seedlings were inhibited to different degrees by exposure to micron-sized polyethylene microplastics compared to the control group.The range of inhibition rate of nitrogen,phosphorus and potassium content in shoots were 5.07～13.68%,2.65～20.50%and 10.72～46.80%,respectively.The range of inhibition rate of nitrogen,phosphorus and potassium content in roots were 2.42～13.68%,2.80～19.58%and 2.42～59.90%,respectively.Millimeter-scale microplastics（3 and 5 mm）promoted the uptake of n potassium by maize seedlings;with the same particle size,the shoot and root nitrogen,phosphorus and potassium contents of maize seedlings varied irregularly with increasing microplastic concentration.Correlation analysis showed that the shoot and root nitrogen,phosphorus and potassium contents of maize seedlings exposed to micron-grade polyethylene microplastic environment were correlated with reduced root volume,surface area,and total root length compared to the control group,indicating that microplastics may have some effect on nutrient elements and root growth of maize.This study reveal the impact of polyethylene microplastic pollution in farmland soil on maize,and it provide a theoretical basis and reference for the ecological risk assessment of microplastics on higher plants.