Study On The Ageing Of Plastics And Effects Evaluation On Plants

Plastic pollution is becoming a serious global problem that may pose a potential threat to ecosystem function and human health.When plastics enter the environment,they undergo aging behaviors such as oxidation,pyrolysis,wind and light,turning them into micro（nano）plastics with smaller particle sizes.Therefore,it is very important to explore the aging behavior and aging process of plastics in the natural environment as well as its ecological environmental effects.In this paper,polyethylene plastic（PE）and polystyrene nanoplastic（PSNP）,which are widely used as research objects,are selected to explore the aging situation of plastics through weight loss rate analysis and scanning electron microscope（SEM）analysis.Meanwhile,Fourier transform infrared spectroscopy（FTIR）and two-dimensional correlation spectroscopy（2D-COS）were combined to analyze the aging characteristics of PE plastics.Based on previous studies,the aging process of plastic in soil was clarified,and plastic may be accumulated in soil in large quantities,accompanied by smaller particle size and changes in physical and chemical properties,which may affect the growth and development of co-existing plants.Therefore,a follow-up study was carried out to evaluate the effects of plastics on plants and their toxic effects.Edible plants（spinach and cucumber）were selected as the research objects,and potted control experiments were conducted to evaluate the effects of PSNP（before and after aging and at different concentrations）on plants under different treatment methods.The main conclusions are as follows:（1）Study the aging behavior and aging process of PE plastic under different conditions（plants,culture time and soil layer）.According to the weight loss rate data,it was found that the aging of PE plastic was not affected by planting plants,while the change of soil layer and culture time had a greater impact on the aging of PE plastic.SEM analysis showed that the PE plastic surface appeared slight cracking after 50 d,and became coarser with pits and cracks after 100 d.The surface structure changes at 0-10 cm were more significant than other soil layers.In addition,there was no significant effect on the surface morphology.FTIR spectral analysis and 2D-COS analysis showed that there were five peaks with obvious changes,located at the wavelength of 969,1364,1629,1733 and 3428 cm^-1,respectively.The peak intensity increased with the increase of time,and the carbonyl peak mainly appeared in the soil layer of 100 d and 0-10 cm.In 0-10 cm and 10-20 cm soil layers,the sequence changes were 969 cm^-1（C-H）>1364 cm^-1（-CH₃）>1629 cm^-1（C=C）>3428 cm^-1（-OH）>1733cm^-1（C=O）;However,no obvious sequence changes were observed in the soil layer of 20-30cm.These results further confirmed that culture time and soil layer played a leading role in the aging of PE plastics.（2）To study the response mechanism of spinach to PSNPs before and after aging.SEM analysis showed that primitive PSNPs and aged PSNPs were observed in the root,stem and leaf of spinach.Physiological and biochemical analysis（biomass,seed germination rate and root-shoot ratio）showed that PSNPs inhibited seed germination and seedling growth before and after aging,and the effect of aging PSNPs was more significant.Metabolomics studies showed that PSNPs in roots significantly affected the metabolic pathways involved in amino acid metabolism,membrane transport,lipid metabolism and carbohydrate metabolism,while PSNPs in leaves significantly affected the metabolic pathways involved in amino acid metabolism.The total metabolism of leaves was less disturbed than that of roots,and the pathway of disturbed metabolism was different from that of roots.Of the leaf metabolites that changed significantly,21 overlapped with the root metabolites that changed significantly.This may indicate that differential metabolites produced in roots exposed to PSNPs are transported to leaves.The content of amino acid metabolites in roots and leaves decreased and increased after PSNPs and PSNPs were applied,respectively.The content of carbohydrate metabolites changed only after PSNPs applied,and the content of saturated fatty acids increased significantly before and after PSNPs applied.However,the content of unsaturated fatty acids changed significantly in roots after aging PSNPs was applied.Therefore,aging PSNPs significantly affected the redistribution of nitrogen and carbon pools in spinach roots,reshaped membrane fluidity,enhanced oxidation resistance and maintained membrane integrity.（3）The response mechanism of cucumber to different concentrations of PSNPs was studied.SEM analysis showed that PSNPs were observed in stomata surface and stomata.Physiological and biochemical data showed no significant effect of PSNPs.Compared with the control group,low concentration of PSNPs significantly affected four metabolic pathways of carbohydrate metabolism.In addition,high concentrations of PSNPs significantly affected the metabolism of five carbohydrates and four amino acids,and high concentrations of PSNPs significantly affected the metabolites of amino acids,carbohydrates and antioxidants.These results suggested that the application of high concentrations of PSNPs could cause a wider range of effects.Transcriptome analysis showed that high concentration of PSNPs resulted in a higher number of differentially expressed genes（DEG）than low concentration of PSNPs.GO enrichment analysis of differentially expressed genes showed that high concentration of PSNPs had a greater impact on biological processes.KEGG enrichment analysis showed that carbohydrate metabolism and signal transduction pathway were almost down-regulated under low concentration exposure,while amino acid metabolism,lipid metabolism and carbohydrate metabolism were almost up-regulated with down-regulation of energy metabolism and signal transduction under high concentration exposure.These results indicate that low concentrations of PSNPs significantly affect plant growth and development,while high concentrations of PSNPs significantly activate plant defense system and regulate plant tolerance to environmental stress.