Effects Of Microplastics On Physiological And Biochemical Characteristics Of Cucumber Seedlings

Microplastics（MPs）have received great attention as a new type of environmental pollutants in recent years,which are mainly derived from the degradation of plastics in various ways.Many studies have shown that MPs pollution in farmland soil environment is more serious than that in marine environment.At present,most of the research on MPs pollution focuses on marine organisms and algae,while the research on the ecological impact of MPs on farmland plants is limited.Although some studies have shown that MPs can affect the growth of farmland plants,the mechanism of its effect is still unclear.In this paper,the physiological and biochemical effects of different particle sizes PS（5,1,0.5 and0.1μm）at 10 mg·kg^-1concentrations and different types of MPs（PVC,PE and PS）at 2 concentrations(0.05 and 0.5 g·kg^-1)on cucumber（Cucumis sativus L.）were studied,and through transcriptome and metabolomics analysis,it tried to explore the mechanism of the effect of MPs on cucumber growth,providing new insights for the effect of MPs on the growth of plants in farmland.The main findings are as follows:1.Effects of different particle sizes PS on the physiological and biochemical effects of cucumbers:PS will inhibit the growth of cucumbers（plant height,aboveground fresh weight,aboveground dry weight,underground fresh weight and underground dry weight）.5,1 and 0.5μm PS significantly reduced chlorophyll a（Chl a）,chlorophyll b（Chl b）and carotenoid content in cucumber leaves.The net photosynthetic rate（Pn）and stomatal conductivity（Gs）of cucumber leaves decreased significantly,while the intercellular CO₂concentration increased significantly or remained unchanged,indicating that the inhibition of photosynthesis by PS on cucumber was non-stomatal restriction,and the larger the particle size,the more obvious the inhibition.The ultrastructure of chloroplast also undergoes large changes,the starch granules of mesophyll cells treated with 5μm became larger,the mesophyll cells inverting at 1 and0.5μm,and inclusions concentrated in the center of the vacuole at 0.1μm.The PSII maximum photochemical efficiency（F_v/F_m）and photosynthetic performance index(PI_abs)of PS-treated cucumber leaves decreased to a certain extent.5μm PS-treated cucumber seedlings had increased membrane permeability,malondialdehyde（MDA）content,CAT activity increased,and POD activity decreased.In addition,the PS treatment significantly decreased the proline content and increased the soluble sugar content of cucumber leaves.The PS treatment also reduced the mineral element content of cucumber leaves to varying degrees.2.Effects of different types of MPs on the physiological and biochemical properties of cucumbers:under the treatment of low concentration(0.05 g·kg^-1)and high concentration(0.5 g·kg^-1)of PVC,PS and PS significantly reduced the underground fresh weight and underground dry weight of cucumber plants.Both PVC and PS increased Chl a,Chl b and carotenoid content in cucumber leaves at both concentrations,while PE only increased Chl b and carotenoid content at high concentrations.The Pn and stomatal limit values（Ls）of cucumber leaves treated with high concentrations of PVC and PS were significantly reduced,while Ci increased or remained unchanged,indicating that non-stomatal limiting factors were the reason for inhibiting photosynthesis in cucumber leaves.PVC,PE,and PS increased the maximum fluorescence yield（F_m）and F_v/F_mvalues of cucumber leaves at low concentrations,while PE and PS decreased PI_absvalues of cucumber leaves at high concentrations,while PVC had no significant effect on chlorophyll fluorescence parameters.At low concentrations,MPs decreased the MDA content SOD activities of cucumber leaves,but increased CAT activity;and the POD and APX activities of PVC and PS treatments decreased at high concentrations,and the CAT activity of PVC treatment was increased,and the Pro content was also increased.At low concentrations,MPs increased the K,Na,Fe and Zn content of cucumber leaves,while reducing Ca and Mg content,while MPs only increased Zn content at high concentrations.3.Through transcriptome analysis,the effects of PS on the physiological and biochemical characteristics of cucumbers were closely related to the phenylpropane biosynthetic pathway,carbohydrate metabolism,amino acid metabolism,signal transduction pathway,and plant pathogen interaction.Phenylpropane biosynthesis affects the morphological structure of the cell wall by regulating lignin synthesis.The genes of butyric acid metabolism,amino sugar and nucleotide sugar metabolism,pyruvate metabolism,and enzymes that play a key role in the TCA cycle are generally downregulated,inhibiting photosynthesis in cucumber leaves.MAPK signaling and plant hormone signaling transduction,as well as downregulation of key genes for plant pathogen interaction,affected the ability of cucumber plants to resist abiotic stress.Through metabolomics analysis,the KEGG pathways enriched by DEMs were flavonoid biosynthesis pathway,isoflavone biosynthesis,carotenoid biosynthesis,biosynthesis of styrene,diarylheptane and gingerol,and plant hormone signaling pathways.Combined analysis of transcriptome and metabolome data also showed that differentially expressed genes and differential metabolites were mainly associated with secondary metabolism such as flavonoid biosynthetic pathways.In summary,the effect of MPs on the growth of cucumber plants has a certain relationship with the particle size and concentration of MPs,and the transcriptome and metabolomics analysis show that the inhibition effect of MPs on the photosynthesis of cucumber leaves and the effect on the disease resistance of cucumber plants are closely related to lignin synthesis,carbohydrate metabolism,plant hormone signal transduction and plant pathogen interaction.Based on this,we speculate that MPs will hinder cucumber lignin synthesis,change plant cell wall structure,and then affect the key enzymes of cucumber leaves to regulate carbohydrates MPs regulate carbohydrate key enzymes,promote the inhibition of photosynthesis of cucumber seedlings,and ultimately affect the growth and disease resistance of cucumber plants.In this paper,the influence mechanism of MPs on the physiological and biochemical characteristics of cucumber seedlings is discussed,which lays a foundation for the in-depth study of the effects of MPs on farmland plants.