Toxic Effects Of Polystyrene Micro/Nanoplastics And Phenanthrene On Rice Seedlings

The continuous emergence of new pollutants creates new and ongoing ecological and environmental problems and is a severe challenge in environmental science research.Microplastics（MPs）and polycyclic aromatic hydrocarbons（PAHs）are important emergent pollutants,and at present,research on the ecological toxicity of micro/nanoplastics（MP/NPs）has mostly focused on marine organisms.In recent years,attention has gradually been paid to terrestrial plants;however,research on the toxicity of MP/NPs with functional groups in higher plants is still scarce,especially in the case of coexistence with organic pollutants,and research on the changes in toxic effects and the mechanism of action is rarely reported.In this study,rice（Oryza sativa L.）was selected as the focal organism,and polystyrene（PS）PS-MPs/PSNPs and phenanthrene（Phe）were selected as the pollutants.The adsorption of PS-MPs/PSNPs on Phe and their absorption and distribution characteristics in rice were studied,and the single and combined toxic effects of PSMPs/PSNPs and Phe at different concentrations,particle sizes,and functional groups on rice were explored.The main results are as follows.1.The adsorption of Phe on different particle sizes（50 nm,5μm）of MP/NPs（PS,PS-COOH,and PS-NH₂）is different.The adsorption of PS on Phe was stronger than that of functional PS,whereas the adsorption of NPs on Phe was higher than that of MPs,and the main mechanism was hydrophobic andπ-πinteractions.Confocal scanning microscopy confirmed that PSNPs were absorbed by roots and transported to the upper ground through the xylem.The amount of NPs absorbed by rice was in the order PS-COOH>PS>PS-NH₂.The root mature zone absorbed more,whereas the root tip absorbed less,which was mainly affected by the hydrophobicity,surface charge,and clustering of NPs.The distribution characteristics of Phe content in roots and shoots were as follows:Phe was significantly higher in roots than in shoots.There were rapid and slow absorption stages of Phe in rice,mainly related to the absorption mode.Due to adsorption,the content of Phe absorbed by rice in the composite treatment group decreased significantly,and the absorption curve of Phe changed.The absorption peak was delayed,mainly due to passive absorption.At the same time,water channel protein-mediated facilitation and diffusion were involved,whereas the active absorption process was affected,which was related to the absorption mode of NPs and the blocked energy transmission of plants.2.Different concentrations and particle sizes of PS had no significant effect on rice seed germination percentage,germination potential,and bud length,but inhibited the seed germination index,vigor index,and root length,which are sensitive indicators of the effect of PS on rice seed germination.Phe had a stronger inhibitory effect on rice seedling seed germination than by PS and had a dose–effect relationship.PS+Phe combined treatment alleviated the toxicity.3.MP/NPs can damage the antioxidant enzyme system of rice,reduce the ascorbic acid content,intensify membrane lipid peroxidation,inhibit photosynthesis,and affect rice growth.Its toxic mechanism mainly involves inhibition of the reactive oxygen species（ROS）metabolic process and the pathway related to the antioxidant system to reduce the response of seedlings to external pressure and stimulation.Higher concentrations have a greater impact on the DNA and photosynthesis of seedlings.The mechanism of influence of MP/NPs is different because of the different particle sizes and functional groups.Moreover,50 nm PS-NH₂showed the strongest inhibitory effect on rice.Its toxic mechanism is severe DNA damage to rice seedling shoots and the toxic effect on cell wall biosynthesis,which is related to the positive electricity of PS-NH₂.PS-COOH mainly affects the photosystem reaction center and thylakoids of young rice seedlings,regulates ion transport and the terpene biosynthesis pathway,and its toxicity to rice seedling leaves is greater than that of PS-COOH.Furthermore,5μm PS-COOH influences rice to accumulate more H₂O₂ and MDA and has a stronger toxic effect on rice than PS-NH₂ and PS.In the same functional group,MPs had less of an effect on rice than NPs,but the opposite was true for PS-COOH.4.Phe significantly inhibited the growth of rice,and the inhibition increased with increasing Phe concentration.The toxic mechanism of Phe is to induce oxidative stress reactions in rice seedlings,reduce the net photosynthetic rate,affect endoplasmic reticulum protein processing,weaken plant disease resistance-related pathways,and affect the growth of rice seedlings.Superoxide dismutase is the main scavenger of ROS accumulation caused by Phe removal in rice seedlings.5.Under the combined treatment of MP/NPs and Phe,compared with that in the control,the combined pollution still had a toxic effect.However,when the concentrations of the two pollutants were low,the combined treatment reduced Phe absorption by rice seedlings,the degree of lipid peroxidation,and the inhibition intensity of antioxidant enzyme activity and photosynthesis and alleviated the toxicity of the two pollutants.When both pollutants were present in high concentrations,their toxicity was enhanced.When the functional groups of MP/NPs were different,Phe+PS-NH₂ increased their surface hydrophobicity,reduced membrane damage to rice seedlings,and alleviated the toxic effects of the two pollutants.However,Phe+PS-COOH enhanced the toxic effects on shoots.It mainly affected rice photosynthesis,reduced chlorophyll content,net photosynthetic rate and transpiration rate.Catalase was the main ROS scavenger in each composite treatment group.According to GO enrichment and KEGG function analysis,the main detoxification mechanisms were the activation of related pathways of carbohydrate and energy metabolism,ROS response,plant hormone signal transmission,and photosynthesis in rice after the combined treatment of Phe and PS.Phe+PS-NH₂ mainly activated the metabolic pathway of the rice cell wall,cell membrane generation and defense,accelerated the induction of plant antitoxin synthesis,enhanced the disease resistance of rice,significantly regulated ribosomal RNA（r RNA）generation,and repaired rice DNA damage.PS-COOH+Phe can affect the metabolism of various amino acids and synthesis of various secondary metabolites in the roots of rice,which may be the reason for mitigating the toxic effect.However,the combined treatment affected the process of terpene metabolism and salicylic acid biosynthesis in the shoots of rice,inhibited the production of chlorophyll in rice,and reduced the ability to resist stress.