Uptake And Transformation Of ZnO Nanoparticles With Different Particle Sizes In Maize (Zea Mays L.) Seedlings And Its Toxicity Effects To Roots

Nanomaterials are new materials that have been widely used in recent years,and the environmental problems caused by a large number of extensive use cannot be ignored.Zinc-Oxide nanoparticles(ZnO NPs),as a typical nano-material,has been studied more and more in plants,especially the transformation in plants.But when the nanomaterial comes into contact with the plant,whether it is a complete particle or a morphological change into the plant body is a dynamic process.It is difficult to evaluate the transfer process of nanoparticles in plants(outside)in the body(outside)of nanoparticles,so it is only meaningful to establish the function of exposure nanomaterials and time gradient.Based on this,this study takes 9 nm and 40 nm ZnO NPs as the research object,takes maize seedlings as the test crops,and uses the corresponding concentration of zinc ions(Zn2+)as the control to study the absorption and accumulation of ZnO NPs in maize seedlings,Transformation mechanism.The main findings are as follows:After treatment with the three types of zinc materials,the zinc concentration in the roots,stems,and leaves of corn seedlings increased with time,and the average value of all treatments was 35 times higher than that of the zinc-free control.Different zinc forms have different effects on zinc content.After 1 day of root exposure,9 nm ZnO NPs treatment had the highest tissue zinc concentration(3138 mg kg-1),which was significantly higher than that of 40 nm ZnO NPs treatment(1920 mg kg-1).The results of buds showed that 9 nm ZnO NPs treatment(737 mg kg-1)and ZnSO4 treatment(1088 mg kg-1)were significantly higher than 40 nm ZnO NPs treatment(229 mg kg-1).The solubility results of the three zincs in Hoagland's solution showed that in the roots treated with 9 nm ZnO NPs,most of the accumulation of zinc was caused by the absorption of intact NP rather than soluble Zn,while 40 nm ZnO NPs were exposed to corn After the seedlings are rooted,transport is hinderedThe results of synchrotron-bas ed X-ray fluorescence microscopy(?-XRF)showed that 9 nm ZnO NPs first entered the apoplast of the root cap parenchymal cells through the root tip cell wall,and then finally reached the central column transport system of the vascular bundle through the endothelial layer.The results of malondialdehyde(MDA)showed that the MDA content in roots treated with 9 nm ZnO NPs for 1 and 3 days was higher than that of Zn2+ treatment.This indicates that the level of oxidative stress is higher than Zn2+.The MDA content in the 40 nm treatment did not increase significantly at the three sampling points.The trends of root activity,superoxide dismutase(SOD)and peroxidase(POD)are consistent with the trend of MDA on maize seedlings.At the same time,the effects of different treatments on biomass are not significant.This makes a theoretical basis for the use of nanomaterials in agricultural engineering.Transmission electron microscopy(TEM)results showed that there were dense particles in 9 nm ZnO NPs treatment and 40 nm ZnO NPs treatment in the vacuole of maize seedling roots.The particles in the vacuole gradually decrease with time until they disappear.Transmission electron microscope energy spectrum(TEM-EDS)and selected area electron dfffraction(SAED)results show that these dense particles contain ZnO This indicates that vacuole is one of the important areas for the accumulation and transformation of ZnO in the root system,and endocytosis and exocytosis is one of the main ways of transporting ZnO NPs in plant roots.The in vitro simulation results show that ZnO NPs are almost insoluble in the simulated solution of neutral pH.The pH value of the solution has the greatest influence on the conversion of ZnO NPs.Among the three substances in the vacuole(under the influence of a single component),phytic acid plays a key role in converting ZnO NPs into amorphous substances,followed by phosphate,and citric acid has the least effect on ZnO NPs.Visual MINTEQ software simulates the reaction of ZnO NPs in a mixed solution(and only considers the chemical composition of the plant).At a concentration that does not exceed the toxicity of plant zinc(300 ppm),the main form of zinc in plants is Zn2+.The results of this study illustrate the effect of ZnO NPs on maize seedlings,have a guiding significance for the application of ZnO NPs in agricultural engineering,and provide scientific evidence for the fertilizer industry to produce nano-fertilizers.