Effect Of Nanomaterials On Maize Growth And Resistance Against Herbivore In The Presence Of Earthworms

A United Nations report predicts that the world’s population will approach 9.8 billion in2050.Rapid population growth is putting enormous pressure on food security.In traditional agricultural production,increasing food production often requires the application of a large amount of chemical fertilizers and pesticides.However,excessive application of chemical fertilizers and pesticides can have a huge negative impact on the environment.With the rapid development of nanotechnology,there are more and more applications of nanomaterials in agriculture.Some nanomaterials（Si O₂ NMs,Ti O₂ NMs）can improve the bioavailability of nutrient elements,regulate plant growth and development,improve grain yield and quality,and enhance plant resistance.Earthworms are“soil ecosystem engineers”,which significantly regulate soil fertility through burrowing,secreting mucus,and casting,thus affecting crop growth and health.However,it has not been clearly studied how the response mechanism of crops will change upon exposure to nanomaterials in the presence of earthworms.Therefore,in order to better guide the safe and sustainable application of nanomaterials in farmland,in this study,Metaphire guillelmi was selected as the experimental soil animal,maize（Zea may L.）was selected as the experimental plant,and Spodoptera frugiperda was selected as the herbivore.In order to clarify the mechanism of nanomaterials in regulating soil nutrient availability,crop growth,and resistance against herbivore in the presence of earthworms,the drilosphere and rhizosphere soil processes,plant phenotypic traits and nutrient metabolism changes,and plant physical and chemical defense were studied.The main research results are as follows:1.Compared with sodium silicate,soil application of equivalent dose(10 mg kg^-1)of Si O₂NMs significantly increased the abundance of silicate solubilizing bacteria in the rhizosphere and drilosphere,which in turn increased the silicic acid in the rhizosphere and drilosphere soil.The application of 10 mg kg^-1 Si O₂ NMs alone had no significant effect on maize growth,but the combined application of 10 mg kg^-1 Si O₂ NMs and earthworms significantly increased the silicon content in maize roots and leaves,and then increased maize biomass.In addition,the results of maize root metabolomics showed that the application of 10 mg kg^-1 Si O₂ NMs alone significantly changed the carbon and nitrogen metabolism of maize roots.Under the combined application of 10 mg kg^-1 Si O₂ NMs and earthworms,the relative abundance of some sugars（maltose and fructose）,amino acids（phenylalanine and glutamic acid）,and small molecular organic acids（caffeic acid and fumaric acid）were further up-regulated.2.The effects of different concentrations of Si O₂ NMs(10,50,100 mg kg^-1)on the growth of maize in the presence of earthworms were studied.By evaluating the biomass and other indexes,it was confirmed that the combined application of 50 mg kg^-1 Si O₂ NMs and earthworms had the best growth-promoting effect.The combined application of earthworms and 50 mg kg^-1 Si O₂ NMs significantly activated the auxin-mediated maize growth and development process,and enhanced the photosynthesis in maize leaves,thus increased the total sugar and sucrose content in maize leaves,promoted the transfer of photosynthates from source to sink,and improved maize yield.Metabolomics was used to further analyze the changes of metabolites in maize grains.The combined application of earthworms and 50 mg kg^-1 Si O₂NMs significantly up-regulated the metabolites contents of glucose-1-phosphate,glucose-6-phosphate,fructose-6-phosphate,glucose,and fructose in maize grains.3.Compared with the lower concentration(10 mg kg^-1),the application of 50 mg kg^-1 Ti O₂NMs significantly reduced the abundance of denitrification function genes in drilosphere soil and suppressed the emission of the greenhouse gas nitrous oxide.At the same time,the application of 50 mg kg^-1 Ti O₂ NMs increased the abundance of nitrogen fixation and nitrification functional genes in maize rhizosphere,and increased the concentration of nitrate nitrogen in maize rhizosphere.By evaluating the maize biomass and net photosynthetic rate,it was confirmed that the combined application of earthworms and 50 mg kg^-1 Ti O₂ NMs achieved the best growth-promoting effect.The optimal culture system was selected to further study the metabolic response of maize leaves,and changes in grain yield and quality.Under the combined application of earthworms and 50 mg kg^-1 Ti O₂ NMs,the relative abundance of some phenolic acids（vanillic acid,ferulic acid,and rutin）,amino acids（leucine,glutamine,tyrosine,and threonine）,and sugars（glucose,fructose,and trehalose）in maize leaves were significantly up-regulated.The grain yield,and essential amino acids and total amino acids contents in maize grains were significantly increased under the combined application of earthworms and 50 mg kg^-1 Ti O₂ NMs.4.Under the stress of Spodoptera frugiperda,by measuring the larvae weight gain,it was confirmed that maize had the best resistance against herbivore under the combined application of earthworms and Si O₂ NMs.Further studies showed that the addition of Si O₂ NMs alone increased the contents of lignin,jasmonic acid,chlorogenic acid,and DIMBOA in maize leaves,and improved the physical and chemical defense of leaves.The addition of earthworm alone increased the contents of jasmonic acid,chlorogenic acid and DIMBOA in maize leaves,and improved the chemical defense of maize leaves.The combined application of Si O₂ NMs and earthworms further increased the content of physical defense and chemical defense substances in maize leaves.The random forest analysis model predicted that the five components of maize leaves that contributed most to the maize resistance against Spodoptera frugiperda larvae were DIMBOA,silicon,jasmonic acid,chlorogenic acid,and lignin.