Mechanistic Study On The Promotion Of Foliar Applied γ-Fe₂O₃ Nanoparticles To The Growth Of Soybean（Glycine Max）

Iron（Fe）is an essential micro-element for human body and plants.Crop yellowing and production loss caused by iron deficiency is a widespread agricultural problem and iron deficiency exists in 30-50%of soil around the world.Meanwhile,diseases such as anemia caused by iron deficiency are very common in clinic and affects almost 2 billion people worldwide.With the continuous development of nanotechnology,nanomaterials have been widely used in agricultural fields such as nanofertilizers,nanopesticides and nanosensors due to its high specific surface area and slow-release properties.Nanofertilizers are expected to become a new type of fertilizer to improve efficiency,reduce environmental impact and achieve the target of green agricultural production.Soybean（Glycine Max L.）is an important food crop in China,which is also a typical plant that uptake iron through reduction strategy.Therefore,it is very sensitive to iron deficiency.Iron deficiency causes huge economic losses due to soybean production loss every year.Research of new type iron fertilizer has become a hot spot currently.In this study,soybean was selected as the test crop,and nano-iron oxide（γ-Fe₂O₃ NPs）foliar application as the treatment method.The effects ofγ-Fe₂O₃ NPs on soybean growth,yield and nutrient elements in grains were explored by testing different concentrations,different particle sizes and different application frequency,so as to establish the optimal growth promoting system of soybean.In addition,as a symbiotic nitrogen fixation crop of soybean,γ-Fe₂O₃ NPs also has a certain effect on the growth of nodules.The promotion effect of different treatments on soybean nodule growth was evaluated to explore the related growth promoting effects and mechanism.The main research results are as follows:（1）The results of assessing promoting effects of different concentrations(10,30,50,100mg·L^-1)and different sizes ofγ-Fe₂O₃ NPs（S-Fe₂O₃ NPs,M-Fe₂O₃ NPs,L-Fe₂O₃ NPs）on soybean growth at seedling stage by foliar application show that 30 mg·L^-1 S-Fe₂O₃ NPs was confirmed as the optimal cultivation system by assessing soybean shoot and root biomass,photosynthesis parameters,and carbohydrate content.（2）30 mg·L^-1 S-Fe₂O₃ NPs is confirmed as the optimal cultivation system to promoting the growth of soybean nodules,the results showed that the number of bacteroids in nano-treatment group significantly higher than the control group.Transcriptome analysis showed that the genes related to carbohydrate assimilation and nitrogen metabolism in soybean nodules were significantly up-regulated.After foliar application ofγ-Fe₂O₃ NPs,poly-β-hydroxybutyrate（PHB）as carbon source accumulate in bacteroids,which is higher than the control group and EDTA-Fe treatment group.Under the treatment ofγ-Fe₂O₃ NPs,nodules conveyed more nitrogen metabolites to roots in the form of ureides.（3）After determining the optimal foliar application concentration and particle size ofγ-Fe₂O₃ NPs,the effects ofγ-Fe₂O₃ NPs on soybean yield and nutrition quality were investigated by setting different application frequencies（3 times and 6 times）.The results showed that soybean yield after 6 times was significantly higher than that after 3 times,EDTA-Fe treatment group and control treatment.The total carbohydrate content in soybean grains under 6 times application was about the same as that in EDTA-Fe treatment group,but the microelements（Fe,Cu,Zn）and protein content were significantly higher than those in other treatment groups,which indicated that 6 times application effectively promoted the accumulation of carbohydrate,protein and microelements in soybean grains.This study determined the optimal growth promoting system of soybean foliar applicationγ-Fe₂O₃ NPs,and revealed the mechanism of promoting soybean and soybean nodules growth byγ-Fe₂O₃ NPs,which provided a theoretical basis forγ-Fe₂O₃ NPs as a new type of iron nanofertilizer in agricultural production to improve soybean yield and the nutrition quality.