Effect Of Nitrate Nitrogen On Nodule Nitrogen Fixation And Carbohydrate Distribution In Soybean

As an important grain and oil crop in China,soybean(Glycine max L.)has excellent nutritional characteristics and wide application.With the rapid development of economy in China,the demand for soybeans shows a rapidly increasing trend.In 2022,the import volume of soybean exceeded 90 million tons,and the self-sufficiency rate of domestic soybeans was only 18.5%.The contradiction between production and demand is prominent.Stable and high yield of soybeans is an important part of food security strategy in China.Soybean and rhizobia fix nitrogen in the air for their own growth through symbiosis.Application of nitrogen fertilizer can improve soybean yield,but excessive fertilizer nitrogen will result in adverse effects on nodule nitrogen fixation.Soil nitrogen and fertilizer nitrogen were mainly absorbed and utilized by soybean in the form of nitrate nitrogen.Nitrate nitrogen with high concentration inhibited nodulation and nitrogen fixation of soybean,which was related to carbohydrate distribution.Currently,the effects of nitrate nitrogen on nodulation,nitrogen fixation and carbohydrate distribution of soybean remain to be further studied.This experiment was carried out in the experimental base of Northeast Agricultural University in 2022.Under sand culture condition,the dual-root soybean with single side nodulated was used to carry out 2 experiments,namely the change of nitrogen supply concentration on the non-nodulated side and the grafting of phloem~+(including epidermis,cortex and phloem)on the non-nodulated side.The distribution of regulatory substances related to nodulation and nitrogen fixation,nitrate nitrogen and carbohydrate were determined,and the microstructure and submicrostructure of nodules were observed to study the effects of nitrate nitrogen on nitrogen fixation and carbohydrate distribution of soybean nodules.The results showed that:(1)Indirect supply of nitrate nitrogen to soybean nodules could systemically regulate nodulation and nitrogen fixation of nodules.In soybean dual-root system,the growth and nitrogen fixation ability of nodules on the nodulated side changed with the change of nitrate supply concentration on the non-nodulated side.The supply of nitrate nitrogen significantly decreased the number,dry weight and nitrogenase activity of nodules.When nitrate nitrogen was withdrawn,the number,dry weight and nitrogenase activity of nodules slowly rose with the increase of withdrawal days.Indirect supply of nitrate nitrogen to soybean nodules inhibited its growth and nitrogen fixation ability,which was reversible.Nitrate nitrogen systemically regulated the growth and nitrogen fixation ability of soybean nodules.(2)Indirect supply of nitrate nitrogen changed the anatomical structure of soybean nodules.When nitrate nitrogen was supplied to the non-nodulated side,the density of infected cells in the center of nodules decreased,and vacuoles appeared in mature infected cells,which gradually became irregular with the increase of nitrogen supply time;After nitrogen supply and nitrogen withdrawal,the structure of mature infected cells did not recover obviously,and the number of infected cells in the nodule center gradually increased;the addition of nitrate nitrogen could lead to the dissolution of symbiotic membrane and the increase of the PHB.With the increase of nitrogen application time,the symbiotic membrane gradually disappeared and the area of the PBS became larger.The ultrastructure of the destroyed nodules did not recover after the withdrawal of nitrate nitrogen.The results showed that the destruction of nitrate nitrogen on the structure of infected cells in nodules was irreversible,and the growth of infected cells in nodule center could resume after nitrogen withdrawal,which was one of the reasons why nitrogenase activity rose with nitrate nitrogen withdrawal.(3)In soybean dual-root system,nitrate nitrogen was absorbed by the root system on the non-nodulated side,then transported to the shoot through root basel and the stem,and then distributed to the roots and nodules on the nodulated side.Both the root basel and stem were the main channels for nitrate nitrogen transportation.The concentration of nitrate nitrogen in the stem,root base,bilateral roots,and nodules increases with the supply of nitrate nitrogen,but decreases with the withdrawal.The concentration of nitrate nitrogen in the stem will continue to increase with the increase of nitrate nitrogen supply time,and a certain concentration of nitrate nitrogen accumulated in the soybean stem.(4)In soybean dual-root system,the addition of nitrate nitrogen could cause carbon competition between underground parts.The contents of starch,sucrose and soluble sugar in lateral roots without nodulation increased with the supply of nitrate nitrogen,and decreased with the withdrawal of nitrate nitrogen.The changes of starch,sucrose and soluble sugar contents in lateral roots and nodules were opposite to those in lateral roots without nodulation.The addition of nitrate nitrogen promoted the distribution of photosynthetic products to non-nodulated and weakened the distribution of photosynthetic products to nodulation.(5)The inhibition of nitrate nitrogen supply on nitrogen fixation in soybean nodules was related to the decrease of carbohydrate in nodules.After phloem~+grafting on the non-nodulated side of soybean daul-root system,the distribution of carbohydrate in nodules was increased,and the nodulation and nitrogen fixation ability of nodules was improved;When nitrate nitrogen was supplied to the non-nodulated side and root phloem~+grafting treatment was carried out,carbohydrate distribution tilted to the nodulated side,which could obviously alleviate the inhibition of nitrate nitrogen on nodulation and nitrogen fixation.