The Regulatory Effect Of Water And Nitrogen Management On Lodging Resistance Of Winter Wheat Stems With Different Planting Densities

Increasing wheat yield is an important way to ensure food security.In the current wheat production,increasing the number of spikes per unit area is the most direct and effective technical measures to achieve high yield of wheat.However,population densification and improper application of fertilizer and water often lead to wheat lodging,which has become the key factors restricting the high yield and efficiency of wheat.Exploring the internal mechanism of wheat stem lodging and optimizing technical measures to improve the ability of wheat stem lodging resistance are the key issues in the research of wheat high-yield and high-efficiency cultivation.The experiment was carried out at Shandong Agricultural University Tai’an Agronomy experimental station.The first experiment was conducted during the 2016-2017 and2017-2018 wheat growing season.The lodging resistant wheat cultivar JM22 and the lodging sensitive wheat cultivar SN16 was used as experimental materials.Two irrigation treatments[rain-fed and spring irrigation（at jointing and anthesis stage）]were set with five planting densities(150×10⁴plants ha^-1,225×10⁴plants ha^-1,300×10⁴plants ha^-1,375×10⁴plants ha^-1,and 450×10⁴plants ha^-1).Lignin accumulation and its monomers（guaiacyl,syringyl,p-hydroxyphenyl）and the gene expression of key lignin synthase in combination with the apparent and microanatomic characteristics of the basal second stems,were determined to elucidate effects of irrigation on the formation of breaking strength in wheat stems at different planting densities.The second experiment was conducted during the 2019-2020 and2020-2021 wheat growing season.Shannong28,a lodging-resistant variety,and Shannong16,a the lodging sensitive variety,were selected as experimental materials.There were four levels of nitrogen fertilizer,which were N4(225 kg ha^-1),N3(202.5 kg ha^-1),N2(180 kg ha^-1),and N1(157.5 kg ha^-1);three planting densities were set,which were 225×10⁴ha^-1（D1）,375×10⁴ha^-1（D2）and 525×10⁴ha^-1（D3）,respectively.The relationships between mechanics,morphology,lignin accumulation,lignin monomer content and proportion,dynamic changes of endogenous hormone content and wheat stem lodging resistance were studied,in order to provide theoretical basis for optimizing cultivation measures to improve stem lodging resistance.The main results are as follows:1.Effect of irrigation on breaking strength of winter wheat stems with different planting densities（1）The mechanism of the difference of stem breaking strength among different wheat varieties and its response to planting density and irrigationThe typical morphological characteristics of lodging-resistant wheat varieties were large area of large and small vascular bundles,large number of large vascular bundles,thick wall,compact arrangement of mechanical tissue and high stem plumpness.High lignin accumulation,high G and S monomer content,low H monomer ratio and high expression of key lignin synthase genes were the typical physiological characteristics of lodging-resistant wheat.Increasing planting density and irrigation,plant height was increased,wall thickness,stem filling degree,G and S subunit contents were decreased,and the H monomer ratio was increased.Under irrigated and densified populations conditions,the decreasing range of wall thickness（-13.53%）,stem filling degree（-16.65%）,and the increasing range of H monomer ratio（15.77%）of high lodging resistance varieties were lower than those of SN16.Therefore,it held better ability to resist adverse environment and maintain lodging resistance.（2）The mechanism of irrigation on the breaking strength of wheat stems was different in planting density.Multiple stepwise regression analysis showed that stem filling degree,wall thickness,total lignin content,G monomer content,S monomer content,H monomer content,small vascular bundle area,and mechanical layer thickness were the main factors affecting the breaking strength of stems.Except the content of H monomer played a negative effect on enhancing the breaking strength of stems,all the other indexes showed a positive effect on elevating the breaking strength.It was found that under low density(150×10⁴plants ha^-1)conditions,irrigation decreased breaking strength of the stem mainly by affecting lignin accumulation and its subunits.However,at medium and high density(225×10⁴plants ha^-1-375×10⁴plants ha^-1),the decrease of stem breaking strength caused by irrigation was the result of the co-regulation of lignin metabolism,stem microscopic and morphological characteristics.At anthesis stage,when irrigated under the condition of low density(150×10⁴plants ha^-1),the content of G subunit was increased by 100μg g^-1,the stem breaking strength was increased by 0.4 N.At medium density(225×10⁴plants ha^-1-300×10⁴plants ha^-1),irrigation increased S subunit content by 100μg g^-1,which resulting in high stem breaking strength by 0.5 N.However,at high density(375×10⁴plants ha^-1-450×10⁴plants ha^-1),along with H subunit content was increased by 10μg g^-1,the stem breaking strength was decreased by 0.14 N.（3）Coordinating cultivation pathways for high-yield inversion resistance in wheatWith the increase of planting density,the yield was increased,and further increased the density,the yield was unchanged or decreased slightly,and the increase of yield benefited from the was increased of the number of effective panicles.In this experiment,on the basis of irrigation（at jointing and anthesis stage）and 375×10⁴plants ha^-1planting density resulted in the highest yield(10,325.03 kg ha^-1)and moderate stem.2.Effects of nitrogen application rate and planting density on lodging resistance of winter wheat stem（1）Effect of nitrogen application rate and density on the mechanical characteristics of wheat stemThe lodging-related morphology mechanism was different with genotypes.SN28 has a smaller lodging index than SN16,which was attributed to its higher breaking moment.The high breaking moment of SN28 depended on high section modulus（amount of stem material）,while the breaking moment of SN16 depended on the bending stress（stem material strength）.With the decreased of nitrogen application rate,the breaking moment and section modulus were increased,while the bending stress was decreased,and finally the lodging index was decreased.With the increased of planting density,the breaking moment,bending moment and section modulus were decreased,and finally the lodging index was increased.The analysis of the pathways showed that the breaking moment was an important parameter to determine the lodging index,and the breaking moment depends on the section modulus.（2）Effect of nitrogen application rate and density on morphological characteristics of wheat stemThe stem diameter and wall thickness of SN28 were larger than those of SN16,so its section modulus was higher than that of SN16.The plant height was increased with the increased of planting density,but showed the opposite trend with the decreased of nitrogen application rate.The plant height was increased with density and nitrogen fertilizer,it was mainly attributed to the elongation of basal internodes（I1+I2）.In addition,the height of center of gravity was increased with the increased of planting density and nitrogen application rate,while stem diameter,wall thickness and stem filling degree showed a downward trend.Correlation analysis showed that lodging index was positively correlated with plant height and center of gravity height,and negatively correlated with stem diameter,wall thickness and stem filling degree.（3）Effect of nitrogen application rate and density on the physiological characteristics of wheat stemsLignin plays a key role in the breaking strength of stems,so the inverted resistant cultivar SN28 had higher G,S,and H subunits and total lignin content than SN16.In this experiment,although the material of SN16 was hard（high bending stress）,but the amount of stem material was small,that is,the lignin content was low（low section modulus）,and finally showed a high lodging index.With the increase of planting density,the content and proportion of G and S lignin monomer were decreased,while that of H monomer was increased,the content of GA was increased,the content of JA was decreased,and the accumulation of stem lignin was decreased.However,reducing the amount of nitrogen application,the content of GA was reduced,the content of JA and the accumulation of lignin were increased,and the lodging resistance of the stem was enhanced.（4）Effect of nitrogen application rate and density on yield and composition of wheatThe grain number per spike and panicle number was decreased with the decrease of N application rate,which was decreased grain yield,but was increased the number of panicles with the increase of planting density,which made up for the loss of grain yield caused by the decrease of N application rate.Under the condition of this experiment,the lodging resistance and stable yield could be achieved by reducing nitrogen to 180 kg ha^-1and planting density of375×10⁴plants ha^-1.