| Nitrogen is one of the essential elements for crop growth,and it plays an important role in increasing grain production.In wheat production,increasing the input of nitrogen fertilizer is often used to increase the biomass of the population to increase yield,but superfluous nitrogen fertilizer input usually causes excessive tillers,hidden populations,poor stalk development,and increased lodging risk.Therefore,in-depth study of the mechanism of nitrogen rate effect on wheat lodging resistance is of great significance for coordinating the contradiction between high yield and lodging resistance in wheat production and formulating reasonable cultivation measures.1.In 2017-2018 and 2018-2019 growing seasons,field experiments were performed by using three wheat varieties jimai22?JM22,lodging-resistance variety?,shannong16?SN16,middle lodging-resistance variety?and taikemai33?lodging-susceptible variety?with four nitrogen levels(0 kg ha-1,120 kg ha-1,240 kg ha-1,360 kg ha-1)at Shandong Agricultural University Agronomy experimental station.The objectives of this study were to investigate the effect of nitrogen rate on wheat lodging resistance,and explain the mechanism of nitrogen on wheat lodging resistance from the terms of stem mechanical characteristics,morphological characteristics,anatomical characteristics,carbohydrates metabolism and accumulation.2.In 2015-2016 and 2016-2017 growing seasons,field experiments were conducted by using‘Jimai 22'with three nitrogen rates(120 kg ha-1,240 kg ha-1,360 kg ha-1)and four irrigation treatments(no irrigation;irrigation at jointing stage;irrigation at jointing stage and anthesis stage;and irrigation at wintering stage,jointing stage and anthesis stage,each irrigation amount was 600 mm3ha-1)at Shandong Agricultural University Agronomy experimental station.The aims of this study were to discussed the nitrogen and irrigation interaction measures to coordinate the high yield and lodging resistance of wheat.The main results are as follows:1.The mechanism of nitrogen rate affecting lodging resistance in different wheat varieties?1?In terms of the mechanical characteristics,compared with SN16 and TK33,the yield of JM22 was higher,but its lodging index and lodging rate were lower due to its higher breaking moment.With nitrogen rate increased,the lodging index was significantly increased,while the breaking moment and bending stress was significantly reduced.In addition,with nitrogen rate increased,the lodging time of SN16 and TK33 were advanced,lodging rate were increased,resulting in a decrease in yield.Path analysis showed that the breaking moment and bending stress were the key parameters that determine the lodging index.Nitrogen rate mainly reduced the bending stress,which was responsible for breaking moment reduction,and finally led to stem strength deterioration.?2?In terms of the morphological characteristics,the breaking moment were negatively correlated with the basal internode length,plant height,and culm wall thickness,while the lodging index was the opposite.Increasing the nitrogen rate,the basal internodes were significant elongation,resulting in the increase of the plant height and the center of gravity height,and the bending moment of the whole plant increased;the outer diameter first increased and then decreased,and the wall thickness decreases significantly along with nitrogen rate increased.Proper nitrogen rate was beneficial to increase the outside diameter,while excessive nitrogen rate resulted in a significant decrease of outer diameter,wall thickness and stem strength.The results showed that shortening the length of basal internodes,decreasing the inner diameter and increasing the outer diameter and wall thickness were beneficial to increase the mechanical strength of stem?3?In terms of the anatomical characteristics,the area of large and small vascular bundles and mechanical tissue thickness were significant positively correlated with the breaking moment.Compared with SN16 and TK33,JM22 had a higher lignification degree,tighter arranged vascular bundle structure,thicker cell wall of mechanical tissue,which were responsible for strong stem strength.With the nitrogen rate increased,the degree of stem lignification decreased,the number of large vascular bundles changed slightly,the area of vascular bundles and the thickness of mechanical tissue decreased significantly,and the thickness of mechanical tissue cell wall decreased,leading to the decline of stem strength.The increase of nitrogen reduced stem strength primarily by reducing lignification of stem,vascular bundles area,mechanical tissue thickness and cell wall thickness.?4?In terms of the carbohydrates metabolism and accumulation,correlation analysis showed that stem density,starch,soluble sugar,cellulose and lignin content were positively correlated with stem bending stress.Compared JM22 with SN16 and TK33,the nonstructural carbohydrate?starch,soluble sugar?content was lower,while the structural carbohydrate?cellulose,lignin?content was higher,thereby enhancing the mechanical strength of the stem.Increasing the nitrogen rate significantly reduced the stem density,the content of nonstructural carbohydrate and structural carbohydrate,causing stem quality became worse.The results showed that the nitrogen rate deteriorated the quality of the stem by reducing the stem density and carbohydrates contents.Further analysis of the genes related to starch degradation,cellulose and lignin synthesis in stem found that with nitrogen application increased,Ta BMY1 was up-regulated,Ta Ces A,Ta PAL,Ta COMT,Ta CCR and Ta CAD were down-regulated,leading to starch degradation increased as well as cellulose and lignin synthesis decreased.In the leaves,increasing the nitrogen rate,the sucrose synthesis-related genes Tatpt,Ta SPS,the starch synthesis-related genes Ta AGP-S and Ta WSSI,and the starch degradation-related gene Ta BMY1 were all up-regulated,while the sucrose transport-related gene Ta SUT2D was not affected by the nitrogen rate.In the leaves,The contents of sucrose,fructose and glucose increased,the content of starch decreased.In the stems,glucose,which was the substrate used for the synthesis of cellulose and lignin,was increased along with nitrogen rate increasing.The above results indicated that increasing the nitrogen rate enhanced the production capacity of leaf assimilation,while the sucrose output capacity of the leaves was not affected by the nitrogen rate,but the capacity of the stem to use the assimilate to synthesize cellulose and lignin was poor.2.The regulation effect of irrigation and nitrogen interactionIncreasing the nitrogen rate increased the bending moment of the whole plant and decreased the breaking moment,resulting in an increase in the lodging index.Reducing the irrigation amount decreased the bending moment of the whole plant,increased the breaking moment,and reduced the lodging risk.Different irrigation and nitrogen management affected the breaking moment by changing the stem morphological and physiological characteristics,and then regulated the mechanical strength.Increasing the nitrogen rate increased plant height and center of gravity height,and decreased the outer diameter,wall thickness,stem and sheath plumpness,nonstructural carbohydrate?starch and soluble sugar?and structural carbohydrate?cellulose and lignin?contents of stem and sheath,resulting in stem strength reduction.Compared W2 treatment with the traditional irrigation W3 treatment,the length of the basal internodes,the plant height and the center of gravity height were reduced,while the outer diameter,the culm wall thickness,the culm density,the nonstructural carbohydrate and structural carbohydrate contents were increased,which were responsible for improving of the breaking moment.In addition,under W2 treatment,the grain number per spike and the 1000-grain weight were increased,which increased the yield by 0.9%-12.6%.Therefore,changing the traditional three times irrigation to twice irrigation and matching a proper nitrogen rate was conducive to achieving high wheat yield and lodging resistance. |
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