Analysis Of QTLs Pyramiding Effects On Low Nitrogen Tolerance Related Traits In Barley

In this study,the RIL population constructed by cultivated barley variety Baudin and local barley variety CN4079 were used as the materials.Different nitrogen treatments were set up in the two-year field experiments.Phenotype identification and QTL mapping analysis of low nitrogen（LN）stress-related traits such as agronomy,yield,N,P,and K efficiency of barley at mature stage were studied,and discovered QTLs for barley low-tolerance traits.Three elite LN-tolerant QTLs clusters from the cultivar Baudin,the local variety CN4079,and the wild material CN4027 were selected for pyramiding effect analysis.Using the two RIL populations constructed by Commander/CN4027//CN4079 recrossing and Fleet/CN4027//CN4079 recrossing as aggregates,we independently developed and screened three QTL cluster-associated SSR molecular markers with parental polymorphism.Using molecular marker-assisted selection technology performs polymorphism screening on the progeny of the pyramid population,and genotypes the progeny of the pyramid population according to different pyramiding methods;Under different nitrogen treatments,the agronomic,yield,N,P,and K efficiency traits related to LN stress tolerance of the pyramid populations in the field were identified.The phenotype and genotype classification were jointly analyzed to explore the three low tolerance,to explore the pyramiding effect of three elite LN-tolerant QTL clusters,and to select new varieties of barley pyramid tolerant to LN stress.The main findings of this study were as follows:（1）In the Baudin/CN4079 population under different nitrogen treatments for two years,low nitrogen stress reduced the yield traits by an average of 49.9%and 36.4%respectively.The negative impact on the number of ears in the population was particularly serious;The plant height,spike length and growth period were reduced by LN stress,while the changes of awn length and lodging resistance were complicated,indicating that the awn length and lodging resistance were complicated by the mechanism of LN stress and were greatly affected by the environment.The nitrogen content and accumulation of various organs of barley were significantly reduced by LN stress,but the three types of nitrogen efficiency increased significantly,with an average increase of 1.43 times and 1.31 times respectively over the past two years,indicating that barley may be able to maintain the growth and production of aboveground to adapt to the environment.The absorption and utilization of phosphorus was hindered by LN stress,but its potassium utilization efficiency increased,indicating that there are differences in the regulation of P and K efficiency in different nitrogen supply environments.（2）A total of 24 QTLs were located at the maturity stage,concentrated on chromosome5H,followed by 2H and 7H.Among them,there were 10,3,and 11 QTLs related to agronomic,yield and nutrient traits,respectively.The range of LOD values and phenotypic variations of all QTLs are 3.09-10.53 and 17.7-49%,respectively.Two QTL clusters were identified on 5H,and most of them contained low-nitrogen specific and stable QTLs,which involved agronomic,yield,nitrogen efficiency and other traits,and provided genetic theoretical possibilities for improving barley nitrogen efficiency and yield simultaneously.Among them,the 5H bpb8462-bpb3241 interval could be used as a target area for further exploration of genes related to the control of LN-tolerance in barley（3）The three elite LN-tolerant QTL clusters used for pyramiding contain 11,8,and 6QTLs for different traits,respectively,which contained the QTLs of harvest index,nitrogen harvest index,potassium harvest index,and potassium utilization efficiency for grain yield commonly.Select SSR molecular markers based on the principle of PCR,design and dev elop primers with QTL peak flanking interval as the target area,a pair of parental polymorphic SSR molecular markers were successfully screened on each QTL cluster interval,and each parameter met the primer design standards.These can be used for polymorphism screening of progeny in the next pyramid population.（4）Molecular marker-assisted selection technology was used to screen for polymorphism in the progeny of two pyramid populations.According to the genotype classification of each single QTL cluster of progeny lines,the population of progeny was further divided into eight genotypes of P1+P2+P3,P1+P2,P1+P3,P2+P3,P1,P2,P3 and P0,eight types of genotypes in the C79F₆ population include 3,3,5,3,8,17,7,and 18 strains,eight types of genotypes in the F79F₈ population include 3,11,10,3,21,10,9,and 40 strains,respectively.（5）The phenotype of the pyramid population at the maturity stage was greatly affected by LN stress,which reduced the yield-related traits,plant height,spike length,lodging resistance,and phenotypic average of the accumulation of NPK in the two pyramid populations.In particular,the dry weight traits of grains decreased significantly.Under different nitrogen levels,the traits related to tolerance to low nitrogen in the pyramid population generally changed greatly,indicating that the genotype gap of the lines of the pyramid population was large,and the genetic information fully mixed the different alleles of the three barley parents,which can be used for further pyramid screening work.Analysis of the phenotypic values of different genotype categories of the progeny of the pyramid population revealed that the four genotypes of P1+P2,P1+P3,P2+P3,and P1+P2+P3 were in terms of yield formation,nitrogen and potassium absorption have been greatly improved,and some average values of lines’ traits under LN stress were even exceeded the traits’ average values under normal nitrogen environment,especially the C79-18,C79-43,C79-55 genotype strains increased the absorption and utilization of nitrogen and potassium under LN stress,the average yield was still 1.8%higher than that of the normal nitrogen level,indicating that the three LN-tolerant QTL cluster pyramid in this study have a favorable improvement effect on barley low-nitrogen tolerance.