Evaluation Of Drought Tolerance Index, Selection Of Drought Tolerant Varieties And Association Analysis Of Important Traits With SSR Markers In Chinese Winter Bread Wheat

Wheat (Triticum aestivum L.) is one of the first plants cultivated by mankind; it has beenextensively planted worldwide owing to its adaptation to a wide range of climates. Wheat isthe third most-produced cereal crop after maize and rice in China; it is mainly planted insemi-humid and semi-arid regions. The arid and semiarid area covers about half of the totalland area in China; and periodic drought, seasonal drought or temporary drought oftenoccurred in semi-humid area, even wet area; drought stress has been one of the mostimportant abiotic constraints greatly limiting wheat production. Therefore, the study ofdrought resistance indicators, the excavation of drought tolerance germplasms, QTL mappingfor important traits, exploitation of allelic variations and maker-assisted selection (MAS) inwheat breeding will play an important role in increasing crop production study. This studyregards90broadly representative Chinese winter wheat accessions as research objects,through normal and drought treatment experiments for two years, screening drought tolerancegermplasms, evaluating drought tolerance of different traits; second, the study regards12bredvarieties as research objects, through normal and drought treatment experiment, analysing therelationships among carbon isotope discrimination, grain yield, photosynthetic traits, stomataldensity of flag leaf, ash content and mineral elements; analysing the genetic variation andbroad sense heritability of the investigated traits for90accessions; to estimate the geneticdiversity, population structure and linkage disequilibrium in Chinese winter wheat with269SSR loci distributed over all21chromosomes; to analyze the association loci with mainagronomic, physiological and root traits using269SSR loci distributed over all21chromosomes. The main results are included as follows:1. Through normal and drought treatment experiments for two years,5accessions withhighly drought tolerance were screened according to the membership function value ofdrought tolerance (MFVD), which combined the drought-tolerant coefficient (DC) of14traitsthat could be easily estimated; these5accessions could be used as materials for droughttolerance improvement in wheat breeding. The analysis, consistency of traits variation todrought stress, indicated that there was a very strong consistency variation between transpiration rate (E) and stomatal conductance (Gs) to drought stress, also a very strongconsistency variation between biological yield per plant (BYPP) and grain yield per plant(GYPP); there were strong consistency variations among plant height (PH), spike length (SL),spikelet number (SN), grain number per spike (GMPS), biological yield per plant (BYPP) andgrain yield per plant (GYPP) under drought stress. The multi-regression analysis was donebetween the membership function value of drought tolerance (MFVD) and thedrought-tolerant coefficient (DC) of each trait. The results revealed that DC of GYPP, E (orGs), PH, UIL and SL could explain more than91%of the total variations in MFVD.Therefore, those five variables could be used as a combination to screen wheat germplasm orlines for drought tolerance in wheat breeding program.2. Through normal and drought treatment experiments in the2011-2012growing seasons,using12bred varieties as materials, the relationship among carbon isotope discrimination,grain yield, photosynthetic traits, stomatal density of flag leaf, ash content and mineralelements was studied. The mean values of8traits, including photosynthetic rate (Pn), carbonisotope discrimination (Δ13C), stomatal conductance (Gs), SPAD value of the flag leafchlorophyll content (SPAD), grain yield (GY), ash content (AC), K content (K) and Mncontent (Mn), significantly decreased under water stressed (WS) condition compared with thewell-watered (WW)(at least P <0.01); on the contrary, Fe content (Fe) significantly increased(P<0.01) under WS. Results of correlation analysis showed that Δ13C was positivelycorrelated with GY under two water conditions (WS and WW); Δ13C was significantlypositively correlated with Ci/Ca under WS (P<0.05), while Δ13C showed significantlynegative correlation with stomatal density of lower epidermis (SDLE) under WW (P<0.05).There were significant positive correlations among Gs, Ci, E and Ci/Ca with each other undertwo water conditions (at least P <0.05); AC was positively correlated with Fe, Zn, Mn and Kcontents under two water conditions; there were positive correlations among Fe, Zn, Mn andK contents with each other under two water conditions. The multiple regression model basedon Δ13C indicated that SDLE and Pn could explain about60.9%of total variations underWW, but multiple regression model could not be built under WS.3. Analysing the genetic variations for the21investigated traits with90Chinese winterwheat accessions, the result showed that the genetic variation of5traits of SL, SN, Pn, Ci andSPAD had lower variability (CVg<10%); other16traits contained abundant genetic variations(CVg>10%). The broad sense heritability for21investigated traits was estimated by geneticvariance and phenotypic variance. The simple traits mainly composed of some morphologicaltraits (SDL, UIL, PH, CL, RD, TKW, SL, SPAD, GNPS, SN, RL and FLL) had relatively highheritability; these traits are available to be selected in early generations. Yield traits (BYPP and GYPP) were of medium heritability; as these traits are the core goal in wheat breeding, sothese traits also are considered to be reliable indexes in early generations. Photosynthetic andother complex traits (Pn, E, Gs, Ci, RS and RV) were of low heritability; especiallyphotosynthetic traits were easily influenced by the environment thus could not be evaluatedaccurately; it is unsuitable for selecting these traits in early generation.4. Ninety winter wheat accessions were analyzed with269SSR markers distributedthroughout the wheat genome. Compared with previous reports, the genetic diversity of the90Chinese winter wheat accessions in this study was at a lower level. Significant difference ingenetic diversity among the three genomes was observed; the B genome showed the highestdiversity and the D genome the lowest. The90Chinese winter wheat accessions could bedivided into three subgroups based on STRUCTURE, UPGMA cluster and principalcoordinate analyses. The population structure derived from STRUCTURE clustering waspositively correlated to some extent with geographic eco-type. LD analysis revealed that therewas a shorter LD decay distance in Chinese winter wheat compared with other wheatgermplasm collections. The whole genome LD decay distance was approximately2.2cM(r~2>0.1, P<0.001), and the LD decay distance in A, B and D genomes of wheat was about2.2,0.6and8.6cM, respectively. Evidence from genetic diversity analyses suggested that wheatgermplasm from other countries should be introduced into Chinese winter wheat and distanthybridization should be adopted to create new wheat germplasm with increased geneticdiversity. The results of this study were expected to provide valuable information for futureassociation mapping by using this Chinese winter wheat collection.5. The marker-trait associations for19traits were studied with90elite wheat genotypesusing269SSR markers distributed throughout the wheat genome. The marker-traitassociation analysis was tested for each water condition in each year with the softwareprogram TASSEL v2.1by using the mixed linear model based on the Q-matrix and thekinship-Matrix. The results showed that a total of138SSR loci were associated with19traits,and274high significant marker-trait associations were detected at the P<0.01level in4environment (11SW,11WW,12SW and12WW). The numbers of SSR association loci with9agronomic traits (PH, UIL, SDL, FLL, FLW, SL, SN, GNPS and TKW) were20,18,18,13,8,13,35,14and20, respectively. The numbers of SSR association loci with5physiologicaltraits (Pn, Gs, E, Ci and SPAD) were16,16,13,27and18, respectively. The numbers of SSRassociation loci with5root traits (CL, RL, RD, RS and RV) were7,4,5,3and6, respectively.Seven SSR loci were associated with4traits, and14high significance marker-traitassociations were simultaneously detected in all4environment. Among138SSR loci,67SSR loci (48.55%of138) were associated with at least two traits and further referred as multi-trait QTL. There were a few loci associated with more traits simultaneously, whichmight be the genetic reason of correlation among traits or pleiotropic phenomena. Some SSRloci were in agreement with mapped QTLs from family-based linkage mapping procedure orassociation analysis, and verified the factcity of results. The results were expected to provideimportant practical value for the fine mapping of QTL and molecular marker for assistedbreeding.