Evaluation Of Submergence Tolerance And Its Morpho-Anatomical Characteristics, Inheritance And QTL Mapping In Soybean

Flooding is often a serious natural disaster in the world.Soybean,originated in China, is planted from the south to the north in the country,including Jiang-Han Plain and the Middle and Lower valleys of Changjiang River.In the two regions,under the control of a subtropical monsoon climate,there often happen some heavy rainstorms during May to September,which cause flooding during soybean growing stages and then cause the reduction of soybean yield.Therefore,breeding for soybeans with flooding tolerance is of great significance for enhancing productivity.Flooding tolerance has been recognized in two categories:tolerance to soil waterlogging and tolerance to submergence.The main objectives of this study were:(1) to establish identification procedure of flooding tolerance,to evaluate the variation of submergence tolerance among soybean germplasm from different eco-regions in China,and to dentify elite accessions with submergence tolerance;(2) to study the morpho-anatomical characteristics of soybeans with different submergence tolerance;(3) to reveal the inheritance mechanism and to map QTLs of submergence tolerance.A total of 749 accessions,composed of 545 accessions of cultivated soybean[Glycine max(L.)Merr.]and 204 accessions of annual wild soybean[Glycine soja Sieb.et Zucc.], sampled from different eco-regions,were used in the pot experiment in 2005 and 2006. According to the preliminary studies,tolerance to submergence with relative death percentage as tolerance index was used in the later experiments due to its better stability and simplicity in comparison with the other indices.The submergence experiment of soybean was conducted at seedling stage with artificial flooding under randomized complete blocks design in the plastic greenhouse at Jiangpu Station,Nanjing Agricultural University.(1) There showed significant genetic differences among the genotypes of both wild and cultivated soybean.A single peak frequency distribution with the majority in the middle part and less frequencies at both sides was observed,the relative death percentage (RDP) of wild and cultivated soybean varied between 0.0%and 207.4%,0.0%and 222.2%, respectively.The frequency of highly tolerant wild soybean was slightly more than that of cultivated soybean,but most part of both cultivated and wild soybean was intermediate and less sensitive to submergence.There existed certain amount of variation of submergence tolerance both within and among eco-regions in G soja,while in G max the among eco-region variation was smaller than the within eco-region variation,of which the CV values were all more than 30.0%.(2) There showed a weak correlation between submergence tolerance and some traits,including geographical latitude,100-seed weight, maturity date,and fat content.(3) Eleven elite accessions with high tolerance to submergence,including eight G soja and three G max accessions,such as N24835.0, N24850.0,N23444.0,N04974.1,et al.were identified and screened out.The relative death percentage of them was 0.0%,and they accounted for 1.5%of total accessions from Eco-regionâ… ,â…¡,â…¢,â…£,andâ…¥.These accessions will be important sources for future submergence tolerance breeding programs and fundamental researches on morphology, anatomy,physiology,biochemistry and genetics.The morpho-anatomical characteristics of submergence tolerance was studied in an artificial flooding experiment in pots with Nannong 1138-2(sensitive,G max),Kefeng No.1(highly tolerant,G.max),PI342618B(highly tolerant,G.soja),PI326582B(highly tolerant,G.soja).(1) Flooding stress caused more adverse effects on underground root system than on aboveground shoot system,with tap root length shorter,lateral roots sparse and total roots reduced,while new leave size reduced,lower part leaves chlorotic,plant height and dry shoot weight increased.(2) The descending order of relative injury rate(RIR) of taproot length,number of lateral roots,dry root weight and root/shoot ratio was Nannong 1138-2,Kefeng No.1,PI326582B,PI342618B,adverse to the order of submergence tolerance.(3) Under flooding,the submerged stem base was thick and hypertrophied,and adventitious roots emerged from the flooded part of the stem above the soil.Aerenchyma formation occurred in adventitious root and tap root cortex of cultivated soybeans(G.max), and highly tolerant Kefeng No.1 had more extensive aerenchyma tissues than sensitive Nannong 1138-2 did.Wild soybeans(G.soja) expressed no aerenchyma formation in adventitious roots,tap roots,stems and leaves.Kefeng No.1(highly tolerant) and Nannong 1138-2(sensitive),with large difference in submergence tolerance between the two parents,were used to study the inheritance of submergence tolerance in soybean.The survival percentage of seedlings of the RIL population NJRIKY derived from Kefeng No.1×Nannong 1138-2 was evaluated under completely submerged condition in pot experiment.Genetic analysis was performed under major gene plus polygene mixed inheritance model in the P₁,P₂,and F_7:11 of the cross 'Kefeng No.1×Nannong 1138-2'.QTL mapping for submergence tolerance in soybean was carried out on the genetic linkage map provided by National Center for Soybean Improvement under the methods of composite interval mapping(CIM) and multiple interval mapping(MIM) of software WinQTL Cartographer Version 2.5.The genetic linkage map contained 488 SSR markers,spanning 25 linkage groups at a total distance 4226.4cM,with an average distance of 8.6cM between the flanking markers.The results from segregation analysis showed that submergence tolerance of soybean was controlled by three major genes with the heritability of 42.40%and no polygene detected.With composite interval mapping(CIM) and multiple interval mapping(MIM),three QTLs associated with submergence tolerance were identified in both CIM and MIM,which located in Satt648～K418₂V,Satt531～A941V and Satt038～Satt275(B53B～Satt038) on linkage groups A1,D1a and G,respectively,explaining 4.4%～7.6%of the total phenotypic variation.The results from segregation analysis and QTL mapping appeared relatively consistent and could verify each other.However,there is some difference between the present results and those in the literature,therefore,further study is to be considered.