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QTLs for delayed canopy wilting from five soybean populations were projected onto the consensus map to identify eight QTL clusters that had QTLs from at least two independent populations. Quantitative trait loci (QTLs) for canopy wilting were identified in five recombinant inbred line (RIL) populations, 93705 KS4895 × Jackson, 08705 KS4895 × Jackson, KS4895 × PI 424140, A5959 × PI 416937, and Benning × PI 416937 in a total of 15 site-years. For most environments, heritability of canopy wilting ranged from 0.65 to 0.85 but was somewhat lower when averaged over environments. Putative QTLs were identified with composite interval mapping and/or multiple interval mapping methods in each population and positioned on the consensus map along with their 95% confidence intervals (CIs). We initially found nine QTL clusters with overlapping CIs on Gm02, Gm05, Gm11, Gm14, Gm17, and Gm19 identified from at least two different populations, but a simulation study indicated that the QTLs on Gm14 could be false positives. A QTL on Gm08 in the 93705 KS4895 × Jackson population co-segregated with a QTL for wilting published previously in a Kefeng1 × Nannong 1138-2 population, indicating that this may be an additional QTL cluster. Excluding the QTL cluster on Gm14, results of the simulation study indicated that the eight remaining QTL clusters and the QTL on Gm08 appeared to be authentic QTLs. QTL × year interactions indicated that QTLs were stable over years except for major QTLs on Gm11 and Gm19. The stability of QTLs located on seven clusters indicates that they are possible candidates for use in marker-assisted selection.

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2421ReseaRch N itrogen is nutritionally important to plants and animals. It is a basic element of energy-transfer molecules (such as adenosine triphosphate [ATP]), nucleic acids (such as DNA), chlorophyll, and a crucial component of amino acids, and hence proteins. In soybean [Glycine max (L.) Merr.], the symbiosis with Bradyrhizobium japonicum allows the crop to grow independently from inorganic N fertilizer. Under drought conditions, however, N 2 fixation is more sensitive than leaf gas exchange (Durand et al., 1987;Kuo and Boersma, 1971;Sinclair, 1986) and photosynthesis (Djekoun and Planchon, 1991;Serraj and Sinclair, 1997). Consequently, reliance on N 2 fixation in soybean makes the plant vulnerable to N deficiency under limited soil-moisture conditions. The initial product of N 2 fixation is NH 3 , and the NH 3 is assimilated into the ureides, allantoin and allantoate, in soybean ABSTRACT Nitrogen (N) fixation in soybean [Glycine max (L.) Merr.] is more sensitive to drought than photosynthesis, and high concentrations of shoot ureide and N are associated with sensitivity of N 2 fixation to drought. Genotypic differences in ureide and N concentration were evaluated using a mapping population of 97 recombinant inbred lines derived from a cross between 'KS4895' and 'Jackson'. For three irrigated environments, broad-sense heritability for ureide and N concentration was 0.73 and 0.59, respectively. Under irrigated conditions, five quantitative trait loci (QTLs) for ureide concentration were identified using composite interval mapping (CIM). Multiple interval mapping (MIM) identified two QTLs with locations similar to those identified with CIM. Four QTLs for N concentration were detected using CIM, and one QTL was identified with MIM with a similar position as that identified with CIM. A QTL on Gm13 for shoot ureide and N appeared to be pleiotropic. In the drought environment, two QTLs were identified using CIM for both shoot ureide and N; a QTL for ureide concentration on Gm19 mapped to the same position as a ureide QTL under irrigated conditions, but the additive effect was opposite in sign. A search for metabolic genes in QTL regions predicted for the pleiotropic effect of N and ureide (Gm13, carbonic anhydrase) and for ureide (Gm19, inosine-uridine nucleoside hydrolase). These QTLs may be useful in selecting lines drought tolerant for N 2 fixation.

Insufficient moisture availability often limits soybean [Glycine max (L.) Merr.] yield. Carbon isotope ratio (δ13C) provides an integrated measure of water use efficiency in C3 plants due to its substantial genetic variance, high heritability, and small genotype × environment interaction (G × E). The objective of this study was to identify quantitative trait loci (QTLs) associated with δ13C using a recombinant inbred line population derived from a cross between ‘KS4895’ and ‘Jackson’. The field experiment was conducted in five environments to evaluate δ13C under rainfed and irrigated conditions. Analysis of variance of δ13C averaged over environment and irrigation treatment showed significant effects of genotype (G), environment (E), and G × E interactions. Heritability of δ13C in different environments and irrigation treatments ranged from 66 to 79%. Averaged over environments and irrigation treatments, heritability was 83%. A total of 24 QTLs associated with δ13C were identified and clustered in nine genomic regions on seven chromosomes. The QTL clusters on Gm05 (1), Gm06 (2) and Gm20 (1) were detected across different environments and irrigation regimes. Collectively, these four QTL clusters accounted for 55% of the phenotypic variation in δ13C. The QTLs on Gm06 and Gm20 also showed additive × additive epistasis that contributed approximately 4.2% to the total phenotypic variation. Several identified δ13C QTLs overlapped with QTLs associated with other physiological traits related to plant water status, biological nitrogen fixation, and plant morphology. The identified genomic regions may be an important resource in genomic selection studies to improve drought tolerance in soybean.