C. Andy King

Inhibition of N2 Fixation in Soybean Is Associated with Elevated Ureides and Amino Acids

Decreased N2 fixation in soybean (Glycine max) L. Merr. during water deficits has been associated with increases in ureides and free amino acids in plant tissues, indicating a potential feedback inhibition by these compounds in response to drought. We evaluated concentrations of ureides and amino acids in leaf and nodule tissue and the concurrent change in N2 fixation in response to exogenous ureides and soil-water treatments for the cultivars Jackson and KS4895. Exogenous ureides applied to the soil and water-deficit treatments inhibited N2 fixation by 85% to 90%. Mn fertilization increased the apparent catabolism of ureides in leaves and hastened the recovery of N2 fixation following exogenous ureide application for both cultivars. Ureides and total free amino acids in leaves and nodules increased during water deficits and coincided with a decline in N2 fixation for both cultivars. N2 fixation recovered to 74% to 90% of control levels 2 d after rewatering drought-stressed plants, but leaf ureides and total nodule amino acids remained elevated in KS4895. Asparagine accounted for 82% of the increase in nodule amino acids relative to well-watered plants at 2 d after rewatering. These results indicate that leaf ureides and nodule asparagine do not feedback inhibit N2 fixation. Compounds whose increase and decrease in concentration mirrored the decline and recovery of N2 fixation included nodule ureides, nodule aspartate, and several amino acids in leaves, indicating that these are potential candidate molecules for feedback inhibition of N2 fixation.

Drought and nitrogen source effects on nitrogen nutrition, seed growth, and yield in soybean

Abstract Drought in soybean [Glycine max (L.) Merr.] decreases yield‐related processes and N2 fixation is more sensitive to drought than are many other of these processes. Therefore, application of nitrogen (N) fertilizer may increase drought tolerance over those plants primarily dependent on N2 fixation. In a field experiment, NH4NO3 applications (+N) to drought‐stressed soybean resulted in biomass and N accumulation rates similar to those rates for an irrigated treatment without N fertilizer (‐N). In contrast, biomass and N accumulation rates were decreased for the ‐N treatment. N fertilization increased seed growth rate and decreased seed fill duration for irrigated and drought treatments. In the drought treatment, N application increased seed number per unit area, which resulted in higher yields. In a greenhouse experiment, fertilization with either KN03 or NH4C1 increased biomass and N accumulation rates during drought over those of plants dependent solely on N2 fixation. It was concluded that applic...

Biomass accumulation and allocation in soybean associated with genotypic differences in tolerance of nitrogen fixation to water deficits

Nitrogen fixation in soybean (Glycine max [L.] Merr.) is more sensitive to water deficits than many physiological processes and may therefore limit yield under nonirrigated conditions. Tolerance of nitrogen fixation to water deficits has been observed in the cultivar Jackson, however, the physiological basis for this is unclear. It was hypothesized that genotypes that could continue biomass production on limited soil water would prolong nitrogen fixation by continued photosynthate allocation to nodules. An initial greenhouse experiment compared biomass and N accumulation in six genotypes over an 8 d water deficit. Low stress intensity minimized genotypic expression of water-deficit tolerance; nevertheless, Jackson was clearly one of the most tolerant genotypes. In a second experiment, Jackson was compared to SCE82-303 at more severe stress levels. Biomass and N accumulation continued during water deficits for Jackson but ceased in SCE82-303. Individual nodule mass tended to increase during water deficits in Jackson and tended to decrease in SCE82-303, indicating greater allocation of photosynthate to Jacksons nodules in response to water deficits. Biomass accumulation of Jackson was contrasted with the USDA plant introduction (PI) 416937, which also has demonstrated tolerance to water deficits. For water-deficit treatments, total biomass accumulation was negligible for PI416937, but biomass accumulation continued at approximately 64% of the control treatment for Jackson. Transpirational losses for Jackson and PI416937 were approximately the same for the water-deficit treatment, indicating that Jackson had higher water use efficiency (WUE). Isotopic discrimination of 13C relative to12 C also indicated that Jackson had superior WUE during water deficits. Carbon-14 allocation in Jackson was compared to KS4895, a cultivar that was identified as sensitive to water deficits in an initial experiment. The comparison of water-deficit treatments of Jackson with KS4895 indicated that Jackson exported significantly greater amounts of14 C from labeled leaves and allocated approximately four times greater amounts of 14C per g of nodule. Results indicated that Jacksons sustained biomass production during water deficits resulted in the continued allocation of photosynthate to nodules and prolonged nitrogenase activity.

Genome-Wide Association Study of Ureide Concentration in Diverse Maturity Group IV Soybean [Glycine max (L.) Merr.] Accessions.

Ureides are the N-rich products of N-fixation that are transported from soybean nodules to the shoot. Ureides are known to accumulate in leaves in response to water-deficit stress, and this has been used to identify genotypes with reduced N-fixation sensitivity to drought. Our objectives in this research were to determine shoot ureide concentrations in 374 Maturity Group IV soybean accessions and to identify genomic regions associated with shoot ureide concentration. The accessions were grown at two locations (Columbia, MO, and Stuttgart, AR) in 2 yr (2009 and 2010) and characterized for ureide concentration at beginning flowering to full bloom. Average shoot ureide concentrations across all four environments (two locations and two years) and 374 accessions ranged from 12.4 to 33.1 µmol g−1 and were comparable to previously reported values. SNP–ureide associations within and across the four environments were assessed using 33,957 SNPs with a MAF ≥0.03. In total, 53 putative loci on 18 chromosomes were identified as associated with ureide concentration. Two of the putative loci were located near previously reported QTL associated with ureide concentration and 30 loci were located near genes associated with ureide metabolism. The remaining putative loci were not near chromosomal regions previously associated with shoot ureide concentration and may mark new genes involved in ureide metabolism. Ultimately, confirmation of these putative loci will provide new sources of variation for use in soybean breeding programs.

Association Mapping of Total Carotenoids in Diverse Soybean Genotypes Based on Leaf Extracts and High-Throughput Canopy Spectral Reflectance Measurements

Carotenoids are organic pigments that are produced predominantly by photosynthetic organisms and provide antioxidant activity to a wide variety of plants, animals, bacteria, and fungi. The carotenoid biosynthetic pathway is highly conserved in plants and occurs mostly in chromoplasts and chloroplasts. Leaf carotenoids play important photoprotective roles and targeted selection for leaf carotenoids may offer avenues to improve abiotic stress tolerance. A collection of 332 soybean [Glycine max (L.) Merr.] genotypes was grown in two years and total leaf carotenoid content was determined using three different methods. The first method was based on extraction and spectrophotometric determination of carotenoid content (eCaro) in leaf tissue, whereas the other two methods were derived from high-throughput canopy spectral reflectance measurements using wavelet transformed reflectance spectra (tCaro) and a spectral reflectance index (iCaro). An association mapping approach was employed using 31,253 single nucleotide polymorphisms (SNPs) to identify SNPs associated with total carotenoid content using a mixed linear model based on data from two growing seasons. A total of 28 SNPs showed a significant association with total carotenoid content in at least one of the three approaches. These 28 SNPs likely tagged 14 putative loci for carotenoid content. Six putative loci were identified using eCaro, five loci with tCaro, and nine loci with iCaro. Three of these putative loci were detected by all three carotenoid determination methods. All but four putative loci were located near a known carotenoid-related gene. These results showed that carotenoid markers can be identified in soybean using extract-based as well as by high-throughput canopy spectral reflectance-based approaches, demonstrating the utility of field-based canopy spectral reflectance phenotypes for association mapping.

Genome-Wide Association Analysis of Diverse Soybean Genotypes Reveals Novel Markers for Nitrogen Traits

Nitrogen is a primary plant nutrient that plays a major role in achieving maximum economic yield. Insufficient availability most often limits soybean [Glycine max (L.) Merr.] crop growth. Symbiotic N2 fixation in soybean is highly sensitive to limited water availability, and breeding for reduced N2 fixation sensitivity to drought is considered an important objective to improve yields under drought. The objective of this study was to identify single nucleotide polymorphism (SNP) markers associated with N traits. A collection of 373 diverse soybean genotypes were grown in four field environments (2 yr and two locations) and characterized for N derived from atmosphere (Ndfa), N concentration ([N]), and C/N ratio. The population structure of 373 soybean genotypes was assessed based on 31,145 SNPs and genome‐wide association analysis using a unified mixed model identified SNPs associated with Ndfa, [N], and C/N ratio. Although the Ndfa, [N], and C/N ratio values were significantly different between the two locations in both years, results were consistent among genotypes across years and locations. While numerous SNPs were identified by association analysis for each trait in only one of the four environments, 17, 19, and 24 SNPs showed a significant association with Ndfa, [N], and C/N ratio, respectively, in at least two environments as well as with the average across all four environments. These markers represent an important resource for pyramiding favorable alleles for drought tolerance and for identifying extremes for comparative physiological studies.

Soybean growth and yield response to saturated soil culture in a temperate environment

Saturated soil culture (SSC) is a production system developed in semiarid tropical Australia that reportedly increases soybean (Glycine max L. (Merr.)) yield above that obtained with conventional irrigation. With SSC, water is maintained in furrows between beds from early vegetative stages until maturity. The objective of this research was to evaluate SSC of soybean as an option in temperate regions for rotations in a rice-based production system. Soybean was sown in 1994 and 1995 at Keiser, Arkansas, on 15 cm high beds, spaced 1 m apart in fields precision-graded to 0.15% slope. In 1994, the cultivars were Hutcheson (MG V) and Manokin (MG IV), and in 1995, the cultivars were Asgrow 6297 (MG VI) (A6297) and Hartz 5545 (MG V) (H5545). In both years, the irrigation treatments were nonirrigated, furrow-irrigated at a 50 mm soil water deficit, and SSC. In agreement with reports from Australia, the SSC treatment resulted in leaf yellowing after treatments were established, this yellowing was associated with a lag in biomass and N accumulation in comparison with the furrow-irrigated treatment in 1994. In 1995, N accumulation rate decreased in SSC during the acclimation period, but there was no effect on biomass accumulation rate. In contrast to reports from Australia, yield was not increased by the SSC treatment. In 1994, seed yield from the SSC treatment averaged across cultivars was 40% below that from the furrow-irrigated treatment. A similar yield decrease for the SSC treatment was observed in 1995 for H5545. For A6297, however, the SSC yields and furrow-irrigated yields did not differ. In 1994 for both cultivars and in 1995 for H5545, there was a strong negative response of seed yield to depth of water in the furrow of the SSC treatment. It was concluded that there is no yield advantage from a SSC management system compared to furrow irrigation at the Arkansas site. A positive yield response of SSC may be limited to arid environments where warm temperatures and high irradiance levels contribute to overcoming the detrimental effects of SSC following treatment establishment.

DIVERSITY AND IMPLICATIONS OF SOYBEAN STEM NITROGEN CONCENTRATION

Soybean [Glycine max (L.) Merr.] shoot nitrogen (N) traits are important for seed production and may hold potential for improving seed yield and quality. Field experiments were established to survey shoot N traits in i) plant introductions, ii) a recombinant inbred line (RIL) population, and iii) modern cultivars. A wide range of N concentrations was observed at beginning seed fill for leaves, petioles, and stems and at maturity for stems. Significant genotypic variations in stem N traits were found in modern cultivars and the RIL population. Molecular marker analysis identified multiple loci associated with stem N concentration. Significant relationships between various tissue N traits and seed yield and quality were also observed. These results illustrate the importance of N dynamics in vegetative tissues for soybean yield and seed composition. The observed variation in N traits indicates that selecting for vegetative N traits could potentially increase yield and improve seed quality.

Association mapping identifies loci for canopy temperature under drought in diverse soybean genotypes

Drought stress is a global constraint for crop production, and improving crop tolerance to drought is of critical importance. Because transpiration cools a crop canopy, a cool canopy under drought indicates a genotype still has access to soil moisture. Because measurements of canopy temperature may be increased in scale in field environments, it is particularly attractive for large-scale, phenotypic evaluations. Our objectives were to identify genomic regions associated with canopy temperature (CT) and to identify extreme genotypes for CT. A diverse panel consisting of 345 maturity group IV soybean accessions was evaluated in three environments for CT. Within each environment CT was normalized (nCT) on a scale from 0 to 1. A set of 31,260 polymorphic single nucleotide polymorphisms (SNPs) with a minor allele frequency ≥ 5% was used for association mapping of nCT. Association mapping identified 52 SNPs significantly associated with nCT, and these SNPs likely tagged 34 different genomic regions. Averaged across all environments, eight genomic regions showed significant associations with nCT. Several genes in the identified genomic regions had reported functions related to transpiration or water acquisition including root development, response to abscisic acid, water deprivation, stomatal complex morphogenesis, and signal transduction. Fifteen of the SNPs associated with nCT were coincident with SNPs for canopy wilting. Favorable alleles from significant SNPs may be an important resource for pyramiding genes, and several genotypes were identified as sources of drought-tolerant alleles that could be used in breeding programs for improving drought tolerance.