Larry C. Purcell

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.

Nitrogen accumulation and nodule activity of field-grown ‘Jackson’ soybean in response to water deficits

Abstract Substantial evidence has accumulated that N 2 fixation in soybean ( Glycine max Merr.) is much more sensitive to drying soil than is leaf gas exchange and plant mass accumulation. In contrast, the soybean cultivar Jackson in controlled environments has been found to have N 2 fixation sensitivity to drying soil that is approximately equivalent to the sensitivity of leaf gas exchange. This study was undertaken to determine if Jackson behaved in a similar way under field conditions. N 2 fixation activity was measured by in situ acetylene reduction rates and N accumulation in shoots. Leaf gas exchange was measured as stomatal conductance and shoot mass accumulation. All measures in this field study indicated that N 2 fixation in Jackson decreased in response to soil drying in about the same proportion as leaf gas exchange. These results with Jackson were in contrast to those with the cultivar Biloxi in which N 2 fixation was decreased to a greater extent by drought than was leaf gas exchange.

Ureide concentration of field‐grown soybean in response to drought and the relationship to nitrogen fixation

Abstract Ureides accumulate in soybean (Glycine max [L.] Merr.) petioles during drought under greenhouse conditions despite decreased N2 fixation and ureide production. Field experiments with water‐deficit treatments were established to examine the possibility of ureide accumulation under natural drought‐stress conditions. Tissue ureide concentration of drought tolerant ‘Jackson’ were compared with either ‘Hutcheson’ or ‘Biloxi’. For mild water‐deficit conditions, petiole ureide levels were greater than a well‐watered treatment at ten sampling dates for Hutcheson compared to three dates for Jackson. At only two dates were there significant differences between genotypes in petiole ureide concentration, in which case petiole ureide concentration of Hutcheson was greater than Jackson. Under more severe water‐deficit conditions, there were greater and more consistent increases in petiole ureide concentration for Jackson and Biloxi. Jackson, however, had lower petiole ureide concentration than Biloxi throughou...

Allantoate amidohydrolase transcript expression is independent of drought tolerance in soybean

Drought is a limiting factor for N2 fixation in soybean [Glycine max (L.) Merr.] thereby resulting in reduced biomass accumulation and yield. Drought-sensitive genotypes accumulate ureides, a product of N2 fixation, during drought stress; however, drought-tolerant genotypes have lower shoot ureide concentrations, which appear to alleviate drought stress on N2 fixation. A key enzyme involved in ureide breakdown in shoots is allantoate amidohydrolase (AAH). It is hypothesized that AAH gene expression in soybean determines shoot ureide concentrations during water-deficit stress and is responsible for the differential sensitivities of the N2-fixation response to drought among soybean genotypes. The objectives were to examine the relationship between AAH transcript levels and shoot ureide concentration and drought tolerance. Drought-tolerant (Jackson) and drought-sensitive (Williams) genotypes were subjected to three water-availability treatments: well-watered control, moderate water-deficit stress, and severe water-deficit stress. Shoot ureide concentrations were examined, in addition to gene expression of AAH and DREB2, a gene expressed during water-deficit stress. As expected, DREB2 expression was detected only during severe water-deficit stress, and shoot ureide concentrations were greatest in the drought-sensitive genotype relative to the drought-tolerant genotype during water-deficit stress. However, expression of AAH transcripts was similar among water treatments and genotypes, indicating that AAH mRNA was not closely associated with drought tolerance. Ureide concentrations in shoots were weakly associated with AAH mRNA levels. These results indicate that AAH expression is probably not associated with the increased ureide catabolism observed in drought-tolerant genotypes, such as Jackson. Further study of AAH at the post-translational and enzymatic levels is warranted in order to dissect the potential role of this gene in drought tolerance.

Soybean (Glycine max) Nodule Physical Traits Associated with Permeability Responses to Oxygen.

Nodule permeability (P) controls the amount of O2 entering the nodule and is an important determinant of N2 fixation. Modulation of water volume in the intercellular spaces of the nodule cortex was hypothesized to change the effective thickness of a diffusion barrier and account for changes in P. This hypothesis was examined by evaluating physical traits of nodules that may affect P. The first test of the hypothesis was to determine whether alterations in P may result in changing both the density and the air space content of nodules as the water content of intercellular spaces was varied. Density of nodules exposed to 21 kPa O2 increased as the time following detachment from the plant increased from 5 to 60 min. Nodules from soybean (Glycine max [L.] Merr.) plants shaded for 48 h had a lower fractional air space content than nodules from control plants. Nodule detachment and prolonged shading decreased P, and the increase in density and decrease in fractional air space content associated with decreased P in these treatments supports the proposed hypothesis. The second test of the hypothesis was to determine whether nodules released water easily in response to water potential gradients. The intrinsic capacitance of nodules determined by pressure-volume analysis was 0.29 MPa-1 and indicated that the tissue can release relatively large amounts of water from the symplast with only small changes in total nodule water potential. Estimates of the bulk modulus of elasticity ranged from 0.91 to 2.60 MPa and indicated a high degree of elasticity. It was concluded that the physical properties of nodules were consistent with P modulation by the release and uptake of intercellular water in the nodule cortex.

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.

Response of Mycorrhizal Infection to Glyphosate Applications and P Fertilization in Glyphosate-Tolerant Soybean, Maize, and Cotton

ABSTRACT Glyphosate and phosphorus (P) fertilizer may alter arbuscular mycorrhizal (AM) fungal infection rates of glyphosate-tolerant cotton, maize, and soybean in low-P soil. Microbial biomass, water soluble P, Mehlich-3 P, and acid and alkaline phosphatase activities were not significantly impacted by glyphosate or P in the greenhouse. Phosphorus fertilization decreased mycorrhizal infection rates in cotton and maize and increased shoot biomass and shoot P in soybean in 2005, and decreased mycorrhizal infection in soybean and increased shoot biomass in cotton and maize and shoot P in all three crops in 2006. In pasteurized soil, glyphosate decreased percent mycorrhizal infection in maize, increased infection in cotton, and did not significantly affect infection in soybean. When soil was not pasteurized, glyphosate did not significantly alter mycorrhizal infection in any crop. The potential for glyphosate to alter AM fungal infection in glyphosate-tolerant plants may depend on whether soil microbial communities are compromised by other factors.

Yield and dry matter productivity of Japanese and US soybean cultivars

Abstract The difference in yields of cultivars may be causing difference in soybean yield between Japan and the USA. The objective of this study was to identify the effect of the cultivar on dry matter production and to reveal the key factors causing the differences in yield by focusing utilization of solar radiation in recent Japanese and US soybean cultivars. Field experiments were conducted during two seasons in Takatsuki, Japan (34°50′), and in a single season in Fayetteville (36°04′), AR, USA. Five Japanese and 10 US cultivars were observed under near-optimal conditions in order to achieve yields as close to their physiological potential as possible. The seed yield and total aboveground dry matter (TDM) were measured at maturity as long as radiation was intercepted by the canopy. The seed yield ranged from 3.10t ha−1 to 5.91t ha−1. Throughout the three environments, the seed yield of US cultivars was significantly higher than that of Japanese cultivars. The seed yield correlated with the TDM rather than the HI with correlation coefficients from .519 to .928 for the TDM vs. .175 to .800 for the HI, for each of the three environments. The higher TDM of US cultivars was caused by a higher radiation use efficiency rather than higher total intercepted radiation throughout the three environments. The seasonal change in the TDM observed in four cultivars indicated that dry matter productivity was different between cultivars, specifically during the seed-filling period.