Increased Nitrogen Fixation Research Articles

First Report of a Phytoplasma Associated with Witches'-Broom of the Giant Coral Tree (Erythrina poeppigiana, Fabaceae) in Costa Rica.

The giant coral tree (Erythrina poeppigiana, Fabaceae) is a common shade tree in coffee plantations in Costa Rica. Coral trees are pruned to decrease fungal infections and increase nitrogen fixation. Recently, severe shoot proliferation, internodes shortening, and leaf reduction were observed in pruned shade trees in the south of San José Province, Costa Rica. Leaf samples from 10 symptomatic E. poeppigiana trees were collected. Also, two samples from symptomless coral trees were collected from areas free of witches'-broom. Total DNA was extracted from 0.5 g of petiole tissue from all samples with the plant extraction mini kit (Qiagen GmbH, Hilden, Germany) with a modified protocol (2) and assayed by nested PCR with phytoplasma universal rDNA primers (P1/P7) (1) and R16F2n/R16R2 (3). All symptomatic trees tested positive for phytoplasmas by PCR, yielding the expected 1.2-kb band. DNA from the symptomless trees was not amplified by PCR. The restriction fragment length polymorphism analyses (HaeIII, AluI, RsaI, BfaI, HpaII, KpnI, HhaI, and MseI) and the sequence of the 1.2-kb PCR fragment (GenBank Accession No. DQ485305) revealed that the phytoplasma associated with coral tree witches'-broom belongs to the aster yellows phytoplasma group (16SrI) (4). To our knowledge, this is the first report of a phytoplasma belonging to the aster yellow group causing witches'-broom in the Erythrina genus.

Study of morphological and symbiotic traits during the ontogeny of supernodular and hypernodular pea mutants

Morphological (plant height and vegetative biomass amount) and symbiotic (number of nodules and nitrogenase activity) traits of six symbiotic pea mutants and the original cultivar Rondo were studied at different vegetation periods. Of the mutants studied, one (K10a) was supemodular and the remaining five (K1a, K2a, K5a, K7a, and K27a) were hypemodular. Essential distinctions in the absolute values and time course of the changes in individual morphological and symbiotic traits of different pea mutants were demonstrated. The supemodular type is inferior to the original cultivar in plant height and production of vegetative biomass, but exceeds it in nodulation and nitrogen fixation. The hypemodular mutants either surpass the original cultivar with respect to the production capacity or display similar results. The symbiotic traits-number of nodules and nitrogen fixation activity--of these mutants are higher compared with the Rondo cultivar. The mutants K1a, K2a, and K27a were demonstrated to be useful in breeding pea for an increase in nitrogen fixation.

Phosphate enrichment in the northern Gulf of Aqaba: Regulation by carbonate sediments and impact on nitrogen elevation

The effect of carbonate sediment in regulating phosphate concentrations in sea water was investigated by laboratory incubation experiments using different sediment types. Incubation experiments were made with two types of sediments: uncontaminated sediment from a marine reserve and contaminated sediment with deposited phosphate powders. Fluxes of inorganic nitrogen and phosphate were estimated from linear regressions of solute concentrations over incubation periods. Ammonium and phosphate fluxes were about twofold higher in the uncontaminated sediment that had significantly lower organic carbon and total phosphate concentrations than in the phosphate-contaminated sediment. To test the effect of dissolved phosphate on increasing nitrogen fixation, additional incubation experiments were carried out using treated carbonate and silicate sediments with added dissolved phosphate (20 μM). Incubations were made under sterile conditions with HgCl2 added to distinguish between biologically enhanced processes and pure physicochemical processes. The adsorption rate of phosphate onto carbonate sediment was about twice that onto silicate sediments. No nitrogen elevation either as ammonium or as nitrate was observed in the soluble phosphate enrichment incubations. In conclusion, this study demonstrates the importance of the regulation of soluble phosphate concentrations in carbonate sediment environments where the carbonate sediment acts as a buffering system keeping soluble phosphate concentrations at certain steady-state levels. The study also demonstrates the lack of evidence on enhancement of nitrogen concentrations due to the increase phosphate concentrations.

Amino acid composition of Brazilian surubim fish ( Pseudoplatystoma coruscans) fed diets with different levels and sources of fat

Surubim ( Pseudoplatystoma coruscans) is a river fish from the Brazilian São Francisco basin which presents a great potential for commercialization. The amino acid composition of carcasses of surubim fed isoproteic diets with variable levels and sources of fat were determined. The increase of soybean oil at levels of 4%, 8% and 12% in the rations promoted a corresponding increase in nitrogen fixation in the fish carcass, reflected by higher amino acid contents. However, the application of 12% added amounts of pig lard, corn oil, linseed oil or soybean oil, as different sources of fat, did not promote much variation in the fish amino acid fixation. The determination of the essential amino acid composition in the fish carcasses and the A/E ratios of individual amino acids provided a pattern of amino acid requirements of surubim fish.

Ecotypic variation in phosphorus-acquisition mechanisms within marine picocyanobacteria

Prochlorococcus and Synechococcus are major prokaryotic primary producers in the oligotrophic oceans that may be affected by the climate-related increases in nitrogen fixation and subsequent phosphorus (P) limitation in some parts of the oceans. Evidence that Prochlorococcus populations in the North Pacific subtropical gyre (NPSG) have increased over the past decades, possibly due to having a competitive advantage under conditions of P limitation, suggests aspects of their P physiology that are important for dictating their in situ success. Here, we compared the physiology of P acquisition and response to P stress (indicated by alkaline phosphatase activity, APA) among isolates of Prochlorococcus and Synechococcus representing different genotypic clades within the marine picophytoplankton lineage (sensu Urbach et al. 1998: J Mol Evol 46:188-201). The 2 Synechococcus isolates examined (WH 8102 and WH 7803) can utilize a wide variety of organic P sources. Of the 3 Prochlorococcus isolates examined, only the HLI genotype, MED4, is capable of growth on a variety of organic P sources. Under conditions of P starvation the 2 Synechococcus strains and Prochlorococcus MED4 exhibit significant increases in APA, above their measurable constitutive activities. The genomes of the Synechococcus strains and Prochlorococcus MED4 indicate the presence of many P-uptake and -regulatory genes required under conditions of P stress, including the phoA gene that encodes for an alkaline phosphatase enzyme. The other isolates of Prochloro- coccus, HLII MIT 9312 and LLIV MIT 9313, have distinctly different P-stress responses. MIT 9312, which contains the same P-uptake and -regulatory genes as MED4, except for psip1 and ptrA, has no constitutive APA, but does exhibit measurable, albeit very low, activity when P starved. MIT 9313, which lacks phoA and a functional phosphate-sensing histidine kinase gene, phoR, exhibits low constitutive activity that decreases when the cells become P-starved. These results show variability in P utilization and in P-stress response between the 2 genera of marine unicellular cyanobacteria, and marked differences among Prochlorococcus genotypes, implying that only certain eco/genotypes of these marine cyanobacteria will have an ecological advantage under conditions of P limitation in the oceans. This further points to the critical need to continue developing genotypic- or cell-specific tools to assess the response of the picophytoplankton community to large-scale changes in nutrient conditions in the oceans.

Regulation of CO2 on heterocyst differentiation and nitrate uptake in the cyanobacterium Anabaena sp. PCC 7120

The aim of the present investigation was to study the effects of different inorganic carbon and nitrogen sources on nitrate uptake and heterocyst differentiation in the culture of cyanobacterium Anabaena sp. PCC 7120. Anabaena was cultivated in media BG11 containing combined nitrogen and supplementary NaHCO3 or CO2. Cell growth, heterocyst differentiation, nitrate reductase (NR, EC 1.7.7.2), glucose-6-phosphate dehydrogenase (G6PDH, EC 1.1.1.49) and NO uptake were analysed. The cells cultivated in BG11(0) medium with aeration were taken as reference. Experimental results showed that the differentiation frequency of heterocysts when the cells were cultivated with elevated CO2 was higher than that of the cells grown with air or bicarbonate. Heterocysts appeared unexpectedly when CO2 was introduced into the medium containing nitrate. However, no heterocysts emerged when CO2 was added to medium containing NH or urea, or when NaHCO3 was supplied to the medium with nitrate. Both nitrate uptake rate and nitrate reduction enzyme activity were depressed by the supplement of CO2 to the culture. The activity of G6PDH was enhanced with the increase in heterocyst differentiation frequency. CO2 might compete with NO for energy and electrons in the uptake process and CO2 appears favoured. This led to a high intracellular C/N ratio and a relative N limitation. So the process of heterocyst differentiation was activated to supplement nitrogen uptake. This study provided an attractive possibility to form more heterocysts by rapid growth of Anabaena cells cultivated in the medium containing nitrate in order to increase nitrogen fixation and hydrogen production.

The Influence of Lipopolysaccharides and Glucans from Two Rhizobium leguminosarum bv. viciae Strains on the Formation and Efficiency of Their Symbioses with Pea Plants

The influence of lipopolysaccharides (LPS), glucans, and their unseparated complexes on nodulation activity of rhizobia and efficiency of their symbioses with pea plants was studied in vegetation experiments. Two Rhizobium leguminosarum bv. viciae strains which differed in their symbiotic properties were used: strain 31 (fix+, efficient, moderately virulent, moderately competitive), and strain 248b (fix-, inefficient, highly virulent, highly competitive). Preparations of LPS-glucan complex and the respective LPS from the highly virulent strain 248b increased the nodulation activity of both strains by 10-26%. Analogous preparations from a less virulent strain 31 did not have this ability. Unseparated LPS-glucan complexes from these strains increased the productivity of plants infected with the efficient strain by 18-23% but did not change it in plants inoculated with the other, inefficient strain. No significant influence of LPS preparations on the symbiosis productivity was observed. Glucans from both strains enhanced the nodulation ability of the highly virulent strain by 36-56%. In addition, treatment of pea plants with glucan from strain 248b increased nitrogen fixation by root nodules by 27% in plants inoculated with strain 31. In general, the formation and efficiency of the symbiosis of R. leguminosarum bv. viciae with pea plants was more influenced by preparations from strain 248b, highly virulent but deficient in nitrogen fixation, than by preparations from the nitrogen fixation-proficient but less virulent strain 31.

Bradyrhizobium japonicum mutants allowing improved nodulation and nitrogen fixation of field-grown soybean in a short season area

In short-season soybean production areas, low soil temperature is potentially a major factor limiting plant growth and yield. Although Bradyrhizobium japonicum strain 532 C is widely used in inoculants in Canadian soybean production, and USDA 110 is widely used in American soybean production, they are both inhibited by low temperatures. Genistein is an important plant-to-bacteria signal compound secreted by soybean roots. The addition of genistein has proven to be an effective means of generating increases in nitrogen fixation and yield but genistein is expensive. We used UV mutagenesis to make 10 mutants from USDA 110 that express nod genes without the addition of plant-to-bacteria signal compounds such as genistein. A field experiment was conducted at the Lods Agronomy Research Centre in southwestern Quebec in 1998 and 1999. The treatments consisted of factorial combinations of inoculant type (no inoculant (control) and inoculants containing the mutants Bj 30050, Bj 30051, Bj 30052, Bj 30053, Bj 30054, Bj 30055, Bj 30056, Bj 30057, Bj 30058, Bj 30059 and the wild type USDA110 or 532 C) and soybean cultivar (Bayfield and Maple Glen). The experiment was structured following a randomized complete block design with four blocks. Data were collected on nodule number, nodule dry weight, shoot nitrogen yield and total nitrogen fixation at five development stages. Averaged over the 2 years of the study, when pods were 2 cm long at one of the four uppermost nodes on the main stem (R4), inoculation with Bj 30055 and Bj 30058 resulted in greater nodule number (23 and 14%, respectively), nodule dry weight (16 and 13%, respectively), shoot nitrogen yield (19 and 21%, respectively) and total nitrogen fixation (10·9 and 12·7%, respectively) than 532 C, which is currently used in Canadian inoculants. The cultivar Bayfield produced more nodules, and higher nodule weight than Maple Glen, but there were no differences between the cultivars for shoot nitrogen yield and total nitrogen fixation.

Interactions among soil biology, nutrition, and performance of actinorhizal plant species in the H.J. Andrews Experimental Forest of Oregon

The study examined the effect of Frankia, macronutrients, micronutrients, mycorrhizal fungi, and plant-growth-promoting fluorescent Pseudomonas sp. on total biomass, nodule weight, and nitrogen fixation of red alder ( Alnus rubra) and snowbrush ( Ceanothus velutinus) under greenhouse conditions. The soil samples were collected from a 10-year-old clearcut on the H.J. Andrews Experimental Forest, Oregon. Within the clearcut, four sampling points were selected along a slope gradient. Red alder and snowbrush plants were greenhouse-grown in a mix of soil–vermiculite–perlite (2:1:1) for 6 and 12 months, respectively. Plants were inoculated with Frankia and a fluorescent Pseudomonas sp. Some of the red alder were also inoculated with ectomycorrhizal Alpova diplophloeus, and some snowbrush with endomycorrhizal Glomus intraradix. There was no interaction between treatment and slope location for either species. There were significant treatment effects for red alder, but not for snowbrush. Red alder seedlings given Frankia and macronutrients produced more biomass and had greater nitrogen fixation than seedlings grown without additions; adding A. diplophloeus increased nitrogen fixation by 33% over that obtained with Frankia plus macronutrients. Frankia, macronutrients, and the mycorrhizal fungus together increased nitrogen fixation by 136% over the control. Adding only micronutrients to Frankia and macronutrients, however, reduced nitrogen fixation by nearly one half; the presence of the mycorrhizal fungus appeared to buffer these negative effects. Pseudomonas inoculation did not affect any of the measured variables. Slope location of soil affected the two plant species differently. Red alder seedlings grown in upper slope soil had greater biomass and nitrogen fixation than those grown in soil from the lower slope. In contrast, snowbrush plants had greater biomass, nodule weight, and nitrogen fixation when grown in bottom slope soil rather than on soil from any of the other slope positions.

Biological productivity during sapropel S5 formation in the Eastern Mediterranean Sea: evidence from stable isotopes of nitrogen and carbon

We determined 15N/14N ratios in modern surface and sapropel S5 sediments of the Mediterranean Sea to clarify differences in the nutrient regime associated with sapropel formation. In the modern situation, high δ15N of unused nitrate (15–20 ‰) remaining in the surface waters during the winter phytoplankton bloom evidences P-limitation of biological production in winter. δ15N of surface sediments decrease towards the east of the basin (5 to >2.5‰). This is a consequence of either eastward increasing nitrogen fixation during the summer months, or of particulate matter being supplied predominantly by the P-limited winter bloom. Very low (−1–1‰) δ15N values in sapropel S-5 from four locations require a very light source of nutrient-N assimilated at a minimum of ten times the modern export flux. Because the isochronous records show no spatial gradient in δ15N, we exclude both Ekman-type upwelling and direct riverine discharge as likely sources of nutrients. Our data are consistent with an anti-estuarine thermohaline circulation in the upper 500m during S5 time, allowing for the trapping of nutrients in the eastern basin. The most likely scenario for S5 is that phosphorus release from a relatively shallow redox boundary resulted in an imbalanced supply of N:P (<16:1) to the photic zone. The result was a slow assimilation of carbon during summer stratification and extensive N2-fixation providing the majority of the export flux from a N-limited system.

Association of Bacillus firmus E3 and Klebsiella terrigena E6 with increased ability for nitrogen fixation

A binary association of Bacillus firmus E3 and Klebsiella terrigena E6 was isolated from the rhizosphere of Dactylus glomerata. This association consisted of a N 2-fixing strain ( K. terrigena) and a ‘promoter’ strain ( B. firmus). Increased nitrogen fixation activity (ca. 110 nmol ethylene mg protein −1 h −1) was detected on the 4th day of growth under aerobic conditions. Living cells of B. firmus, cell-free culture filtrates, and two compounds excreted by B. firmus, phenol and p-hydroxybenzoic acid, increased nitrogenase activity of K. terrigena. This increase was accompanied by the enhancement of K. terrigena respiration (O 2-consumption) rate and was probably due to respiration protection of nitrogenase induced by these phenolics.

Continuous CO2 enrichment leads to increased nodule biomass, carbon availability to nodules and activity of carbon‐metabolising enzymes but does not enhance specific nitrogen fixation in pea

Recent research has shown that nodule nitrogen fixation is limited under a wide range of environmental constraints by lowered carbon flux within the nodule due to down‐regulation of sucrose synthase activity. The aim of this work was to elucidate whether an increase in both carbon flux and activity of enzymes of carbon metabolism in nodules may lead to an increased nitrogen fixation. We report the effects caused by a continuous exposure to atmospheric CO2 enrichment in nodulated pea plants. CO2 enrichment led to an enhanced whole‐plant growth and increased nodule biomass. Moreover, nodules of plants grown at increased CO2 showed a higher sugar content as well as enhancement of some activities related to nodule carbon metabolism, such as sucrose synthase, UDP glucose pyrophosphorylase and phosphoenolpyruvate carboxylase. Indeed, acetylene reduction activity, measured by the classical technique, was increased more than four times. However, when specific nitrogen fixation was determined as hydrogen evolution, no significant differences were detected, consistent with the lack of changes of enzymes involved in nitrogen metabolism such as glutamate synthase and aspartate aminotransferase. These results are discussed in the context of the regulation of nitrogen fixation and nodule metabolism.

Agricultural management of grain legumes: has it led to an increase in nitrogen fixation?

Biological nitrogen (N 2) fixation is an important aspect of sustainable and environmentally friendly food production and long-term crop productivity. The amount of N 2 fixed is primarily controlled by four principal factors: (1) the effectiveness of the rhizobia–host plant symbiosis, (2) the strength of the sink, i.e., the ability of the host plant to accumulate N, (3) the amount of available soil N and (4) environmental constraints to N 2 fixation. Much of the N fixed by grain legumes is removed at harvest, the remainder becomes available to subsequent crops following mineralization, may be incorporated into the soil organic matter, or as with fertilizer N, may be lost from the cropping system. This paper reviews some of the agronomic management practices that affect N 2 fixation by grain legumes, asking whether grain legumes can provide an overall net N benefit to the soil when grown in rotation with other crops. A survey of long-term trends in N 2 fixation by selected grain legumes is included, and some possible explanations for the observed stagnation in efforts to increase N 2 fixation under field conditions are presented.

Elevated CO2 increases nitrogen fixation and decreases soil nitrogen mineralization in Florida scrub oak

SummaryWe report changes in nitrogen cycling in Florida scrub oak in response to elevated atmospheric CO2 during the first 14 months of experimental treatment. Elevated CO2 stimulated above‐ground growth, nitrogen mass, and root nodule production of the nitrogen‐fixing vine, Galactia elliottii Nuttall. During this period, elevated CO2 reduced rates of gross nitrogen mineralization in soil, and resulted in lower recovery of nitrate on resin lysimeters. Elevated CO2 did not alter nitrogen in the soil microbial biomass, but increased the specific rate of ammonium immobilization (NH4+ immobilized per unit microbial N) measured over a 24‐h period. Increased carbon input to soil through greater root growth combined with a decrease in the quality of that carbon in elevated CO2 best explains these changes. These results demonstrate that atmospheric CO2 concentration influences both the internal cycling of nitrogen (mineralization, immobilization, and nitrification) as well as the processes that regulate total ecosystem nitrogen mass (nitrogen fixation and nitrate leaching) in Florida coastal scrub oak. If these changes in nitrogen cycling are sustained, they could cause long‐term feedbacks to the growth responses of plants to elevated CO2. Greater nitrogen fixation and reduced leaching could stimulate nitrogen‐limited plant growth by increasing the mass of labile nitrogen in the ecosystem. By contrast, reduced nitrogen mineralization and increased immobilization will restrict the supply rate of plant‐available nitrogen, potentially reducing plant growth. Thus, the net feedback to plant growth will depend on the balance of these effects through time.

Delivery of N2 to bacteroids in simulated soybean nodule cells: consideration of gradients of concentration of dissolved N2 in cell walls, cytoplasm, and symbiosomes

The previously published simulation of physiological functions occurring in infected cells of soybean nodules has been extended to include consideration of the diffusion of N2 from the outside of a nodule to the nitrogen-fixing bacteroids, in relation to published values for the apparentKm(N2) for nitrogen fixation in the soybean nodule system. Nitrogen fixation is driven by bacteroid respiration, so increases in the average relative oxygenation (Y) of cytoplasmic leghaemoglobin lead to increased bacteroid respiration, increased nitrogen fixation, and greater differences in concentration of dissolved N2 between the cell surface and the innermost bacteroids (d[N2]). Over the range ofY considered, values for d[N2] were from 5.2- to 6.2-fold greater than the corresponding values for d[O2], because of facilitation of O2 flux by cytoplasmic leghaemoglobin. Gradients of [N2] within symbiosomes are small relative to cytoplasmic values and at the symbiosome surface [N2] was greater than 0.4 mol/m3 at the greatest rates of nitrogen fixation calculated. Therefore, it is unlikely that values for [N2] anywhere in the infected cell are low enough to affect rates of nitrogen fixation significantly, unless low external atmospheric N2 pressures are used experimentally.

The Role of Nitrogen Fixation in Crop Production

Summary Biological nitrogen fixation is an important process for agricultural productivity in many cropping systems because of direct inputs of atmospheric nitrogen, and rotational effects such as disease control. Advances in molecular biology techniques provide new opportunities to understand the ecology of root nodule bacteria and may improve the selection of elite strains for inoculation. An understanding of the genetic basis of nodulation in grain and pasture legumes may improve inoculation technologies. Temperate and tropical pastures may be improved through effective inoculation, removal of nutritional constraints, and use of alternate legume species. Increases in nitrogen fixation in crop legumes may result from addressing problems in the legume host, the microsymbiont and the environment.

Plant growth promoting rhizobacteria accelerate nodulation and increase nitrogen fixation activity by field grown soybean [Glycine max (L.) Merr.] under short season conditions

A 3 × 2 × 2 factorial field experiment, organized in a randomized complete block split-plot with four replications, was conducted in 1994 to evaluate the effect of two plant growth-promoting rhizobacteria (PGPR) strains (Serratia liquefaciens 2-68 or Serratia proteamaculans 1-102) on nodulation, nitrogen fixation, and total nitrogen yield by two soybean cultivars in a short season area. The experiments were conducted at the Emile A. Lods Research Centre, McGill University, Macdonald Campus, Montreal, Canada, and performed at two adjacent sites. One site was fumigated with methyl bromide (50 g m-2). Another site was kept unfumigated. Co-inoculation of soybean with B. japonicum and PGPR increased soybean nodulation and hastened the onset of nitrogen fixation, when the soils were still cool. Total fixed N, fixed N as a percentage of total plant N, and protein and N yield were also increased by PGPR inoculation. AC Bravor tended to be more responsive to both PGPR treatments for total fixed N and N yields than Maple Glen, suggesting that inoculation with PGPR was more effective for cultivars with higher yield potentials.

Grazing effects on nitrogen fixation in coral reef algal turfs

This study addressed whether grazing by the sea urchin Diadema antillarum influenced rates of nitrogen fixation by algal turf communities on Caribbean coral reefs. Because the turfs were nitrogen-limited, we also assessed whether newly-fixed nitrogen was important for supporting net primary productivity by the turfs. We measured acetylene reduction in turfs grown in treatments excluding or including D. antillarum in the presence of other herbivores at 3 m water depth on Tague Bay forereef, St. Croix, U.S. Virgin Islands. These were the first measurements of acetylene reduction on coral reefs under quasi-natural conditions of high water-flow and photosynthetic oxygen generation. Rates of acetylene reduction under these conditions were as high as any measured previously in coral reef communities (mean 7.6 nmol C2H4 cm−2 h−1). Algal turfs grazed by D. antillarum and other herbivores had chlorophyll-specific acetylene reduction rates up to three times higher than when D. antillarum was excluded. High rates of nitrogen fixation by the turfs were sufficient to meet <2% of the nitrogen required to support net chlorophyll-specific primary productivity over 24 h. Grazer-mediated increases in nitrogen fixation do not appear responsible for a parallel enhancement of net primary productivity. Algal turfs at this site must be dependent primarily on external sources of nitrogen.

Distribution of a bacterium (γ-1,2,3,4,5,6-hexachlorocyclohexane-decomposing Sphingomonas paucimobilis) among soil aggregates

A great variety of microorganisms lives in soil with a heterogeneous structure. Identification and characterization of microhabitats for survival of individual soil microorganisms are important not only for gaining a better understanding of soil microbial ecology, but also for controling the biological functions of soil such as increase in nitrogen fixation, prevention of soil-borne diseases, and microbial degradation of hazardous waste materials.

Application of gibberellic acid to the surface of soybean seed (t Glycine max (L.) Merr.) and symbiotic nodulation, plant development, final grain and protein yield under short season conditions

In short-season soybean production areas, low soil temperature is the major factor limiting soybean establishment, nodulation and nitrogen fixation. Gibberellic acid (GA) pretreatment of crop seeds can overcome low soil temperature inhibition of seed germination and seedling development. However, previous studies have found that the application of GAs decreased legume nodulation and nitrogen fixation under optimal growth conditions. A field experiment was conducted under short season conditions in eastern Canada to determine whether the application of GA3 to soybean seed could accelerate germination, and increase plant nodulation and nitrogen fixation. The results indicated that GA3 application accelerated seedling emergence but decreased plant nodulation and nitrogen accumulation at early plant growth stages. However, these initial negative effects were overcome as the plants developed. Gibberellic acid applied to soybean seed at the time of planting did not influence final grain and protein yield.