scispace - formally typeset
Search or ask a question

Showing papers on "Nitrogen fixation published in 1992"


Journal ArticleDOI
TL;DR: Genetic studies indicate that the controls of heterocyst development and nitrogenase synthesis are closely interrelated and that the expression of N2 fixation (nif) genes is regulated by pO2.

523 citations


Journal ArticleDOI
TL;DR: Increased BNF in mixed legume/grass pastures is being obtained through selection or breeding of legumes for increased productivity and/or to minimise effects of nutrient limitations, low soil moisture, soil acidity, and pests and disease.
Abstract: Biological nitrogen fixation (BNF) in mixed legume/grass pastures is reviewed along with the importance of transfer of fixed nitrogen (N) to associated grasses. Estimates of BNF depend on the method of measurement and some of the advantages and limitations of the main methods are outlined. The amounts of N fixed from atmospheric N2 in legume/grass pastures throughout the world is summarised and range from 13 to 682 kg N ha-1 yr-1. the corresponding range for grazed pastures, which have been assessed for white clover pastures only, is 55 to 296 kg N ha-1 yr-1.

480 citations



Book ChapterDOI
TL;DR: The actual levels of N2 fixation attained by legume and non-legume associations are reviewed and their role as a source of N in tropical and sub-tropical agriculture is assessed.
Abstract: Inputs of biologically fixed N into agricultural systems may be derived from symbiotic relationships involving legumes and Rhizobium spp., partnerships between plants and Frankia spp. or cyanobacteria, or from non-symbiotic associations between free-living diazotrophs and plant roots. It is assumed that these N2-fixing systems will satisfy a large portion of their own N requirements from atmospheric N2, and that additional fixed N will be contributed to soil reserves for the benefit of other crops or forage species. This paper reviews the actual levels of N2 fixation attained by legume and non-legume associations and assesses their role as a source of N in tropical and sub-tropical agriculture. We discuss factors influencing N2 fixation and identify possible strategies for improving the amount of N2 fixed.

366 citations


Journal ArticleDOI
TL;DR: Cereal/legume intercropping increases dry matter production and grain yield more than their monocultures and the distance between the cereal and legume root systems is important because N is transferred through the intermingling of root systems.
Abstract: Cereal/legume intercropping increases dry matter production and grain yield more than their monocultures. When fertilizer N is limited, biological nitrogen fixation (BNF) is the major source of N in legume-cereal mixed cropping systems. The soil N use patterns of component crops depend on the N source and legume species. Nitrogen transfer from legume to cereal increases the cropping system's yield and efficiency of N use. The use of nitrate-tolerant legumes, whose BNF is thought to be little affected by application of combined N, may increase the quantity of N available for the cereal component. The distance between the cereal and legume root systems is important because N is transferred through the intermingling of root systems. Consequently, the most effective planting distance varies with type of legume and cereal. Mutual shading by component crops, especially the taller cereals, reduces BNF and yield of the associated legume. Light interception by the legume can be improved by selecting a suitable plant type and architecture. Planting pattern and population at which maximum yield is achieved also vary among component species and environments. Crops can be mixed in different proportions from additive to replacement or substitution mixtures. At an ideal population ratio a semi-additive mixture may produce higher gross returns.

267 citations



Journal ArticleDOI
TL;DR: Two new diazotrophic bacteria, Listonella anguillarum and Vibrio campbellii and one non-nitrogen-fixing bacterium, Staphylococcus sp.
Abstract: Two new diazotrophic bacteria, Listonella anguillarum and Vibrio campbellii , and one non-nitrogen-fixing bacterium, Staphylococcus sp., were isolated from the rhizosphere of mangrove trees. Strains of these newly-defined diazotrophs are known as pathogenic bacteria in fish and shellfish. During the purification of diazotrophic species from the entire rhizosphere population, N 2 -fixation of the bacterial mixtures decreased. When grown in vitro in mixed cultures, the non-fixing bacterium Staphylococcus sp. increased the nitrogen-fixing capacity of L. anguillarum by 17% over the pure culture; the nitrogen-fixing capacity per bacterial cell increased 22%. This interaction was not due to a change in O 2 concentration. Staphylococcus sp. decreased the nitrogen-fixing capacity of V. campbellii by 15%. These findings indicate that (i) other species of rhizosphere bacteria, apart from the common diazotrophic species, should be evaluated for their contribution to the nitrogen-fixation process in mangrove communities; and (ii) the nitrogen-fixing activity detected in the rhizosphere of mangrove plants is probably not the result of individual nitrogen-fixing strains, but the sum of interactions between members of the rhizosphere community.

150 citations


Journal ArticleDOI
TL;DR: Symbiotic nitrogen fixation in nodules of legumes depends on the complex interaction between the legume plant and (Brady)Rhizobium bacteria, and mutants with altered symbiotic performance are considered to be useful for investigatings on the interaction in the nodule formation processes.
Abstract: Symbiotic nitrogen fixation in nodules of legumes depends on the complex interaction between the legume plant and (Brady)Rhizobium bacteria. Nodule formation and nitrogen fixation are closely regulated by both the host plant and the microsymbiont. Plant mutants with altered symbiotic performance are considered to be useful to gain a better understanding of the plant—microbe interactions in the legume—(Brady)Rhizobium symbiosis (Jacobsen 1984; Carroll et al 1985a, b; Park and Buttery 1988; Duc and Messager 1989; Gremaud and Harper 1989). Recently, Carroll et al. (1985a, b) have isolated the supernodulating mutants of the soybean cv. “Bragg,” which display a very large number of nodules and “nitrate-tolerant-symbiotic” (nts) characteristics. More recently, Gremaud and Harper (1989) have also isolated similar mutants from the soybean cv. “Williams.” These mutants not only provide materials that are useful for investigatings on the interaction in the nodule formation processes but also for agricultur...

128 citations


Journal ArticleDOI
TL;DR: In the plant kingdom a great variety of pathogenic, saprophytic and symbiotic interactions between plants and microorganisms occur, and several of these interactions have been the subject of intensive research.
Abstract: In the plant kingdom a great variety of pathogenic, saprophytic and symbiotic interactions between plants and microorganisms occur, and several of these interactions have been the subject of intensive research. One of the best studied interactions is the symbiosis of Rhizobium, Bradyrhizobium or Azorhizobium bacteria with legume plants, which results in the formation of root nodules, in which bacteria are able to fix atmospheric nitrogen into ammonia. This process of symbiotic N2 fixation is the major naturally occurring mechanism by which nitrogen is reduced and the ecological and agricultural importance of this process has been an important incentive to study this plant-microbe relation. The process of symbiotic N2 fixation has been discussed in several recent reviews [33].

128 citations


Journal ArticleDOI
TL;DR: An analysis of data compiled from the literature confirms a strong inverse relationship between annual rates of nitrogen fixation and the soil nitrogen content in agricultural and pastoral ecosystems, and provides important support for the hypothesis that phosphorus availability is a key regulator of nitrogen biogeochemistry.
Abstract: An analysis of data compiled from the literature confirms a strong inverse relationship between annual rates of nitrogen fixation and the soil nitrogen content in agricultural and pastoral ecosystems However, this inverse relationship is strongly modified by the rate of application of phosphorus fertilizer, which strongly influences the activities of both symbiotic and non-symbiotic nitrogen fixing organisms In the case of symbiotic legumes, the response of N-fixation to N and P is in part a result of changes in legume dominance within the plant community These results, as well as supporting data presented from a review of experiments on nitrogen fixation in a variety of other terrestrial and aquatic ecosystems, provide important support for the hypothesis that phosphorus availability is a key regulator of nitrogen biogeochemistry

117 citations


Journal ArticleDOI
TL;DR: In this article, the process of ammonia fixation has been studied in three well characterized and structurally diverse fulvic and humic acid samples, and liquid phase 15N NMR spectrometry was performed on the samples before and after reaction with ammonium hydroxide.

Journal ArticleDOI
TL;DR: Applied N had the largest impact on N accumulation rates before R2 and from the second sampling to R7, both periods of minimal N2 fixation, and 15N uptake and extractable soil N data indicate that common bean derived more N from the mineral source than soybean because of more efficient uptake.
Abstract: Nitrogen derived from symbiotic and mineral sources by a legume is determined by the interactions between mineral N supply, plant N demand, and N assimilation traits. These interactions need to be understood to maximize legume N2 fixation and yield, and to identify plant traits supporting high N2-fixation. These interactions were examined in inoculated soybean [Glycine max Merr. (L.)] and common bean (Phaseolus vulgaris L.) by varying N supply (9, 120, and 900 kg N ha-1) at two field sites. Nitrogen fixation was measured by 15N dilution method. Plants were sampled at full bloom (R2), 21 to 25 d from R2, and physiological maturity (R7). Total N of both legumes at R7 was 25% greater with 900 than 9 kg N ha(-1). With 900 kg N ha(-1), soybean N accumulation at R7 (271 kg N ha-1) was 42% more than common bean but was 22% less at R2 (78 kg N ha-1). Applied N had the largest impact on N accumulation rates before R2 and from the second sampling to R7, both periods of minimal N2 fixation. Rate of N accumulation by common bean (0.19 g N m(-2) d-1) was more uniform over the growth cycle than by soybean (0.26 g N m(-2) d-1) which peaked (0.58 g N m(-2)d-1) between R2 and the second sampling. This peak also coincided with maximum N2 fixation rate. Our 15N uptake and extractable soil N data indicate that common bean derived more N from the mineral source than soybean because of more efficient uptake. Greenhouse data indicated greater root weight and uptake of mineral N per unit root weight for common bean than soybean. Maximizing both N2 fixation and yield might entail timing the mineral N supply during early vegetative and late reproductive phases. The limited N2 fixation capacity of common bean might be due to N assimilation traits favoring mineral N uptake.

Journal ArticleDOI
TL;DR: It is now clear that Mo is not an essential metal for N2 fixation, and the soil bacterium Azotobacter chroococcum has an Mo-containing nitrogenase with properties very similar to those found in many other diazotrophic organisms, but it also has a vanadium- containing nitrogenase which functions in the presence of V ifMo is not available.
Abstract: Biological nitrogen fixation, the reduction of dinitrogen to ammonia, is catalysed by nitrogenases. These enzymes are found in relatively few groups of bacteria and until recently it appeared that N2 fixation occurred by a single route involving a molybdenum-containing enzyme. However, it is now clear that Mo is not an essential metal for N2 fixation. The soil bacterium Azotobacter chroococcum has an Mo-containing nitrogenase with properties very similar to those found in many other diazotrophic organisms, but it also has a vanadium-containing nitrogenase which functions in the presence of V if Mo is not available. The closely related species A. vinelandii has, in addition to Mo and V nitrogenases, a third nitrogenase which functions only under conditions where Mo and V levels are very low [1].

Journal ArticleDOI
TL;DR: The levels ofnitrogenase activities in the tubercles in situ were low, but they may indicate a significant contribution to the nitrogen dynamics of these nitrogen-limited Douglas-fir forests over a long-term period.
Abstract: Nitrogenase activities, measured by acetylene reduction, were detected under microaerophilic field conditions in Douglas-fir tuberculate ectomycorrhizae. Tuberculate ectomycorrhizae consist of densely packed clusters of ectomycorrhizal rootlets enclosed in a supplementary fungal peridium-like layer. Nitrogenase activity was primarily in the external layer and was greatly enhanced with added sucrose. The bacterium isolated, a nitrogen-fixing, spore-forming Bacillus sp., is an aerobe but requires anaerobic conditions for nitrogenase activity. Respiration in the tuberculate complex by the fungus, roots, and associated mycorrhizosphere microbes probably contributes to maintaining a microaerophilic niche where nitrogen fixation can take place. Water extracts of peridium or mycorrhizal root tips enhanced nitrogenase activity of this associative Bacillus sp., thereby indicating a close nutritional relationship between this bacterium and the tuberculate mycorrhizae. Thiamine more significantly enhanced bacterial nitrogenase activity than biotin; no activity was detected with p-aminobenzoic acid. Even though the levels ofnitrogenase activities in the tubercles in situ were low, as measured by the present methods, they may indicate a significant contribution to the nitrogen dynamics of these nitrogen-limited Douglas-fir forests over a long-term period.

Journal ArticleDOI
TL;DR: Results show that alterations in Rm6963 which include LPS changes lead to an altered symbiotic phenotype during the association with alfalfa that affects the timing of nodule emergence, the progress of nitrogen fixation, and the strain competitiveness for nodulation.
Abstract: A transposon Tn5-induced mutant of Rhizobium meliloti Rm2011, designated Rm6963, showed a rough colony morphology on rich and minimal media and an altered lipopolysaccharide (LPS). Major differences from the wild-type LPS were observed in (i) hexose and 2-keto-3-deoxyoctonate elution profiles of crude phenol extracts chromatographed in Sepharose CL-4B, (ii) silver-stained sodium dodecyl sulfate (SDS)-polyacrylamide gel electrophoresis patterns of crude and purified LPS fractions, and (iii) immunoreactivities otherwise present in purified LPS of the parental strain Rm2011. In addition, Rm6963 lost the ability to grow in Luria-Bertani medium containing the hydrophobic compounds sodium deoxycholate or SDS and showed a decrease in survival in TY medium supplemented with high calcium concentrations. The mutant also had altered symbiotic properties. Rm6963 formed nodules that fixed nitrogen but showed a delayed or even reduced ability to nodulate the primary root of alfalfa without showing changes in the position of nodule distribution profiles along the roots. Furthermore, 2 to 3 weeks after inoculation, plants nodulated by Rm6963 were smaller than control plants inoculated with wild-type bacteria in correlation with a transient decrease in nitrogen fixation. In most experiments, the plants recovered later by expressing a full nitrogen-fixing phenotype and developing an abnormally high number of small nodules in lateral roots after 1 month. Rm6963 was also deficient in the ability to compete for nodulation. In coinoculation experiments with equal bacterial numbers of both mutant and wild-type rhizobia, only the parent was recovered from the uppermost root nodules. A strain ratio of approximately 100 to 1 favoring the mutant was necessary to obtain an equal ratio (1:1) of nodule occupancy. These results show that alterations in Rm6963 which include LPS changes lead to an altered symbiotic phenotype during the association with alfalfa that affects the timing of nodule emergence, the progress of nitrogen fixation, and the strain competitiveness for nodulation.


Journal ArticleDOI
TL;DR: It is suspected that flexibility of the endosymbiont load can reduce the metabolic cost to the diatom if the endOSYmbionts are dependent on the di Atom for a resource.
Abstract: Diatoms of the family Epithemiaceae possess a unicellular nitrogen-fixing cyanobacterial endosymbiont. We investigated the potential of extracellular nitrogen and phosphorus concentrations to affect the endosymbiont load of Rhopalodia gibba O. Mull, and Epithemia turgida Ehr. in field and culture populations. In a growth chamber experiment, monoclonal cultures of R. gibba were exposed to three levels of nitrate-nitrogen. Nutrient-diffusing substrates were used in a lake environment to create nine microhabitats of varying nitrogen and phosphorus ratios for natural populations of R. gibba and E. turgida. The number of endosymbionts per diatom increased as ambient nitrogen became limiting; mean endosymbiont volume increased as nitrogen increased. The mean endosymbiont surface area: volume ratio decreased with increasing nitrogen. Total endosymbiont volume per diatom (the product of the number of endosymbionts per diatom and their individual biovolumes) did not have a simple response to increasing nitrogen. Phosphorus limitation uncoupled the relationship between endosymbiont load and nitrogen. We suspect that flexibility of the endosymbiont load can reduce the metabolic cost to the diatom if the endosymbionts are dependent on the diatom for a resource.

Book ChapterDOI
TL;DR: The general principle of integrated uses of BNF in rice-farming systems is shown and the inoculation of cyanobacteria has been long recommended, but its effect is erratic and unpredictable.
Abstract: This paper summarizes recent achievements in exploiting new biological nitrogen fixation (BNF) systems in rice fields, improving their management, and integrating them into rice farming systems. The inoculation of cyanobacteria has been long recommended, but its effect is erratic and unpredictable. Azolla has a long history of use as a green manure, but a number of biological constraints limited its use in tropical Asia. To overcome these constraints, the Azolla-Anabaena system as well as the growing methods were improved. Hybrids between A. microphylla and A. filiculoides (male) produced higher annual biomass than either parent. When Anabaena from high temperature-tolerant A. microphylla was transferred to Anabaena-free A. filiculoides, A. filiculoides became tolerant of high temperature. Azolla can have multiple purposes in addition to being a N source. An integrated Azolla-fish-rice system developed in Fujian, China, could increase farmers' income, reduce expenses, and increase ecological stability. A study using Azolla labeled with 15N showed the reduction of N losses by fish uptake of N. The Azolla mat could also reduce losses of urea N by lowering floodwater-pH and storing a part of applied N in Azolla. Agronomically useful aquatic legumes have been explored within Sesbania and Aeschynomene. S. rostrata can accumulate more than 100kg N ha-1 in 45 d. Its N2 fixation by stem nodules is more tolerant of mineral N than that by root nodules, but the flowering of S. rostrata is sensitive to photoperiod. Aquatic legumes can be used in rainfed rice fields as N scavengers and N2 fixers. The general principle of integrated uses of BNF in rice-farming systems is shown.

Journal ArticleDOI
TL;DR: It is concluded that great variation exists among Rhizobium spp.
Abstract: The diversity of rhizobia that form symbioses with roots of black locust (Robinia pseudoacacia L.), an economically important leguminous tree species, was examined by inoculating seedling root zones with samples of soil collected from the United States, Canada, and China. Bacteria were isolated from nodules, subcultured, and verified to be rhizobia. The 186 isolates varied significantly in their resistance to antibiotics and NaCl, their growth on different carbohydrates, and their effect on the pH of culture media. Most isolates showed intermediate antibiotic resistance, the capacity to use numerous carbohydrates, and a neutral to acid pH response. Isolates had greater similarity within sampling locations than among sampling locations. The isolates were grouped by using numerical taxonomy techniques, and representative strains of 37 groups were selected. The mean generation times of these isolates ranged from 3 to 9 h, and the protein profile of each of the 37 isolates was unique. Nitrogen fixation, total nitrogen accumulation, and plant growth varied significantly among black locust seedlings inoculated with the representative isolates. We conclude that great variation exists among Rhizobium spp. that nodulate black locust, and selection of strains for efficiency of the symbiotic association appears possible.

Journal ArticleDOI
TL;DR: Both plant and bacterial components of effective root nodules have unique adaptive features for maximizing carbon and nitrogen metabolism in an O 2 -limited environment.
Abstract: Because legume root nodules have high rates of carbon and nitrogen metabolism, they are ideal for the study of plant physiology, biochemistry and molecular biology. Many plant enzymes involved in carbon and nitrogen assimilation have enhanced activity and enzyme protein in nodules as compared to other plant organs. For all intents and purposes the interior of the root nodule is O 2 limited. Both plant and bacterial components of effective root nodules have unique adaptive features for maximizing carbon and nitrogen metabolism in an O 2 -limited environment

Journal ArticleDOI
TL;DR: Three wheat cultivars with different tolerances against free aluminium were grown monoxenically in association with Azospirillum brasilense and much higher concentrations of low molecular dicarboxylic acids were found in the exudates of aluminium-tolerant plants.
Abstract: Three wheat cultivars with different tolerances against free aluminium were grown monoxenically in association with Azospirillum brasilense. In situ nitrogen fixation, measured with the acetylene reduction assay, was higher by the aluminium-tolerant cultivars than by the sensitive cultivar. The transfer of fixed nitrogen to the host plant, determined by the 15N dilution technique, was also significantly higher in the aluminium-resistant wheat plants. The total accumulation of fixed nitrogen in the host plants due to an A. brasilense inoculation varied from approximately 13% to 17% of the total nitrogen in the root and 2.9% to 3.9% of the nitrogen in the shoot. The quantity and quality of exudates released in liquid nutrient solution were analysed separately for two of the wheat cultivars, one aluminium-tolerant and one aluminium-sensitive. After 29 days of growth the aluminium-tolerant plants exudated significantly higher total amounts of carbon than aluminium-sensitive plants. No differences between the two cultivars existed in the carbon exudation rate per gram dry root. Much higher concentrations of low molecular dicarboxylic acids i.e. succinic, malic and oxalic acid, were found in the exudates of aluminium-tolerant plants. Dicarboxylic acids are potential chelating compounds for positively charged metals such as aluminium and they may play an important role in protecting the plant against aluminium incorporation. They are also very suitable substrates for Azospirillum spp. It is therefore suggested that these factors may be causing the higher associative nitrogen fixation rates which was found in the aluminium-tolerant wheat cultivars.

Journal ArticleDOI
01 Feb 1992-Ecology
TL;DR: A comparison of typical nitrogenase activities measured at several sites suggested that rates of N2 fixation were highest in L. lepidus growing at disturbed low N sites, and adult lupine C and N composition also varied during the growing season, with trends correlated with seasonal patterns of nitrogenase activity.
Abstract: We measured the timing and magnitude of nitrogenase activity and N2 fixation by lupines colonizing early successional volcanic sites at Mount St. Helens. Ni- trogenase activity (measured by acetylene reduction) in Lupinus lepidus growing at a py- roclastic site exhibited significant diurnal trends, with lowest ethylene evolution rates at night. Nitrogenase activity also followed seasonal trends, with high rates in June, very low levels in August, the dry warm part of the season, and a partial recovery of nitrogenase activity in September after precipitation resumed. A comparison of typical nitrogenase activities measured at several sites suggested that rates of N2 fixation were highest in L. lepidus growing at disturbed low N sites. Adult lupine C and N composition also varied during the growing season, with trends correlated with seasonal patterns of nitrogenase activity. Seasonal N2 fixation in L. lepidus and L. latifolius was measured using 15N isotope. Both species fixed ;60% of their N during the first season of growth with some evidence of preferential allocation to aboveground biomass. N fixation by Lupinus lepidus individuals was - 18.1 mg/g biomass or an average of 15.4 mg per plant, while L. latifolius fixed an average of 16.3 mg/g biomass, equivalent to 22.9 mg per plant. Average net C fixation during the same period was 355 and 589 mg per plant for L. lepidus and L. latifolius, respectively. Despite these rates, the current distribution of L. lepidus into a few, small patches that occupy < 1% of the surface area indicates that annual N inputs by lupines are <0.05 kg/ha and thus probably not the primary source of N input into developing Mount St. Helens pyroclastic sites except at a local scale.

Journal ArticleDOI
TL;DR: The results suggest that heliobacteria, like phototrophic purple bacteria, are active N2-fixing bacteria and that despite their gram-positive phylogenetic roots, heliOBacteria retain the capacity to control nitrogenase activity by a “switch-off” type of mechanism.
Abstract: Three species of anoxygenic phototrophic heliobacteria, Heliobacterium chlorum, Heliobacterium gestii, and Heliobacillus mobilis, were studied for comparative nitrogen-fixing abilities and regulation of nitrogenase. Significant nitrogenase activity (acetylene reduction) was detected in all species grown photoheterotrophically on N2, although cells of H. mobilis consistently had higher nitrogenase activity than did cells of either H. chlorum or H. gestii. Nitrogen-fixing cultures of all three species of heliobacteria were subject to “switch-off” of nitrogenase activity by ammonia; glutamine also served to “switch-off” nitrogenase activity but only in cells of H. mobilis and H. gestii. Placing photosynthetically grown heliobacterial cultures in darkness also served to “switch-off” nitrogenase activity. Dark-mediated “switch-off” was complete in lactate-grown heliobacteria but in pyruvate-grown cells substantial rates of nitrogenase activity continued in darkness. In all heliobacteria examined ammonia was assimilated primarily through the glutamine synthetase/glutamate synthase (GS/GOGAT) pathway although significant levels of alanine dehydrogenase were present in extracts of cells of H. gestii, but not in the other species. The results suggest that heliobacteria, like phototrophic purple bacteria, are active N2-fixing bacteria and that despite their gram-positive phylogenetic roots, heliobacteria retain the capacity to control nitrogenase activity by a “switch-off” type of mechanism. Because of their ability to fix N2 both photosynthetically and in darkness, it is possible that heliobacteria are significant contributors of fixed nitrogen in their paddy soil habitat.

Journal ArticleDOI
TL;DR: The results suggest that recently fixed nitrogen is incorporated into glutamine in the N2-fixing trichomes and may be passed as glutamate to non-N2- fixing trchomes and the previously observed absence of appreciable uptake of NH4+, NO3-, or urea by Trichodesmium spp.
Abstract: We examined freshly collected samples of the colonial planktonic cyanobacterium Trichodesmium thiebautii to determine the pathways of recently fixed N within and among trichomes. High concentrations of glutamate and glutamine were found in colonies. Glutamate and glutamine uptake rates and concentrations in cells were low in the early morning and increased in the late morning to reach maxima near midday; then uptake and concentration again fell to low values. This pattern followed that previously observed for T. thiebautii nitrogenase activity. Our results suggest that recently fixed nitrogen is incorporated into glutamine in the N2-fixing trichomes and may be passed as glutamate to non-N2-fixing trichomes. The high transport rates and concentrations of glutamate may explain the previously observed absence of appreciable uptake of NH4+, NO3-, or urea by Trichodesmium spp. Immunolocalization, Western blots (immunoblots), and enzymatic assays indicated that glutamine synthetase (GS) was present in all cells during both day and night. GS appeared to be primarily contained in cells of T. thiebautii rather than in associated bacteria or cyanobacteria. Double immunolabeling showed that cells with nitrogenase (Fe protein) contained levels of the GS protein that were twofold higher than those in cells with little or no nitrogenase. GS activity and the uptake of glutamine and glutamate dramatically decreased in the presence of the GS inhibitor methionine sulfoximine. Since no glutamate dehydrogenase activity was detected in this species, GS appears to be the primary enzyme responsible for NH3 incorporation.

Journal ArticleDOI
TL;DR: Three strains of Bradyrhizobium, 280A, 2209A and 32H1, that nodulated peanuts (Arachis hypogaea L.), were tested for their ability to grow and survive at elevated temperatures of up to 42°C in laboratory culture to determine the effect of high root temperature on nodulation, growth and nitrogen fixation of peanut.
Abstract: Three strains of Bradyrhizobium, 280A, 2209A and 32H1, that nodulated peanuts (Arachis hypogaea L), were tested for their ability to grow and survive at elevated temperatures of up to 42°C in laboratory culture Strain 32H1 was unable to grow at 37°C and was more sensitive to elevated temperatures than the other two strains All three produced heat-shock proteins of molecular weights 17 kDa and 18 kDa Two greenhouse experiments were conducted to determine the effect of high root temperature on nodulation, growth and nitrogen fixation of peanut Two peanut varieties (Virginia cv NC7 and Spanish cv Pronto) were inoculated and exposed to root temperatures of 30°, 37° and 40°C Nodulation and nitrogen fixation were strongly affected by root temperature but there was no variety × temperature interaction At a constant 40°C root temperature no nodules were formed Nodules were formed when roots were exposed to this temperature with diurnal cycling but no nitrogen fixation occurred Highest plant dry weight, shoot nitrogen content and total nitrogen were observed at a constant root temperature of 30°C Increasing root temperature to 37°C reduced average nitrogen content by 37% and total nitrogen by 49% but did not reduce nodulation The symbiotic performance of the strains corresponded to their abilities to grow and survive at high temperature in culture

Journal ArticleDOI
TL;DR: The general conclusions are that increases in nitrogen-fixation potential should be possible in all grain legumes through both plant breeding and strain improvement and major increases in the quantity of nitrogen fixed could be achieved.
Abstract: This review considers the main factors determining the level of nitrogen fixation in the major grain-legume crops (chickpea, common bean, cowpea, faba bean, lentil, pea, peanut, pigeon pea and soybean) and discusses the possibilities and means for improving symbiotic nitrogen fixation. Methods of determining whether nitrogen is limiting growth and yield are discussed, and responses of the various crops to both the addition of nitrogen fertilizer and inoculation with effective strains of rhizobia are described. The present situation and future prospects for increasing nitrogen fixation through plant breeding, development of superior strains, superior host–strain combinations, improved inoculation techniques, bioengineering and other non-traditional methods are discussed. The general conclusions are that increases in nitrogen-fixation potential should be possible in all grain legumes through both plant breeding and strain improvement. Major increases in the quantity of nitrogen fixed could be achieved simpl...

Journal Article
TL;DR: A noticeable decline in nitrogenase activity and total nitrogen concentration in the plant under saline conditions could be attributed to nodule senescence, as shown by the lowering trend of leghaemoglobin, soluble protein and carbohydrate contents of cytosol and bacteroids.
Abstract: Faba bean cultivar Giza 3 inoculated with Rhizobium leguminosarum RCR 1001 was grown in a pot experiment and irrigated with saline water ( mixture of NaCl and CaC12 0.25 Ca:Na on molar basis). Salinity levels of 5.8, 8.8, 11.6 and 14.6 d Sm \"! (equivalent to 50, 75, 100 and 125 mM NaCl) significantly decreased nodule number, nodule fresh weight and total nitrogenase activity. Salinity inhibited specific nitrogenase activity, protein, leghaemoglobin and carbohydrate content of the nodules at 11.6 and 14.6 d Sm \"! (100 and 125 mM NaCl). Salinity levels of 8.8, 11.6 and 14.6 dSm-1 (75, 100 and 125 mM NaCl) caused a significant reduction in dry weights of roots, stems, leaves and total plant nitrogen. The results indicated that Rhizobium leguminosarum RCR 1001 formed an infective and effective symbiosis with faba bean under saline conditions. A noticeable decline in nitrogenase activity and total nitrogen concentration in the plant under saline conditions could be attributed to nodule senescence, as shown by the lowering trend of leghaemoglobin, soluble protein and carbohydrate contents of cytosol and bacteroids.

Journal ArticleDOI
TL;DR: The results suggested that the nitrogenase activity was mostly produced by Azospirillum, and occurred at the expense of the degradation and fermentation products of the pectin.
Abstract: Cocultures of different Azospirillum species with Bacillus polymyxa or Bacillus subtilis allow the efficient utilization of pectin as carbon and energy sources for nitrogen fixation. The nitrogenase activity obtained with cocultures was as high as 30–80 nmol C2H4 h−1 mL−1, a much higher value than that obtained with pure cultures of either Azospirillum (up to 13 nmol C2H4 h−1 mL−1) or B. polymyxa (up to 2 nmol C2H4 h−1 mL−1) alone. To establish to what extent each partner contributed to nitrogenase activity, acetylene reduction was assayed as a function of time and it was also measured on Azospirillum cultivated in the cultures filtrates of the Bacillus. The results suggested that the nitrogenase activity was mostly produced by Azospirillum. The nitrogenase activity occurred at the expense of the degradation and fermentation products of the pectin. The new pectinolytic species, Azospirillum irakense, utilized both degradation and fermentation products of pectin, whereas the nonpectinolytic strains (Azospi...

Journal ArticleDOI
TL;DR: The effect of different rotational systems on symbiotic N2 fixation by, and yield of, peanut was determined over two growing seasons in southern Queensland and suggested that soil N reserves had been depleted by 27-37 kg N/ha as a result of that season's peanut cropping.
Abstract: The effect of different rotational systems on symbiotic N2 fixation by, and yield of, peanut (Arachis hypogaea L. cv. Virginia Bunch) was determined over two growing seasons in southern Queensland. Estimates of N2fixation were obtained with the 15N natural abundance procedure by using a non-nodulating peanut genotype as a non-N2-fixing reference plant. Three rotational treatments were compared in both years. Crops were rainfed and dependent solely upon indigenous rhizobia for nodulation in the first season, but in the second, supplementary irrigation was supplied and portions of the trial were inoculated with three different rhizobial strains. Dry matter, crop N and pod yield were greatest in both years following a period of grass ley and lowest in a continuous peanut system. However, the proportion of the crop N derived from N2 fixation (PN2) was the same across all rotational and inoculation treatments, despite pre-sowing differences in the levels of soil nitrate and numbers of native rhizobia, and despite seasonal differences in the amounts of N mineralized. Proportions and amounts of N2 fixed were lower (22-31% and 32-57 kg N/ha, respectively) during the first year than under the better moisture conditions and lower concentrations of soil nitrate that prevailed in following season (44-48s and 82-120 kg N/ha). A greater proportion of fixed N was located in the pods and kernels (PN2 62-70%) than in the supporting shoot (PN2 13-21%) in the second year. When estimates of N2 fixation were compared with N removal in the pods at harvest, the net N balance suggested that soil N reserves had been depleted by 27-37 kg N/ha as a result of that season's peanut cropping.

Journal ArticleDOI
TL;DR: This phenotype was found for tested host plants of NGR234 with either determinate- or indeterminate-type nodules, confirming for the first time that symbiosis-specific uptake of dicarboxylates is a prerequisite for nitrogen fixation in tropical legume symbioses.
Abstract: To investigate the role of dicarboxylate transport in nitrogen-fixing symbioses between Rhizobium and tropical legumes, we made a molecular genetic analysis of the bacterial transport system in Rhizobium sp. NGR234. This braod host range strain fixes nitrogen in association with evolutionarily divergent legumes. Two dicarboxylate transport systems were cloned from Rhizobium NGR234. One locus was chromosomally located, whereas the other was carried on the symbiotic plasmid (pSym) and contained a dctA carrier protein gene, which was analyzed in detail. Although the DNA and derived amino acid sequences of the structural gene were substantially homologous to that of R. meliloti, its promoter sequences was quite distinct, and the upstream sequence also exhibited no homology to dctB, which is found at this position in R. meliloti. A site-directed internal deletion mutant in dctA of NGR234 exhibited a (unique) exclusively symbiotic phenotype that could grow on dicarboxylates ex planta, but could not fix nitrogen in planta. This phenotype was found for tested host plants of NGR234 with either determinate- or indeterminate-type nodules, confirming for the first time that symbiosis-specific uptake of dicarboxylates is a prerequisite for nitrogen fixation in tropical legume symbioses.