Showing papers on "Nitrogen fixation published in 1999"

Rhizobium-Legume Symbiosis and Nitrogen Fixation under Severe Conditions and in an Arid Climate

Abstract: Biological N2 fixation represents the major source of N input in agricultural soils including those in arid regions. The major N2-fixing systems are the symbiotic systems, which can play a significant role in improving the fertility and productivity of low-N soils. The Rhizobium-legume symbioses have received most attention and have been examined extensively. The behavior of some N2-fixing systems under severe environmental conditions such as salt stress, drought stress, acidity, alkalinity, nutrient deficiency, fertilizers, heavy metals, and pesticides is reviewed. These major stress factors suppress the growth and symbiotic characteristics of most rhizobia; however, several strains, distributed among various species of rhizobia, are tolerant to stress effects. Some strains of rhizobia form effective (N2-fixing) symbioses with their host legumes under salt, heat, and acid stresses, and can sometimes do so under the effect of heavy metals. Reclamation and improvement of the fertility of arid lands by application of organic (manure and sewage sludge) and inorganic (synthetic) fertilizers are expensive and can be a source of pollution. The Rhizobium-legume (herb or tree) symbiosis is suggested to be the ideal solution to the improvement of soil fertility and the rehabilitation of arid lands and is an important direction for future research.

Symbiotic N2 fixation response to drought

Abstract: Symbiotic nitrogen fixation is highly sensitive to drought, which results in decreased N accumulation and yield of legume crops. The effects of drought stress on N 2 fixation usually have been perceived as a consequence of straightforward physiological responses acting on nitrogenase activity and involving exclusively one of three mechanisms: carbon shortage, oxygen limitation, or feedback regulation by nitrogen accumulation. The sensitivity of the nodule water economy to the volumetric flow rate of the phloem into the nodule offers a common framework to understand each of these mechanisms. As these processes are sensitive to volumetric phloem flow into the nodules, variations in phloem flow as a result of changes in turgor pressure in the leaves are likely to cause rapid changes in nodule activity. This could explain the special sensitivity of N 2 fixation to drying soils. It seems likely that N feedback may be especially important in explaining the response mechanism in nodules. A number of studies have indicated that a nitrogenous signal(s), associated with N accumulation in the shoot and nodule, exists in legume plants so that N 2 fixation is inhibited early in soil drying. The existence of genetic variation in N 2 fixation response to water deficits among legume cultivars opens the possibility for enhancing N 2 fixation tolerance to drought through selection and breeding.

Sucrose Synthase in Legume Nodules Is Essential for Nitrogen Fixation

Abstract: The role of sucrose synthase (SS) in the fixation of N was examined in the rug4 mutant of pea (Pisum sativum L.) plants in which SS activity was severely reduced. When dependent on nodules for their N supply, the mutant plants were not viable and appeared to be incapable of effective N fixation, although nodule formation was essentially normal. In fact, N and C resources invested in nodules were much greater in mutant plants than in the wild-type (WT) plants. Low SS activity in nodules (present at only 10% of WT levels) resulted in lower amounts of total soluble protein and leghemoglobin and lower activities of several enzymes compared with WT nodules. Alkaline invertase activity was not increased to compensate for reduced SS activity. Leghemoglobin was present at less than 20% of WT values, so O2 flux may have been compromised. The two components of nitrogenase were present at normal levels in mutant nodules. However, only a trace of nitrogenase activity was detected in intact plants and none was found in isolated bacteroids. The results are discussed in relation to the role of SS in the provision of C substrates for N fixation and in the development of functional nodules.

Phylogenetic diversity of nitrogen fixation genes in the symbiotic microbial community in the gut of diverse termites.

Abstract: Nitrogen fixation by the microorganisms in the gut of termites is one of the crucial aspects of symbiosis, since termites usually thrive on a nitrogen-poor diet. The phylogenetic diversity of the nitrogen-fixing organisms within the symbiotic community in the guts of various termite species was investigated without culturing the resident microorganisms. A portion of the dinitrogenase reductase gene (nifH) was directly amplified from DNA extracted from the mixed population in the termite gut. Analysis of deduced amino acid sequences of the products of the clonally isolated nifH genes revealed the presence of diverse nifH sequences in most of the individual termite species, and their constituents were considerably different among termite species. A majority of the nifH sequences from six lower termites, which showed significant levels of nitrogen fixation activity, could be assigned to either the anaerobic nif group (consisting of clostridia and sulfur reducers) or the alternative nif methanogen group among the nifH phylogenetic groups. In the case of three higher termites, which showed only low levels of nitrogen fixation activity, a large number of the sequences were assigned to the most divergent nif group, probably functioning in some process other than nitrogen fixation and being derived from methanogenic archaea. The nifH groups detected were similar within each termite family but different among the termite families, suggesting an evolutionary trend reflecting the diazotrophic habitats in the symbiotic community. Within these phylogenetic groups, the sequences from the termites formed lineages distinct from those previously recognized in studies using classical microbiological techniques, and several sequence clusters unique to termites were found. The results indicate the presence of diverse potentially nitrogen-fixing microbial assemblages in the guts of termites, and the majority of them are as yet uncharacterized.

Stimulation of nitrogen fixation and induction of flavonoid‐like compounds by rhizobacteria

Abstract: Bacteria belonging to fluorescent Pseudomonas and to the spore-forming Bacillus groups, predominantly found in the rhizosphere and rhizoplane of healthy chickpea plants, were studied in order to determine their effect on effective strains of Rhizobium sp. 'Cicer' (Ca181 and Ca313) and their plant growth-promoting ability under aseptic and soil conditions. Co-inoculation of the rhizobacteria with effective Rhizobium strains of chickpea resulted in a significant increase in nodule weight, root and shoot biomass and total plant nitrogen when grown either in sterilized chillum jars or under pot culture conditions. The Rhizobium stimulatory Pseudomonas sp. 'CRP55b' showed maximum increase in all the symbiotic parameters. On co-inoculation with 'Ca181' and 'Ca313', Pseudomonas sp. 'CRP55b' and 'CRS68' resulted in significant increases in nodule weight, root and shoot biomass and total plant nitrogen. The nodule stimulating rhizobacteria enhanced levels of flavonoid-like compounds in roots on seed bacterization. Also, ethyl acetate extracts of culture supernatant fluids when applied to seeds resulted in enhancement of flavonoids in roots, suggesting that the rhizobacteria have a direct influence on root flavonoids which might be an additional factor in nodule promotion by these bacteria. These rhizobacteria also produced fluorescent compounds with absorption maxima at 252 nm, similar to those of plant flavonoids but having a different Rf value. Whether such fluorescent compounds act as signal molecules for induction of plant flavonoids in plant roots requires further study.

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

Abstract: Summary We 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.

Nutrient control of cyanobacterial blooms in the baltic sea

Abstract: Cyanobacterial blooms in the Baltic Sea were investigated with respect to growth Limitation and nitrogen fixation. The community was composed predominantly of Synechococcus spp., and large, heterocystous, nitrogen-fixing cyanobacteria (Aphanizomenon spp, and Nodularia spp.), that usually formed buoyant macroscopic aggregates. Although conspicuous, these aggregates often represented less than 20 to 30% of the total chlorophyll a. Nitrogenase activity was not Limited by molybdate availability, but, instead, by high concentrations of sulfate. This may explain inhibition of nitrogenase activity at high salinities. Inhibition of nitrogenase activity at high salinity did not occur when sulfate concentration was kept low. Nitrogen fixation and growth of the diazotrophic cyanobacteria were limited by iron. Synechococcus spp. was primarily nitrogen Limited but iron appeared to be the secondary limiting substrate, particularly when these organisms depended on nitrate as the source of nitrogen. Nutrient limitation of the picoplanktonic community was particularly apparent when a wind- induced mixing event occurred. These organisms responded by a subsequent doubling of their biomass within 24 h. Mixing of the water column apparently transported nutrients from greater depth into the euphotic zone, causing a temporary relieve of nitrogen limitation. [KEYWORDS: Baltic Sea; cyanobacteria; bloom; picoplankton; nutrients; iron; nitrogen; nitrogen fixation; molybdate; sulfate Nitrogen-fixation; planktonic cyanobacteria;marine-phytoplankton; iron limitation; gas vesicles; cell-size; ocean; eutrophication; synechococcus; communities]

Isolation and characterization of diazotrophic bacteria from banana and pineapple plants

Abstract: Banana and pineapple fruit crops are widely cultivated in tropical areas where high amounts of fertilizers are applied, principally nitrogen. Over 200 kg N.ha-1.yr-1 is often applied to these crops. Nevertheless, developing countries face the problem of high costs of chemical fertilizers. As already demonstrated for other tropical crops, like sugar cane, the utilization of nitrogen-fixing bacteria may support the growth of these fruit plants. In this work, we demonstrate the association of nitrogen-fixing bacteria with banana and pineapple. Samples from roots, stems, leaves and fruits of different genotypes showed the occurrence of diazotrophic bacteria, when evaluated in semi-specific semi-solid media. These isolates could be separated into seven different groups according to their morphological and physiological characteristics. Additional, phylogenetic assignments were performed with group- and species-specific oligonucleotide probes. Bacteria related to the groups of Azospirillum amazonense, Azospirillum lipoferum, Burkholderia sp. and a group similar to the genus Herbaspirillum could be detected in samples of both crops. However, Azospirillum brasilense and another two groups of Herbaspirillum-like bacteria were detected only in banana plants. Two isolates of the latter group were identified as Herbaspirillum seropedicae, whereas the other isolates may represent a new Herbaspirillum species.

Comparative study of nitrogen fixation and carbon metabolism in two chick-pea (Cicer arietinum L.) cultivars under salt stress

Abstract: salinity is one of the major environmental constraints on agriculture in many regions of the world (Boyer, 1982;

Isolation and characterization of alfalfa-nodulating rhizobia present in acidic soils of central argentina and uruguay

Abstract: Over 4 million ha of land throughout Argentina and Uruguay are used for the production of alfalfa (Medicago sativa L.) (38). Therefore, it is important to manage the N2-fixing symbiosis to maximize the production of this crop. An important constraint to this aim results from the moderately low soil pH that affects the establishment of an effective symbiosis with indigenous and inoculated rhizobia. Large areas of arable lands in the central region of Argentina have progressively acidified over the last 10 to 20 years (21, 34), where the continuous cultivation over time without crop rotation has been identified as one of the main factors that favored the acidification of soils (21, 34). It has been shown that the poor symbiosis at low pH results from a variety of influences upon the host plant (24, 27), the population of rhizobia (31), and the symbiotic interaction itself (11, 36, 41). Early studies by Munns (36, 37) compared the progress of symbiosis under neutral and acid conditions, concluding that early steps during preinfection are the more acid-sensitive events (36). This observation is in agreement with results reported by Caetano Anolles et al. in 1989, who showed a negative influence of low pH on the bacterial attachment to roots. Most of the fundamental research has sought to characterize the physiology of the interaction (11, 23, 25, 27, 28) and the effects of acidity in laboratory and in field experiments (1a, 39) and only more recently to address the identification of the bacterial determinants of acid tolerance (18, 43, 44). Particularly, Sinorhizobium meliloti strains are among the more acid-sensitive rhizobia (6, 19, 20). Most S. meliloti isolates tolerate acidity in the range between pH 5.5 and 6.0 (25). Although there is no basis to support that a higher acid tolerance of the bacteria will correspond to a better symbiotic performance under acidic conditions, it was found that acid-tolerant (AT) S. meliloti strains isolated from nodulated Medicago spp. collected in Sardinia enhanced the establishment of medic pastures in mildly acidic soils from Western Australia (23, 25). In any case, although symbiotic proficiency and acid tolerance of rhizobia are both desirable bacterial traits, they are not necessarily linked (17, 22, 25, 30, 31). While basic aspects of symbiosis have been extensively characterized, further work is still needed in order to increase our knowledge on the rhizobial ecology under suboptimal environmental conditions such as acidity. The characterization of the populations of alfalfa-nodulating rhizobia from acid soils had shown the presence of alfalfa-specific nodulating S. meliloti and another lineage represented by strain Or191 isolated from Oregon which also nodulates the common bean (13, 15). Strain Or191 was also shown to be more tolerant to acidity on agar plates (pH 5.2). Results of the genetic characterization indicate that strain Or191 is related to a previously unrecognized taxonomic group that includes strains of Rhizobium phaseoli type I (15), since renamed Rhizobium etli (32, 45). The AT strain Or191 was ineffective in alfalfa (13) but had measurable levels of symbiotic nitrogen fixation (15). So far, a detailed examination of the composition of native populations of alfalfa-nodulating rhizobia in soils from Argentina and Uruguay has not yet been carried out. In this work, we present results on the isolation and characterization of alfalfa-nodulating rhizobia from local acid soils and demonstrate the presence of two rhizobial populations with marked differences in their acid tolerance and symbiotic properties.

Effects of mycorrhizal colonization on biomass production and nitrogen fixation of black locust (Robinia pseudoacacia) seedlings grown under elevated atmospheric carbon dioxide

Abstract: Interactive effects of elevated atmospheric CO2 and arbuscular mycorrhizal (AM) fungi on biomass production and N2 fixation were investigated using black locust (Robinia pseudoacacia). Seedlings were grown in growth chambers maintained at either 350 μmol mol−1 or 710 μmol mol−1 CO2. Seedlings were inoculated with Rhizobium spp. and were grown with or without AM fungi. The 15N isotope dilution method was used to determine N source partitioning between N2 fixation and inorganic fertilizer uptake. Elevated atmospheric CO2 significantly increased the percentage of fine roots that were colonized by AM fungi. Mycorrhizal seedlings grown under elevated CO2 had the greatest overall plant biomass production, nodulation, N and P content, and root N absorption. Additionally, elevated CO2 levels enhanced nodule and root mass production, as well as N2 fixation rates, of non- mycorrhizal seedlings. However, the relative response of biomass production to CO2 enrichment was greater in non-mycorrhizal seedlings than in mycorrhizal seedlings. This study provides strong evidence that arbuscular mycorrhizal fungi play an important role in the extent to which plant nutrition of symbiotic N2-fixing tree species is affected by enriched atmospheric CO2.

Nitrogen fixation and growth of non-crop legume species in diverse environments

Abstract: Of the 643 legume genera, few have been exploited in agriculture and 40% have not even been evaluated for their ability to nodulate and fix nitrogen. Most of these are in tropical/subtropical regions, with habitats ranging from extremely dry to flooded. Recent work in some of these areas shows that plants can nodulate under conditions previously thought to be disadvantageous. The accepted dogma that nitrogen fixing legumes have a high demand for P is challenged and examples of how legumes can extract P from soils with low available P described. Species tolerant to shading and high soil Al are cited, although the mechanisms of adaptation are not yet clear. Some tropical soils have high nitrate levels and, contrary to perceived wisdom, there are legumes which can nodulate under such conditions. Many tropical tree legumes prefer ammonium to nitrate and are able to fix nitrogen and assimilate ammonium at the same time. In all these cases, there are genotypic differences, both within and among species. Large areas of tropical fresh water, such as the Brazilian Pantanal and the Orinoco floodplain have nodulated legumes predominant in their flora. The ecological potential of these has not been evaluated. One of the sites of nodule evolution is likely to have been in such areas. Modes of infection of legumes by rhizobia vary with taxonomic tribe and may represent evolution for survival in different environments. As more legumes, from more ecosystems are studied, a wider range of adaptations is likely to be found. Work is urgently needed to study these, especially in areas being cleared for agriculture or by logging.

Genetics of nitrogen regulation in Methanococcus maripaludis.

Abstract: We have used genetic methods in Methanococcus maripaludis to study nitrogen metabolism and its regulation. We present evidence for a "nitrogen regulon" in Methanococcus and Methanobacterium species containing genes of nitrogen metabolism that are regulated coordinately at the transcriptional level via a common repressor binding site sequence, or operator. The implied mechanism for regulation resembles the general bacterial paradigm for repression, but contrasts with well-known mechanisms of nitrogen regulation in bacteria, which occur by activation. Genes in the nitrogen regulons include those for nitrogen fixation, glutamine synthetase, (methyl)ammonia transport, the regulatory protein GlnB, and ammonia-dependent NAD synthetase, as well as a gene of unknown function. We also studied the function of two novel GlnB homologues that are encoded within the nif gene cluster of diazotrophic methanogens. The phenotype resulting from a glnB null mutation in M. maripaludis provides direct evidence that glnB-like genes are involved in "ammonia switch-off," the post-transcriptional inhibition of nitrogen fixation upon addition of ammonia. Finally, we show that the gene nifX is not required for nitrogen fixation, in agreement with findings in several bacteria. These studies illustrate the utility of genetic methods in M. maripaludis and show the enhanced perspective that studies in the Archaea can bring to known biological systems.

Nutrient Limitation to Nitrogen Fixation in Young Volcanic Sites

Abstract: I used measures of 15N natural abundance and of nitrogenase activity (acetylene reduction) to examine whether the supply of non-N nutrients limits rates of N2 fixation on young volcanic substrates in Hawaii. Leaves of the dominant tree (Metrosideros polymorpha, a nonfixer) were strongly depleted in 15N in control plots (–10.8 to –11.10/00). More than 5 y of repeated fertilization with P increased δ15N to –8.9 to –9.90/00, and the addition of all other essential plant nutrients (except N) together with P further increased 15N to –8.1 to –9.30/00. This pattern is consistent with enhanced N2 fixation, because newly fixed N would have a δ15N near 00/00. Assays of nitrogenase activity in the experimental plots demonstrated that potential N fixation associated with nonvascular plants and with tree and fern litter were increased significantly by additions of P and by the combined nutrient treatment; when these were added together, the increase in nitrogenase activity was 6- to 11-fold over control plots. The supply of P and other weathering-derived nutrients constrains rates of N2 fixation in these young volcanic sites and thereby contributes to the maintenance of N limitation to primary production and other ecosystem processes.

Nitrogen nutrition in nodulated field plants of the shrub tea legume Aspalathus linearis assessed using 15N natural abundance

Abstract: Provision of N, P, and Ca to field plants of A. linearis markedly (P<0.05) increased growth and N nutrition in a very acidic nutrient-poor soil. Application of P and Ca promoted a significant increase in %N derived from fixation and amounts of N fixed compared to those receiving no nutrients. N2 fixation measured under field conditions ranged from 3.8 g N plant-1 in unfertilized control to 7.1 g N plant-1 in fertilized plants. Overall, about 85% increase in N2 fixation was observed with P supply. The high N2-fixing activity in P-treated plants was confirmed by their lower (more negative) ∂15N values. Age of plants also influenced growth and symbiotic activity as the ∂15N values, %N derived from fixation, and N fixed were lower in 1- and 2-year-old plants compared to 3-year-old. The contribution of symbiotic fixation in unfertilized A. linearis to the N economy of the ecosystem ranged from 105 kg N ha-1 in 1-year-old plants to 128 kg N ha-1 in 3-year-old plants, clearly indicating the remarkable adaptation of this symbiosis to the very nutrient-poor, low pH conditions of Cedarberg soils.

Conversion factor between acetylene reduction and nitrogen fixation in free-living cyanobacteria from high arctic habitats

Abstract: The conversion factor between acetylene reduction and 15N incorporation in free-living cyanobacteria was determined in different high arctic habitats in the area of Ny-Alesund (78.5°N, 11.6°E), Spitsbergen, in the summer of 1994. The experiments were carried out under constant conditions, 19°C and 200 µE·m-2·s-1. The nitrogen-fixation activities, measured as 15N-incorporation, were in the range 4.01-6.54 mg N2 fixed·gdw-1·day-1 (dw, dry weight) in sheets of Nostoc commune and 778-1206 mg N2 fixed·m-2·day-1 in the cyanobacterial crusts. The acetylene reduction activities were in the range 0.72-1.91 mg ethylene produced·gdw-1·day-1 of N. commune and 12.8-63.7 mg ethylene produced·m-2·day-1 in the cyanobacterial crusts. The conversion factor of N. commune ranged from 0.11 to 0.48 for ethylene produced to nitrogen fixed, whereas the cyanobacterial crusts covering the soil surface gave conversion factors in the range 0.022-0.073 for ethylene produced to nitrogen fixed. An Anabaena sp., isolated from one of the...

Environmental factors influencing the nitrogen fixation activity of free-living terrestrial cyanobacteria from a high arctic area, Spitsbergen

Abstract: The influence of environmental factors on the nitrogen fixation activity of free-living, terrestrial cyanobacteria from a high arctic area were investigated using experimental manipulations with tw...

Do top-down and bottom-up controls interact to exclude nitrogen-fixing cyanobacteria from the plankton of estuaries? An exploration with a simulation model

Abstract: Explaining the nearly ubiquitous absence of nitrogen fixation by planktonic organisms in strongly nitrogen-limited estuaries presents a major challenge to aquatic ecologists. In freshwater lakes of moderate productivity, nitrogen limitation is seldom maintained for long since heterocystic, nitrogen-fixing cyanobacteria bloom, fix nitrogen, and alleviate the nitrogen limitation. In marked contrast to lakes, this behavior occurs in only a few estuaries worldwide. Primary production is limited by nitrogen in most temperate estuaries, yet no measurable planktonic nitrogen fixation occurs. In this paper, we present the hypothesis that the absence of planktonic nitrogen fixers from most estuaries is due to an interaction of bottom-up and top-down controls. The availability of Mo, a trace metal required for nitrogen fixation, is lower in estuaries than in freshwater lakes. This is not an absolute physiological constraint against the occurrence of nitrogen-fixing organisms, but the lower Mo availability may slow the growth rate of these organisms. The slower growth rate makes nitrogen-fixing cyanobacteria in estuaries more sensitive to mortality from grazing by Zooplankton and benthic organisms.

Simulation of nitrogen uptake, fixation and leaching in a grass/white clover mixture

Abstract: To represent nitrogen cycling in a low input grass/legume pasture system, a previously developed, weather-driven grass/white clover growth model has been adapted to become the crop growth component of the soil nitrogen dynamics model SOILN. This provides a means of simulating nitrogen uptake by a grass/white clover crop, an important component of the overall nitrogen balance in low-input grassland systems. Crop growth is represented by a photosynthesis equation adapted to take account of competition between the two crops for resources of light, water and nitrogen in the soil. Water shortage is represented by linked simulations with the soil water and heat model SOIL, and nitrogen shortage by links with the SOILN model. Nitrogen fixation has been introduced according to an equation for potential fixation reduced by environmental factors, particularly temperature. Transfer of nitrogen-rich clover plant material to the soil nitrogen pools of SOILN (from where it becomes available as a nutrient for grass) is also represented. The model is tested by comparing simulated cut crop yields and nitrogen content of cut material with measured data from perennial ryegrass/white clover at a test site. Soil nitrogen processes in the model are tested by comparing simulated and measured nitrate in drainflows. Apart from some discrepancies between simulated and measured results attributable to the inherent instability of a mixed crop system, agreement is reasonable by the standards of biological system models, indicating that the combined model gives a realistic representation of carbon and nitrogen processes in grassland with a grass, legume mixed crop.

The impact of elevated atmospheric CO2 and nitrate supply on growth, biomass allocation, nitrogen partitioning and N2 fixation of Acacia melanoxylon

Abstract: The interactive effects of nitrate supply and atmospheric CO2 concentration on growth, N2 fixation, dry matter and nitrogen partitioning in the leguminous tree Acacia melanoxylon R.Br. were studied. Seedlings were grown hydroponically in controlled-environment cabinets for 5 weeks at seven 15N-labelled nitrate levels, ranging from 3 to 6400 mmol m–3. Plants were exposed to ambient (~350 µmol mol–1) or elevated (~700 µmol mol–1) atmospheric CO2 for 6 weeks. Total plant dry mass increased strongly with nitrate supply. The proportion of nitrogen derived from air decreased with increasing nitrate supply. Plants grown under either ambient or elevated CO2 fixed the same amount of nitrogen per unit nodule dry mass (16.6 mmol N per g nodule dry mass) regardless of the nitrogen treatment. CO2 concentration had no effect on the relative contribution of N2 fixation to the nitrogen yield of plants. Plants grown with ≥50 mmol m–3 N and elevated CO2 had approximately twice the dry mass of those grown with ambient CO2 after 42 days. The rates of net CO2 assimilation under growth conditions were higher per unit leaf area for plants grown under elevated CO2. Elevated CO2 also decreased specific foliage area, due to an increase in foliage thickness and density. Dry matter partitioning between plant organs was affected by ontogeny and nitrogen status of the plants, but not by CO2 concentration. In contrast, plants grown under elevated CO2 partitioned more of their nitrogen to roots. This could be attributed to reduced nitrogen concentrations in foliage grown under elevated CO2.

Isolation of Nitrogen-Fixing Bacteria Containing Molybdenum-Independent Nitrogenases from Natural Environments

Abstract: Seven diazotrophs that grow well under Mo-deficient, N2-fixing conditions were isolated from a variety of environments. These isolates fall in the γ subdivision of the classProteobacteria and have genes that encode the Mo nitrogenase (nitrogenase 1) and the V nitrogenase (nitrogenase 2). Four of the isolates also harbor genes that encode the iron-only nitrogenase (nitrogenase 3).

Silicon promotes nodule formation and nodule function in symbiotic cowpea (Vigna unguiculata)

Abstract: Applying silicon in the form of metasilicic acid (H4SiO3) or silicic acid (H4SiO3) to Bradyrhizobium-infected, hydroponically grown cowpea seedlings resulted in a significant (P0.05) increase in the number of nodules, nodule dry matter, and nitrogen fixed on a per plant basis. Total dry matter of plants increased with silicon supply, and the differences were significant (P0.05) at the higher silicon concentrations. Cowpea plants cultured in sand were also assessed for their response to silicic acid. Nodule number and nodule mass increased with silicon supply to sand cultured plants, though nitrogen fixation was unaltered. Although silicon is not essential for growth of cowpea, it is important for nodule formation and nodule functioning in hydroponically grown plants. Consequently, data collected and conclusions drawn from earlier glasshouse experiments, which have excluded silicon from nutrient solutions, may be flawed. Future studies on nodulation and nitrogen fixation using legumes in liquid culture must therefore include silicon as a nutrient element.

Characterization of high temperature-tolerant rhizobia isolated from Prosopis juliflora grown in alkaline soil.

Abstract: A method was developed for the fast screening and selection of high-temperature tolerant rhizobial strains from root nodules of Prosopis juliflora growing in alkaline soils The high-temperature tolerant rhizobia were selected from 2,500 Rhizobium isolates with similar growth patterns on yeast mannitol agar plates after 72 h incubation at 30 and 45°C, followed by a second screening at 475°C Seventeen high-temperature tolerant rhizobial strains having distinguishable protein band patterns were finally selected for further screening by subjecting them to temperature stress up to 60°C in yeast mannitol broth for 6 h The high-temperature tolerant strains were NBRI12, NBRI329, NBRI330, NBRI332, and NBRI133 Using this procedure, a large number of rhizobia from root nodules of P juliflora were screened for high-temperature tolerance The assimilation of several carbon sources, tolerance to high pH and salt stress, and ability to nodulate P juliflora growing in a glasshouse and nursery of the strains were studied All five isolates had higher plant dry weight in the range of 299 to 886% in comparison with uninoculated nursery-grown plants It was demonstrated that it is possible to screen in nature for superior rhizobia exemplified by the isolation of temperature-tolerant strains, which established effective symbiosis with nursery-grown P juliflora These findings indicate a correlation between strain performance under in vitro stress in pure culture and strain behavior under symbiotic conditions Pure culture evaluation may be a useful tool in search for Rhizobium strains better suited for soil environments where high temperature, pH, and salt stress constitutes a limitation for symbiotic biological nitrogen fixation

Effects of herbicides on nodulation, symbiotic nitrogen fixation, growth and yield of pea ( Pisum sativum )

Abstract: Two pot experiments were performed to study the effects of three pre-emergence herbicides (terbutryn/terbuthylazine, trietazine/simazine and prometryn) and a post-emergence herbicide (bentazone) on nodulation, symbiotic nitrogen fixation, growth and yield of pea (Pisum sativum L.) grown in perlite under nitrogen-free conditions. All pre-emergence herbicides decreased nodulation, total nitrogenase activity, net photosynthesis, leaf area, root and shoot dry weight, nitrogen content and seed yield of peas. The effects of herbicides increased with increase in rate of application. Of the herbicides tested, terbutryn/terbuthylazine and trietazine/simazine had the greatest adverse effects. Pea plant biomass (root plus shoot) was correlated with plant nitrogen content but not total nitrogenase activity. The results of the experiments suggested that the decreased growth of herbicide-treated plants was due to direct effects of the herbicides on peas and not due to indirect effects of the herbicides on rhizobia.