Showing papers on "Nitrogen fixation published in 2003"

Plant Growth-Promoting Effects of Diazotrophs in the Rhizosphere

Abstract: Because of their ability to transform atmospheric N2 into ammonia that can be used by the plant, researchers were originally very optimistic about the potential of associative diazotrophic bacteria to promote the growth of many cereals and grasses. However, multiple inoculation experiments during recent decades failed to show a substantial contribution of Biological Nitrogen Fixation (BNF) to plant growth in most cases. It is now clear that associative diazotrophs exert their positive effects on plant growth directly or indirectly through (a combination of) different mechanisms. Apart from fixing N2, diazotrophs can affect plant growth directly by the synthesis of phytohormones and vitamins, inhibition of plant ethylene synthesis, improved nutrient uptake, enhanced stress resistance, solubilization of inorganic phosphate and mineralization of organic phosphate. Indirectly, diazotrophs are able to decrease or prevent the deleterious effects of pathogenic microorganisms, mostly through the synthesis of anti...

Nitrogenase gene diversity and microbial community structure: a cross-system comparison.

Abstract: Biological nitrogen fixation is an important source of fixed nitrogen for the biosphere. Microorganisms catalyse biological nitrogen fixation with the enzyme nitrogenase, which has been highly conserved through evolution. Cloning and sequencing of one of the nitrogenase structural genes, nifH, has provided a large, rapidly expanding database of sequences from diverse terrestrial and aquatic environments. Comparison of nifH phylogenies to ribosomal RNA phylogenies from cultivated microorganisms shows little conclusive evidence of lateral gene transfer. Sequence diversity far outstrips representation by cultivated representatives. The phylogeny of nitrogenase includes branches that represent phylotypic groupings based on ribosomal RNA phylogeny, but also includes paralogous clades including the alternative, non-molybdenum, non-vanadium containing nitrogenases. Only a few alternative or archaeal nitrogenase sequences have as yet been obtained from the environment. Extensive analysis of the distribution of nifH phylotypes among habitats indicates that there are characteristic patterns of nitrogen fixing microorganisms in termite guts, sediment and soil environments, estuaries and salt marshes, and oligotrophic oceans. The distribution of nitrogen-fixing microorganisms, although not entirely dictated by the nitrogen availability in the environment, is non-random and can be predicted on the basis of habitat characteristics. The ability to assay for gene expression and investigate genome arrangements provides the promise of new tools for interrogating natural populations of diazotrophs. The broad analysis of nitrogenase genes provides a basis for developing molecular assays and bioinformatics approaches for the study of nitrogen fixation in the environment.

Amino-acid cycling drives nitrogen fixation in the legume– Rhizobium symbiosis

Abstract: The biological reduction of atmospheric N2 to ammonium (nitrogen fixation) provides about 65% of the biosphere's available nitrogen. Most of this ammonium is contributed by legume–rhizobia symbioses1, which are initiated by the infection of legume hosts by bacteria (rhizobia), resulting in formation of root nodules. Within the nodules, rhizobia are found as bacteroids, which perform the nitrogen fixation: to do this, they obtain sources of carbon and energy from the plant, in the form of dicarboxylic acids2,3. It has been thought that, in return, bacteroids simply provide the plant with ammonium. But here we show that a more complex amino-acid cycle is essential for symbiotic nitrogen fixation by Rhizobium in pea nodules. The plant provides amino acids to the bacteroids, enabling them to shut down their ammonium assimilation. In return, bacteroids act like plant organelles to cycle amino acids back to the plant for asparagine synthesis. The mutual dependence of this exchange prevents the symbiosis being dominated by the plant, and provides a selective pressure for the evolution of mutualism.

Nitrogen fixation in perennial forage legumes in the field

Abstract: Nitrogen acquisition is one of the most important factors for plant production, and N contribution from biological N2 fixation can reduce the need for industrial N fertilizers. Perennial forages are widespread in temperate and boreal areas, where much of the agriculture is based on livestock production. Due to the symbiosis with N2-fixing rhizobia, perennial forage legumes have great potential to increase sustainability in such grassland farming systems. The present work is a summary of a large number of studies investigating N2 fixation in three perennial forage legumes primarily relating to ungrazed northern temperate/boreal areas. Reported rates of N2 fixation in above-ground plant tissues were in the range of up to 373 kg N ha−1 year−1 in red clover (Trifolium pratense L.), 545 kg N ha−1 year−1 in white clover (T. repens L.) and 350 kg N ha−1 year−1 in alfalfa (Medicago sativa L.). When grown in mixtures with grasses, these species took a large fraction of their nitrogen from N2 fixation (average around 80%), regardless of management, dry matter yield and location. There was a large variation in N2 fixation data and part of this variation was ascribed to differences in plant production between years. Studies with experiments at more than one site showed that also geographic location was an important source of variation. On the other hand, when all data were plotted against latitude, there was no simple correlation. Climatic conditions seem therefore to give as high N2 fixation per ha and year in northern areas (around 60°N) as in areas with a milder climate (around 40°N). Analyzing whole plants or just above-ground plant parts influenced the estimate of N2 fixation, and most reported values were underestimated since roots were not included. Despite large differences in environmental conditions, such as N fertilization and geographic location, N2 fixation (Nfix; kg N per ha and year) was significantly (P<0.001) correlated to legume dry matter yield (DM; kg per ha and year). Very rough, but nevertheless valuable estimations of Nfix in legume/grass mixtures (roots not considered) are given by Nfix = 0.026ċDM + 7 for T. pratense, Nfix = 0.031ċDM + 24 for T. repens, and Nfix = 0.021ċDM + 17 for M. sativa.

Endophytic colonization of plant roots by nitrogen-fixing bacteria

Abstract: Nitrogen-fixing bacteria are able to enter into roots from the rhizosphere, particularly at the base of emerging lateral roots, between epidermal cells and through root hairs. In the rhizosphere growing root hairs play an important role in symbiotic recognition in legume crops. Nodulated legumes in endosymbiosis with rhizobia are amongst the most prominent nitrogen-fixing systems in agriculture. The inoculation of non-legumes, especially cereals, with various non-rhizobial diazotrophic bacteria has been undertaken with the expectation that they would establish themselves intercellularly within the root system, fixing nitrogen endophytically and providing combined nitrogen for enhanced crop production. However, in most instances bacteria colonize only the surface of the roots and remain vulnerable to competition from other rhizosphere micro-organisms, even when the nitrogen-fixing bacteria are endophytic, benefits to the plant may result from better uptake of soil nutrients rather than from endophytic nitrogen fixation. Azorhizobium caulinodans is known to enter the root system of cereals, other non-legume crops and Arabidopsis, by intercellular invasion between epidermal cells and to internally colonize the plant intercellularly, including the xylem. This raises the possibility that xylem colonization might provide a non-nodular niche for endosymbiotic nitrogen fixation in rice, wheat, maize, sorghum and other non-legume crops. A particularly interesting, naturally occurring, non-nodular xylem colonising endophytic diazotrophic interaction with evidence for endophytic nitrogen fixation is that of Gluconacetobacter diazotrophicus in sugarcane. Could this beneficial endophytic colonization of sugarcane by G. diazotrophicus be extended to other members of the Gramineae, including the major cereals, and to other major non-legume crops of the World?

Defining new roles for plant and rhizobial molecules in sole and mixed plant cultures involving symbiotic legumes

Abstract: Summary The view that symbiotic legumes benefit companion and subsequent plant species in intercrop and rotation systems is well accepted. However, the major contributions made separately by legumes and their microsymbionts that do not relate to root-nodule N2 fixation have been largely ignored. Rhizobia (species of Rhizobium, Bradyrhizobium, Azorhizobium, Allorhizobium, Sinorhizobium and Mesorhizobium) produce chemical molecules that can influence plant development, including phytohormones, lipo-chito-oligosaccharide Nod factors, lumichrome, riboflavin and H2 evolved by nitrogenase. When present in soil, Nod factors can stimulate seed germination, promote plant growth and increase grain yields of legume and nonlegume crops, as well as stimulate increased photosynthetic rates following plant leaf spraying. Very low concentrations of lumichrome and H2 released by bacteroids also promote plant growth and increase biomass in a number of plant species grown under field and glasshouse conditions. Rhizobia are known to suppress the population of soil pathogens in agricultural and natural ecosystems and, in addition to forming nodule symbioses with rhizobia, the legume itself releases phenolics that can suppress pathogens and herbivores, solubilize nutrients, and promote growth of mutualistic microbes. Phytosiderophores and organic acid anions exuded by the host plant can further enhance mineral nutrition in the system. This review explores new insights into sole and mixed plant cultures with the aim of identifying novel roles for molecules of legume and microbial origin in natural and agricultural ecosystems.

Contribution of biological nitrogen fixation to cowpea: a strategy for improving grain yield in the semi-arid region of Brazil

Abstract: Nodulating bacteria from the family Rhizobiaceae are common in the semi-arid tropics around the world. The Brazilian semi-arid region extends over 95 million hectares of which only 3% is suitable for irrigation, therefore leaving an immense dryland area to be exploited by peasant farmers, who often lack appropriate technologies for sustainable management. Cowpea is an important crop in this area, representing the staple protein source for human nutrition. This work aimed to identify rhizobial strains capable of guaranteeing sufficient nitrogen derived from biological fixation for cowpea cultivated in dryland areas, evaluating not just efficiency but also the ecological parameters of competitiveness and survival in the soil. Grain yield and nodulation parameters showed that strain BR 3267 is capable of establishing efficient nodulation, improving both yield and total N accumulated in grain. Cowpea inoculated with strain BR 3267 showed grain productivity similar to plants receiving 50 kg of N per hectare, which is the amount of fertilizer commonly used in the north-east region. These characteristics associated with previously determined ecological properties makes strain BR 3267 an important resource for the optimization of biological nitrogen fixation in cowpea in the dryland areas of the semi-arid tropics. Data on the dynamics of rhizobial populations in such areas have shown that (1) the naturalized rhizobium population is very small and, by themselves, do not promote proper nodulation and, (2) the inoculant rhizobia do not persist between crops. Such characteristics represent an opportunity for the introduction of superior rhizobia strains, such as BR 3267, during the cowpea crop.

Effects of drought on nitrogen fixation in soybean root nodules

Abstract: Soybean plants [Glycine max (L.) Merr.] were grown in silica sand and were drought stressed for a 4 week period during reproductive development and without any mineral N supply in order to maximize demand for fixed nitrogen. A strain of Bradyrhizobium japonicum that forms large quantities of polysaccharide in nodules was used to determine whether or not the supply of reduced carbon to bacteroids limits nitrogenase activity. A depression of 30-40% in nitrogen content in leaves and pods of stressed plants indicated a marked decline in nitrogen fixation activity during the drought period. A 50% increase in the accumulation of bacterial polysaccharide in nodules accompanied this major decrease in nitrogen fixation activity and this result indicates that the negative impact of drought on nodules was not due to a depression of carbon supply to bacteroids. The drought treatment resulted in a statistically significant increase in N concentration in leaves and pods. Because N concentration and chlorophyll concentration in leaves were not depressed, there was no evidence of nitrogen deficiency in drought-stressed plants, and this result indicates that the negative impact of drought on nodule function was not the cause of the depression of shoot growth. At the end of the drought period, the concentration of carbohydrates, amino nitrogen, and ureides was significantly increased in nodules on drought-stressed plants. The overall results support the view that, under drought conditions, nitrogen fixation activity in nodules was depressed because demand for fixed N to support growth was lower.

Genetic resources for improving nitrogen fixation in legume-rhizobia symbiosis

Abstract: Leguminous crops are genetically polymorphous for the balance between symbiotrophic and combined types of nitrogen nutrition. In pea, polebean, alfalfa and fenugreek the wild-growing populations and local varieties exceed the agronomically advanced cultivars in the activity of N2 fixation that occurs in symbiosis with nodule bacteria (rhizobia). Combined nitrogen nutrition ensures higher productivity than symbiotrophic one in the “old” leguminous crops (pea, alfalfa, common vetch, polebean, soybean), while the symbiotrophic type dominates in some “young” crops (hairy vetch, kura clover, goat's rue). An importance is emphasized of using the symbiotically active wild-growing genotypes as the initial material for breeding the legume cultivars. The data on high heritability (broad sense, narrow sense, realized) of the legume symbiotic activity demonstrate that the plant selection for this activity may be highly effective. A range of methods to select the legumes for an improved symbiotic activity is available including plant growth in N-depleted substrates, analysis of nodulation scores, direct (“isotopic”) and indirect (acetylene reduction) estimation of nitrogenase activity. Analysis of the specificity of interactions between different plant genotypes and bacterial strains (via two-factor analysis of variance) demonstrates the strain-specific plant polygenes are of a special importance in controlling the intensity of nitrogen fixation. Therefore, a coordinated plant-bacteria breeding is required to create the optimal combinations of partners' genotypes. Selection and genetic construction of the commercially attractive rhizobia strains should involve improvement of nitrogen fixing, nodulation and competitive abilities expressed in combination with the symbiotically active plant genotypes, Breeding of the leguminous crops for the preferential nodulation by highly active rhizobia strains, for the ability to support N2 fixation under moderate N fertilization levels and to ensure a sufficient energy supply of symbiotrophic nitrogen nutrition is required

Nitrogen binding to the FeMo-cofactor of nitrogenase.

Abstract: Density functional calculations are presented to unravel the first steps of nitrogen fixation of nitrogenase. The individual steps leading from the resting state to nitrogen binding at the FeMo-cofactor with a central nitrogen ligand are characterized. The calculations indicate that the Fe-Mo cage opens as dinitrogen binds to the cluster. In the resting state, the central cage is overall neutral. Electrons and protons are transferred in an alternating manner. Upon dinitrogen binding, one protonated sulfur bridge is broken. An axial and a bridged binding mode of dinitrogen have been identified. Adsorption at the Mo site has been investigated but appears to be less favorable than binding at Fe sites.

Effect of phosphorus nutrition on the nodulation, nitrogen fixation and nutrient - use efficiency of bradyrhizobium japonicum - soybean (glycine max l. merr.) symbiosis

Abstract: Summary. Characterization of nodule growth and functioning, phosphorus status of plant tissues and host- plant growth of nodulated soybean (Glycine max L. Merr.) plants grown under different phosphorus conditions was studied in order to evaluate the role of phosphorus in symbiotic nitrogen fixation. Phosphorus deficiency treatment decreased the whole plant fresh and dry mass, nodule weight, number and functioning. Under conditions of phosphorus oversupply the decrease in plant growth, nodulation and acetylene reduction was stronger. Phosphorus deficiency significantly affected all phosphorus metabolites. Contents of different phosphorus fractions were decreased under the conditions of phosphorus deficiency.

Nitrogen Fixation in Bryophytes, Lichens, and Decaying Wood along a Soil-age Gradient in Hawaiian Montane Rain Forest1

Abstract: We determined rates of acetylene reduction and estimated total nitrogen fixation associated with bryophytes, lichens, and decaying wood in Hawaiian montane rain forest sites with underlying substrate ranging in age from 300 to 4.1 million years. Potential N fixation ranged from ca 0.2 kg/ha annually in the 300-year-old site to ca 1 kg/ha annually in the 150,000-year-old site. Rates of acetylene reduction were surprisingly uniform along the soil-age gradient, except for high rates in symbiotic/associative fixers at the 150,000-year-old site and in heterotrophic fixers at the 2100-yearold site. Low fixation at the youngest site, where plant production is known to be N-limited, suggests that demand for N alone does not govern N fixation. Total N fixation was highest in sites with low N:P ratios in leaves and stem wood, perhaps because epiphytic bryophytes and lichens depend on canopy leachate for mineral nutrients and because heterotrophic fixation is partly controlled by nutrient supply in the decomposing substrate; however, differences in substrate cover, rather than in fixation rates, had the largest effect on the total N input from fixation at these sites.

The Lotus japonicus Sen1 gene controls rhizobial differentiation into nitrogen-fixing bacteroids in nodules

Abstract: A Lotus japonicus mutant, Ljsym75, which forms ineffective symbiotic nodules and defines a new locus involved in the process of nitrogen fixation, was characterized in detail in order to identify the stage of developmental arrest of the nodules. No nitrogen-fixing activity was detectable in Ljsym75 nodules at any stage during plant development, and plant growth was markedly retarded. Ljsym75 plants formed twice as many nodules as the wild-type Gifu, and this phenotype was not influenced by the application of low concentrations of nitrate. Although the ineffective nodules formed on Ljsym75 were anatomically similar to effective Gifu nodules, Ljsym75 nodules senesced prematurely. Microscopic examination revealed that bacteria endocytosed into Ljsym75 nodules failed to differentiate into bacteroids. Moreover, the bacteria contained no nitrogenase proteins, whereas leghemoglobin was detected in the cytosol of the nodules. These results indicate that Ljsym75 is required for bacterial differentiation into nitrogen-fixing bacteroids in nodules, and thus the Ljsym75 gene was renamed sen1 (for stationary endosymbiont nodule). Linkage analysis using DNA markers showed that Sen1 is located on chromosome 4.

Alder and lupine enhance nitrogen cycling in a degraded forest soil in Northern Sweden

Abstract: Positive effects of legumes and actinorhizal plants on N-poor soils have been observed in many studies but few have been done at high latitudes, which was the location of our study. We measured N2 fixation and several indices of soil N at a site near the Arctic Circle in northern Sweden. More than 20 years ago lupine (Lupinus nootkatensis Donn) and gray alder (A Inns incana L. Moench) were planted on this degraded forest site. We measured total soil N, net N mineralization and nitrification with a buried bag technique, and fluxes of NH 4 + and NO 3 − as collected on ion exchange membranes. We also estimated N2 fixation activity of the N2-fixing plants by the natural abundance of 15N of leaves with Betula pendula Roth. as reference species. Foliar nitrogen in the N2-fixing plants was almost totally derived from N2 fixation. Plots containing N2-fixing species generally had significantly higher soil N and N availability than a control plot without N2-fixing plants. Taken together, all measurements indicated that N2-fixing plants can be used to effectively improve soil fertility at high latitudes in northern Sweden.

Effects of drought stress on legume symbiotic nitrogen fixation: physiological mechanisms.

Abstract: Drought stress is one of the major factors affecting nitrogen fixation by legume-rhizobium symbiosis. Several mechanisms have been previously reported to be involved in the physiological response of symbiotic nitrogen fixation to drought stress, i.e. carbon shortage and nodule carbon metabolism, oxygen limitation, and feedback regulation by the accumulation of N fixation products. The carbon shortage hypothesis was previously investigated by studying the combined effects of CO2 enrichment and water deficits on nodulation and N2 fixation in soybean. Under drought, in a genotype with drought tolerant N2 fixation, approximately four times the amount of 14C was allocated to nodules compared to a drought sensitive genotype. It was found that an important effect of CO2 enrichment of soybean under drought was an enhancement of photo assimilation, an increased partitioning of carbon to nodules, whose main effect was to sustain nodule growth, which helped sustain N2 rates under soil water deficits. The interaction of nodule permeability to O2 and drought stress with N2 fixation was examined in soybean nodules and led to the overall conclusion that O2 limitation seems to be involved only in the initial stages of water deficit stresses in decreasing nodule activity. The involvement of ureides in the drought response of N2 fixation was initially suspected by an increased ureide concentration in shoots and nodules under drought leading to a negative feedback response between ureides and nodule activity. Direct evidence for inhibition of nitrogenase activity by its products, ureides and amides, supported this hypothesis. The overall conclusion was that all three physiological mechanisms are important in understanding the regulation of N2 fixation and its response of to soil drying.

Sulfate inhibition of molybdenum-dependent nitrogen fixation by planktonic cyanobacteria under seawater conditions: a non-reversible effect

Abstract: The trace element molybdenum is a central component of several enzymes essential to bacterial nitrogen metabolism, including nitrogen fixation. Despite reasonably high dissolved concentrations (for a trace metal) of molybdenum in seawater, evidence suggests that its biological reactivity and availability are lower in seawater than in freshwater. We have previously argued that this difference is related to an inhibition in the uptake of molybdate (the thermodynamically stable form of molybdenum in oxic natural waters) by sulfate, a stereochemically similar ion. Low molybdenum availability may slow the growth rate of nitrogen-fixing cyanobacteria, and in combination with an ecological control such as grazing by Zooplankton, keep fixation rates very low in even strongly nitrogenlimited coastal marine ecosystems. Here we present results from a seawater mesocosm experiment where the molybdenum concentration was increased 10-fold under highly nitrogen-limited conditions. The observed effects on nitrogen-fixing cyanobacterial abundance and nitrogen-fixation inputs were much smaller than expected. A follow-up experiment with sulfate and molybdenum additions to freshwater microcosms showed that sulfate (at seawater concentrations) greatly reduced nitrogen fixation by cyanobacteria and that additions of molybdenum to the levels present in the seawater mesocosm experiment only slightly reversed this effect. In light of these results, we re-evaluated our previous work on the uptake of radio-labeled molybdenum by lake plankton and by cultures of heterocystic cyanobacteria. Our new interpretation indicates that sulfate at saline estuarine levels (>8–10 mM) up to seawater (28 mM) concentrations does inhibit molybdenum assimilation. However, the maximum molybdenum uptake rate ( Vmax) was a function of the sulfate concentration, with lower Vmax values at higher sulfate levels. This indicates that this inhibition is not fully reversed at some saturating level of molybdenum, as assumed in a simple competitive inhibition model. A multi-enzyme, mixed kinetics model with two or more uptake enzyme systems activated in response to the environmental sulfate and molybdate conditions may better explain the repressive effect of sulfate on Mo-mediated processes such as nitrogen fixation.

Can introduced and indigenous rhizobial strains compete for nodule formation by promiscuous soybean in the moist savanna agroecological zone of Nigeria

Abstract: Promiscuous soybean lines have been bred on the basis that they would nodulate freely without artificial inoculation. However, our recent studies have demonstrated that the indigenous rhizobia are not able to meet their full nitrogen (N) requirement. Rhizobia inoculation might be necessary. We examined the competition for nodule formation among native Rhizobia spp. and two inoculated Bradyrhizobia strains (R25B indigenous strain and a mixture of R25B+IRj 2180A indigenous strain from soybean lines in the savanna of northern Nigeria), their effect on N fixation, and their contribution to the yield of four soybean cultivars, grown in the field in three different agroecological zones in the moist savanna of Nigeria. About 34% of nodules were formed by the mixture of introduced R25B+IRj 2180A, while R25B formed only about 24% of the nodules but did not influence biomass and grain yield production. The indigenous rhizobia strains that nodulated the soybean varieties fixed up to 70% of their accumulated total N, confirming the promiscuous nature of these soybean varieties. Even though these varieties fixed about 75 kg N ha−1; this was not enough to sustain their optimum grain yield, as earlier reported. However, the grain yield from inoculated soybean was not significantly higher than that from the uninoculated soybean, showing a degree of competitiveness among the introduced rhizobial strains and the native rhizobia population.

Nitrogen uptake and fixation in the cyanobacterium Cylindrospermopsis raciborskii under different nitrogen conditions

Abstract: Ammonium and nitrate uptake and N2-fixation of the heterocystous cyanoprokaryote Cylindrospermopsis raciborskii was examined in continuous cultures under different nitrogen concentrations and dilution rates using the 15N technique. It was found that at luxury phosphorus supply (5 mg PO4-P l−1) the biomass was similar in all cultures irrespective of the amount and portioning (continuous or pulsed) of available nitrogen forms. The added ammonium and nitrate was fully taken up by C. raciborskii and the remaining nitrogen demand was met by N2-fixation. Different ammonium concentrations (300, 750, 1500 and 3000 μg 15N l−1) added at the same dilution rate did not affect the growth of C. raciborskii. In the culture supplied with pulsed ammonium, N2-fixation was detected prior to ammonium addition only. After the ammonium pulse, the N2-fixation continued for a while then decreased and stopped. In addition, the inflowing ammonium was fully taken up by the organism. The rate of nitrogen fixation reached its original level after 8–24 hours, depending on the dilution rate. It can be suggested that the nitrogen fixation system stopped and was then activated again depending on the nitrogen content of the cells.

Nodulation and dinitrogen fixation of legume trees in a tropical freshwater swamp forest in French Guiana

Abstract: Nodulated legume trees comprised 43% of the stand basal area in the low, most frequently flooded microsites, and 23% in higher, drier microsites in a tropical freshwater swamp forest in French Guiana. Dinitrogen fixation in Pterocarpus officinalis, Hydrochorea corymbosa and Inga pilosula was confirmed by acetylene reduction assay (ARA), presence of leghaemoglobin in nodules and the 15N natural abundance method. The results for Zygia cataractae were inconclusive but suggested N2 fixation in drier microsites. Nodulated Inga disticha had a 15N-to-14N ratio similar to non-N2-fixing trees, but ARA indicated nitrogenase activity and leghaemoglobin was present in nodules. All bacterial strains were identified as Bradyrhizobium spp. according to the partial 16S rDNA sequences, and they were infective in vitro in the model species Macroptilium atropurpureum. About 35-50% of N in the leaves of P. officinalis, H. corymbosa and I. pilosula was fixed from the atmosphere. Dinitrogen fixation was estimated to contribute at least 8-13% and 17-28% to whole-canopy N in high and low microsites, respectively. Symbiotic N2 fixation appears to provide both a competitive advantage to legume trees under N-limited, flooded conditions and an important N input to neotropical freshwater swamp forests. (Resume d'auteur)

Influence of boron and calcium on the tolerance to salinity of nitrogen-fixing pea plants

Abstract: The effects of different levels of B (from 9.3 to 93 μM B) and Ca (from 0.68 to 5.44 mM Ca) on plant development, nitrogen fixation, and mineral composition of pea (Pisum sativum L. cv. Argona) grown in symbiosis with Rhizobium leguminosarum bv. viciae 3841 and under salt stress, were analysed. The addition of extra B and extra Ca to the nutrient solution prevented the reduction caused by 75 mM NaCl of plant growth and the inhibition of nodulation and nitrogen fixation. The number of nodules recovered by the increase of Ca concentration at any B level, but only nodules developed at high B and high Ca concentrations could fix nitrogen. Addition of extra B and Ca during plant growth restored nodule organogenesis and structure, which was absolutely damaged by high salt. The increase in salt tolerance of symbiotic plants mediated by B and Ca can be co-related with the recovery of the contents of some nutrients. Salinity produced a decrease of B and Ca contents both in shoots and in nodulated roots, being increased by the supplement of both elements in the nutrient solution. Salinity also reduced the content in plants of other nutrients important for plant development and particularly for symbiotic nitrogen fixation, as K and Fe. A balanced nutrition of B and Ca (55.8 μM B, 2.72 mM Ca) was able to counter-act the deficiency of these nutrients in salt-stressed plants, leading to a huge increase in salinity tolerance of symbiotic pea plants. The necessity of nutritional studies to successfully cultivate legumes in saline soils is discussed and proposed.