Comparison of common bean (Phaseolus vulgaris L.) genotypes for nitrogen fixation tolerance to soil drying

Aims

Common bean is a major source of protein for many people worldwide. However, the crop is often subjected to drought conditions and its advantage in undertaking symbiotic nitrogen fixation can be severely decreased. The primary objective of this study was to compare the resistance of nitrogen fixation of 12 selected genotypes to soil drying.

Methods

Twelve common bean genotypes of diverse genetic background were compared. Plants were grown in pots and subjected to soil drying over about 2 weeks. Nitrogen fixation was measured daily using a flow-through acetylene reduction technique. The plants were exposed to acetylene for only a short time period allowing repeated measures. The acetylene reduction rate of plants on drying soil was normalized against the rates measured for well-watered plants.

Results

Substantial variability was identified among genotypes in the threshold soil water content at which nitrogen fixation was observed to decrease. Genotypes SER 16, SXB 412, NCB 226, and Calima were found to have the greatest delay in their decrease in nitrogen fixation rates based on soil water content. These four genotypes expressed substantial tolerance of nitrogen fixation to soil drying. These experiments also resulted in data on the threshold soil water contents at which transpiration rates decreased. A decrease in transpiration rates at high soil water contents is potentially advantageous since it allows soil water conservation for use as the severity of the drought increases. There was a general trend of those genotypes with sustained nitrogen fixation rates to low soil water contents also expressing decreased transpiration rates at high soil water contents.

Conclusions

This study identified genetic variation among common bean genotypes in their response of nitrogen fixation and transpiration to soil drying. Five genotypes (SER 16, SXB 412, NCB 226, Calima, and SEA 5) expressed the desired traits for water-limited conditions, which might be exploited in breeding efforts.     

Soybean genotypic differences in sensitivity of symbiotic nitrogen fixation to soil dehydration

Nitrogen fixation activity by soybean (Glycine max (L.) Merr.) nodules has been shown to be especially sensitive to soil dehydration. Specifically, nitrogen fixation rates have been found to decrease in response to soil dehydration preceding alterations in plant gas exchange rates. The objective of this research was to investigate possible genetic variation in the sensitivity of soybean cultivars for nitrogen fixation rates in response to soil drying. Field tests showed substantial variation among cultivars with Jackson and CNS showing the least sensitivity in nitrogen accumulation to soil drying. Glasshouse experiments confirmed a large divergence among cultivars in the nitrogen fixation response to drought. Nitrogen fixation in Jackson was again found to be tolerant of soil drying, but the other five cultivars tested, including CNS, were found to be intolerant. Experiments with CNS which induced localized soil drying around the nodules did not result in decreases in nitrogen fixation rates, but rather nitrogen fixation responded to drying of the entire rooting volume. The osmotic potential of nodules was found to decrease markedly upon soil drying. However, the decrease in nodule osmotic potential occurred after significant decreases in nitrogen fixation rates had already been observed. Overall, the results of this study indicate that important genetic variations for sensitivity of nitrogen fixation to soil drying exist in soybean, and that the variation may be useful in physiology and breeding studies.     

Comparison of inhibition of N2 fixation and ureide accumulation under water deficit in four common bean genotypes of contrasting drought tolerance

Background and Aims

Drought is the principal constraint on world production of legume crops. There is considerable variability among genotypes in sensitivity of nitrogen fixation to drought, which has been related to accumulation of ureides in soybean. The aim of this study was to search for genotypic differences in drought sensitivity and ureide accumulation in common bean (Phaseolus vulgaris) germplasm that may be useful in the improvement of tolerance to water deficit in common bean.

Methods

Changes in response to water deficit of nitrogen fixation rates, ureide content and the expression and activity of key enzymes for ureide metabolism were measured in four P. vulgaris genotypes differing in drought tolerance.

Key Results

A variable degree of drought-induced nitrogen fixation inhibition was found among the bean genotypes. In addition to inhibition of nitrogen fixation, there was accumulation of ureides in stems and leaves of sensitive and tolerant genotypes, although this was higher in the leaves of the most sensitive ones. In contrast, there was no accumulation of ureides in the nodules or roots of stressed plants. In addition, the level of ureides in the most sensitive genotype increased after inhibition of nitrogen fixation, suggesting that ureides originate in vegetative tissues as a response to water stress, probably mediated by the induction of allantoinase.

Conclusions

Variability of drought-induced inhibition of nitrogen fixation among the P. vulgaris genotypes was accompanied by subsequent accumulation of ureides in stems and leaves, but not in nodules. The results indicate that shoot ureide accumulation after prolonged exposure to drought could not be the cause of inhibition of nitrogen fixation, as has been suggested in soybean. Instead, ureides seem to be produced as part of a general response to stress, and therefore higher accumulation might correspond to higher sensitivity to the stressful conditions.     

N2Fixation Response to Drought in Common Bean (Phaseolus vulgarisL.)

Nitrogen fixation activity in common bean is generally thoughtto be low and sensitive to soil drying and, consequently, droughtcan have important negative effects on N accumulation and yieldpotential. The objectives of this research were to examine theresponse of N₂fixation to drought stress in common bean, andto test the hypothesis that drought sensitivity of N₂fixationin common bean is linked to ureide levels in the plants. Twoglasshouse experiments were conducted to compare the responsesof leaf transpiration and acetylene reduction activity (ARA)to soil water contents. ARA decrease during soil dehydrationwas found to lag behind the decline in transpiration. This indicatesthat ARA is relatively less sensitive to soil dehydration comparedto leaf gas exchange. Further, in comparing two cultivars therewas no consistent difference in the relative response of ARAand transpiration to soil drying. The ureide concentrationsmeasured in common bean plants were low, ranging from 0.1 to1.0 mmol l^-1in xylem sap exudates. Ureide concentrations inthe sap exudate varied significantly among the two genotypeseven though there was no difference in ARA response to drought.It was concluded that in common bean, the lower sensitivityof N₂fixation to drought compared to leaf gas exchange couldbe related to low ureide concentrations in petioles and xylemsap.Copyright 1998 Annals of Botany Company Phaseolus vulgaris,nitrogen fixation, drought stress, nodules, ureides.     

An overview of rhizosphere processes related with plant nutrition in major cropping systems in China

Rhizosphere processes of individual plants have been widely investigated since 1904 when the term “rhizosphere” was first put forward. However, little attention has been paid to rhizosphere effects at an agro-ecosystem level. This paper presents recent research on the rhizosphere processes in relation to plant nutrition in main cropping systems in China. In the peanut (Arachis hypogaea L.)/maize (Zea mays L.) intercropping system, maize was found to improve the Fe nutrition of peanut through influencing its rhizosphere processes, suggesting an important role of phytosiderophores released from Fe-deficient maize. Intercropping between maize and faba bean (Vicia faba L.) was found to improve nitrogen and phosphorus uptake in the two crops compared with corresponding sole crop. There was a higher land equivalent ratio (LER) in the intercropping system of maize and faba bean than the treatment of no root interactions between the two crops. The increased yield of maize intercropped with faba bean resulted from an interspecific facilitation in nutrient uptake, depending on interspecific root interactions of the two crops. In the rotation system of rice (Oryza sativa L.)-wheat (Triticum aestivum L.) crops, Mn deficiency in wheat was caused by excessive Mn uptake by rice and Mn leaching from topsoil to subsoil due to periodic cycles of flooding and drying. However, wheat genotypes tolerant to Mn deficiency tended to distribute more roots to deeper soil layer and thus expand their rhizosphere zones in the Mn-deficient soils and utilize Mn from the subsoil. Deep ploughing also helped root penetration into subsoil and was propitious to correcting Mn deficiency in wheat rotated with rice. In comparison, oilseed rape (Brassica napus L.) took up more Mn than wheat through mobilizing sparingly soluble soil Mn due to acidification and reduction processes in the rhizosphere. Thus, oilseed rape was tolerant to the Mn-deficient conditions in the rice-oilseed rape rotation. Oxidation reactions on root surface of rice also resulted in the formation of Fe plaque in the rice rhizosphere. Large amounts of Zn were accumulated on the Fe plaque. Zinc uptake by rice plants increased as Fe plaque formed, but decreased at high amounts of Fe plaque. It is suggested that to fine-tune cropping patterns and optimize nutrient management based on a better understanding of rhizosphere processes at an agro-ecosystem level is crucial for increasing nutrient use efficiency and developing sustainable agriculture in China.     

Nitrogen Fixation (Acetylene Reduction) in a Salt Marsh Amended with Sewage Sludge and Organic Carbon and Nitrogen Compounds

Seasonal distribution of nitrogen fixation by Spartina alterniflora epiphytes and in surface and soil samples was investigated in a Georgia salt marsh which was amended with sewage sludge or with glucose and/or ammonium nitrate. There was no significant difference between the rates of fixation in the unamended and sewage sludge plots. Additional perturbation experiments suggested that nitrogen addition indirectly stimulates nitrogen fixation by enhancing Spartina production and root exudation. Glucose additions, on the other hand, suppressed nitrogen fixation on a long-term basis. It is suggested that the microbial population in the soil out-competed the plants for the available nitrogen and in turn suppressed plant production and possibly root exudation. A comparison of nitrogen fixation in clipped and unclipped Spartina plots substantiated the suggestion that root exudation probably supports nitrogen fixation. Fixation in the clipped plots was significantly lower (P < 0.05) than the rates in the unclipped plots.     

Combined inoculation with Glomus intraradices and Rhizobium tropici CIAT899 increases phosphorus use efficiency for symbiotic nitrogen fixation in common bean (Phaseolus vulgaris L.)

This study compared the response of common bean (Phaseolus vulgaris L.) to arbuscular mycorrhizal fungi (AMF) and rhizobia strain inoculation. Two common bean genotypes i.e. CocoT and Flamingo varying in their effectiveness for nitrogen fixation were inoculated with Glomus intraradices and Rhizobium tropici CIAT899, and grown for 50 days in soil–sand substrate in glasshouse conditions. Inoculation of common bean plants with the AM fungi resulted in a significant increase in nodulation compared to plants without inoculation. The combined inoculation of AM fungi and rhizobia significantly increased various plant growth parameters compared to simple inoculated plants. In addition, the combined inoculation of AM fungi and rhizobia resulted in significantly higher nitrogen and phosphorus accumulation in the shoots of common bean plants and improved phosphorus use efficiency compared with their controls, which were not dually inoculated. It is concluded that inoculation with rhizobia and arbuscular mycorrhizal fungi could improve the efficiency in phosphorus use for symbiotic nitrogen fixation especially under phosphorus deficiency.     

Soybean cultivar variability for nodule formation and growth under drought

Both nodulation and nitrogen fixation in soybean [Glycine max (L.) Merr.] are sensitive to soil drying, which can have important negative effects on yield. An exception to this general response has been the identification of the cultivar Jackson as being drought tolerant for N₂ fixation. The objectives of this research were to examine nodule formation and growth in Jackson among other soybean cultivars in response to soil drying under field conditions. Two field experiments were conducted to examine the genetic variation in the sensitivity of nodule numbers and dry weights to soil drying. Substantial variation among soybean lines was found, and the drought-tolerance trait was demonstrated again in Jackson. Greenhouse experiments were conducted to further analyze the variation of nodulation response to soil water content. The differences among cultivars observed in the field were confirmed in the greenhouse. Importantly, the relative drought insensitivity of N₂ fixation in cultivar Jackson was associated with high individual nodule dry weight under drought conditions, relative to well-watered plants. It was concluded that large variation in nodulation sensitivity to water deficit exists among soybean cultivars and that the response of N₂ fixation rates to drought is related in part to nodule formation and growth.     

Measurement of nitrogen fixation by soybean in the field using the ureide and natural N abundance methods

Nitrogen fixation by field-grown soybean (Glycine max [L.] Merrill) was assessed by the natural ¹⁵N abundance and ureide methods. The field sites (five) and genotypes (six, plus two levels of inoculation on Bragg) were chosen to provide a range of proportions of plant N derived from nitrogen fixation (P). Genotypes K466, K468, nts1007, and nts1116 and Davis were included on the basis of their reported tolerance of the suppressive effects of nitrate on nodulation and nitrogen fixation. Bragg was included as a `nitrate-sensitive' genotype. Seeds of all genotypes were inoculated at sowing with Bradyrhizobium japonicum CB1809 (USDA136). Amounts of nitrate in the soil profile (0-1.2 meter depth) at sowing ranged from 70 (site 3) to 278 kilograms per hectare (site 5), resulting in large effects on plant nodulation, on the δ¹⁵N values of nodulated plants, on the relative abundance of ureide-N in vacuum-extracted sap (VES) and stem extracts, and finally on the estimates of P. There was no relationship between amount of soil nitrate at sowing and the δ¹⁵N of the plant-available soil N. Correlation matrices of the measured and calculated parameters indicated generally weak correlations between crop growth (dry matter and N) and the parameters of symbiotic activity (nodule weight, δ¹⁵N, relative ureide-N); correlations were strong and highly significant between nodulation and the measures of nitrogen fixation (δ¹⁵N, relative ureide-N; r = 0.79-0.92). Estimates of P ranged between 0 and 68% (δ¹⁵N) and between 6 and 56% (ureide) and were highly correlated (r = 0.97). Results indicated that the ureide method can be used with confidence to assess P by field-grown crops of soybean.     

Agrobacterium tumefaciens Is a Diazotrophic Bacterium

This is the first report that Agrobacterium tumefaciens can fix nitrogen in a free-living condition as shown by its abilities to grow on nitrogen-free medium, reduce acetylene to ethylene, and incorporate ¹⁵N supplied as ¹⁵N₂. As with most other well-characterized diazotrophic bacteria, the presence of NH₄⁺ in the medium and aerobic conditions repress nitrogen fixation by A. tumefaciens. The system requires molybdenum. No evidence for nodulation was found with pea, peanut, or soybean plants. Further understanding of the nitrogen-fixing ability of this bacterium, which has always been considered a pathogen, should cast new light on the evolution of a pathogenic versus symbiotic relationship.     

高肥力土壤条件下不同基因型花生对氮素利用的差异

在桶栽条件下,利用¹⁵N示踪技术,选用20个基因型花生为供试材料,研究了高肥力土壤条件下不同基因型花生对氮素利用的差异.结果表明:高肥力土壤条件下花生氮素营养以土壤氮为主,根瘤固氮次之,肥料氮最低.不同基因型间花生对全氮、肥料氮、土壤氮和根瘤固氮的吸收和积累均存在显著差异,基因型间遗传变异以根瘤固氮最大,肥料氮和土壤氮相当.氮素荚果生产效率和氮肥利用率基因型间差异显著,最高值分别为最低值的3.6和2.1倍.全氮、肥料氮、土壤氮和根瘤固氮的氮素收获指数基因型间均存在显著遗传变异,且以根瘤固氮的氮素收获指数基因型间遗传变异最大.花生荚果产量与不同氮源氮素积累量及氮素收获指数、氮素荚果生产效率和氮肥利用率呈显著或极显著正相关.依据花生对不同氮源氮素吸收积累和荚果产量筛选出全氮高积累高产型、肥料氮高积累高产型、土壤氮高积累高产型和根瘤固氮高积累高产型四大类型花生,其中四大类型特征兼有的有4个花生基因型.     

Identification of the sym plasmid of Rhizobium leguminosarum strain 1001 and its transfer to and expression in other Rhizobia and Agrobacterium tumefaciens

A plasmid of 150 Mdal from Rhizobium leguminosarum RCC1001 was found to be a Sym plasmid (pSym1) carrying genes for root nodulation and nitrogen fixation on plants of the pea vetch cross-inoculation group. The plasmid was expressed not only in different R. leguminosarum and R. trifolii hosts, but also in Agrobacterium tumefaciens and R. meliloti, although in root nodules induced by A. tumefaciens and R. meliloti hosts no nitrogen was fixed. The host range for root nodule induction appeared to be determined by pSym1 and only included plants of the pea vetch cross-inoculation group; in contrast, the host range for the induction of root hair deformations, which was found also to be determined by pSym1 was less restricted and included besides plants of the pea vetch group in addition plants of the clover group. This corroborates previous findings that host specificity for nodulation and nitrogen fixation is exerted at a stage after the induction of root hair deformations.     

Diverse symbiovars nodulating cowpea (Vigna unguiculata L. Walp.) in highly adaptable agro-ecological zones in Mozambique

The presence of effective microsymbionts in the soil and their compatibility with the host plant are the key determinants to the N₂ fixation process. In Sub-Saharan Africa, nitrogen fixation in locally adapted cowpea and the distribution of their symbiovars are not well understood. The Aim of the study was to assess the distribution and symbiotic phylogenetic position of cowpea microsymbionts. Root nodules were sampled from various cowpea genotypes planted in Agro-Ecological Zone 7 and 8 (AEZ 7 and AEZ 8). Root-nodule bacteria were isolated and their molecular characterization was conducted. Physicochemical properties of soil were recorded. Enterobacterial Repetitive Intergenic Consensus (ERIC) distribution patterns in rhizobial genomes resulted in genetically diverse rhizobial population in Northern Mozambique. Principal component analysis showed that location-specific soil environment determined the presence of particular microsymbionts. Based on 16S rRNA and symbiotic gene analysis many diverse symbiovars were found in Mozambican soils. With few discrepancies, the results further confirmed the coevolution of the nifH, nodD, nodC and nodY/K genes, which was indicative of natural events such as vertical/horizontal gene transfer. The results suggested that ecological and phylogenetic studies of the microsymbionts are necessary to better reflect symbiovar identification and the ecological adaptation of the cowpea-nodulating rhizobial community.     

Nodulation and nitrogen-fixation byHippophaë rhamnoides L. in the field

Summary 1. The extent of nodulation and nitrogen fixation by a population ofHippophaë rhamnoides bushes occurring on a sand dune system on the east coast of England has been studied.2. Nodules were found on all plants (the latter ranged from 3 to 16 years in age) and using the isotope N¹⁵ it was confirmed that these nodules fixed nitrogen. The nodules on the youngest plants were most active in fixation but because of the greater total nodule mass older plants fixed the largest quantities of nitrogen.3. Large increases in total nitrogen (i.e. soil plus plant nitrogen) occurred with increase in age of the plants, such nitrogen increases ranging from 27 kg per hectare per annum in the presence of bushes 0 to 3 years old to 179 kg per hectare per annum in the presence of bushes 13 to 16 years old.4. The N¹⁵-technique indicated that a proportion of the nitrogen accumulated was due to fixation byHippophaë but nitrogen contributions from other sources could not be ruled out.5. The levels of nitrate-nitrogen and ammonium-nitrogen in the soil underHippophaë showed a marked seasonal variation, increasing to a maximum in the winter and to a minimum in the summer. These levels were in general inversely related to the level of average monthly rainfall. Because of the low levels of combined nitrogen in the soil during the summer months it seems unlikely that combined nitrogen should markedly inhibit fixation in this situation.     

Nitrogen Fixation in Peanut Nodules during Dark Periods and Detopped Conditions with Special Reference to Lipid Bodies

The peanut plant (Arachis hypogaea L.), unlike other known legumes, can sustain nitrogen fixation when prolonged periods of darkness or detopping curtail the supply of photosynthate to the nodule. This ability to withstand photosynthate stress is attributed to the presence of lipid bodies in infected nodule cells. In both dark-treated and detopped plants, the lipid bodies show a gradual decrease in numbers, suggesting their utilization as a source of energy and carbon for nitrogen fixation. Lipolytic activity can be localized in the lipid bodies, and the existence of β-oxidation pathway and, glyoxylate cycle is shown by the release of ¹⁴CO₂ from ¹⁴C lineoleoyl coenzyme A by the nodule homogenate.     

Relationships between nitrogen fixation and photosynthesis as the main components of the productivity in alfalfa

Alfalfa (Medicago sativa L.) plants were inoculated with Sinorhizobium meliloti Tn-5 mutants featuring various nitrogen-fixing effectiveness and then grown in sand culture to study relations between CO₂ exchange, plant productivity, and nitrogen fixation. At the flowering stage, the relationship between nitrogen fixation and photosynthesis of whole alfalfa plants was described with the logarithmic curve. At the same stage of plant development, a close relationship was observed between nitrogen fixation rate and plant weight; this relationship showed a trend toward saturation at high rates of nitrogen fixation. The increase in nitrogenase activity of root nodules was accompanied by stimulation of root respiration; the relation of respiration to nitrogen-fixing activity was manifested stronger than its relation to the total root weight. It is concluded that highly effective strains of root nodule bacteria can realize their potential only in combination with complementary plant genotypes featuring active photosynthesis that provides a balanced supply of assimilates for both the symbiotic apparatus and growth processes in the macrosymbiont.     

Effect of peanut mixed cropping with gramineous species on micronutrient concentrations and iron chlorosis of peanut plants grown in a calcareous soil

To gain a better understanding of the mechanisms of improvement of iron nutrition of peanut (Arachis hypogaea L.) intercropped with maize (Zea mays L.) in calcareous soil, both greenhouse and field experiments were conducted to investigate the rhizosphere (phytosiderophores) effects from maize, barley, oats and wheat with different phytosiderophores release rates on iron nutrition and other micronutrients in calcareous soil. Six cropping treatments were examined in a greenhouse experiment: peanut grown separately in monoculture, normal peanut/maize intercropping (two genotypes: Danyu13, Zhongdan12), peanut/barley intercropping, peanut/oats intercropping, and peanut/wheat intercropping. Additionally, we investigated in a field experiment the same five cropping systems as the greenhouse experiment (maize/peanut intercropping not including Zhongdan12). Our results show that the chlorophyll and active Fe concentrations in the young leaves of the peanut in the intercropping system with different gramineous species were much higher than those of the peanut in monoculture. In greenhouse conditions, the Fe concentration in the shoots of peanut plants grown in the intercropping systems of two maize genotypes separately were 1.40–1.44, 1.47–1.64 and 1.15–1.42 times higher respectively than those of peanut plants grown in monocropping at 55, 60 and 70 days. In particular, the Fe concentration in shoots of peanut plants grown in the intercropping systems of barley, oats and wheat were not only higher than those in monocropping but also higher than those in peanut intercropped cropping with maize. In the field, the concentration of Fe in shoot of intercropped peanut plants in rows 1–3 from gramineous species were significantly higher than in monocropping at the flowering stage. Simultaneously with iron nutrition variation in peanut, Zn and Cu concentrations of intercropped grown peanut increased significantly compared to those in monocropping in the greenhouse experiment, and different intercropping treatments generally increased the Zn and Cu content in the shoot of peanut in the field. Systemic mechanisms may be involved in adaptation to nutrient stresses at the whole plant level. The study suggests that a reasonable intercropping system of nutrient efficient species should be considered to prevent or mitigate iron and zinc deficiency of plants in agricultural practice.     

N2 Fixation by Azospirillum brasilense and Its Incorporation into Host Setaria italica

Growth and nitrogen fixation were followed during the life cycle of Setaria italica (foxtail millet) inoculated with Azospirillum brasilense in controlled-environment growth chambers. The plants were fertilized at seeding with a limiting amount of combined nitrogen and maintained with an N-free mineral solution. During maturation of the plants, substantial nitrogenase activity, measured by acetylene reduction, developed in the rhizosphere, with total fixation estimated to be equivalent to 20% of the N in the inoculated plants. The peak of this activity coincided with depletion of soluble nitrogen from the system, which in turn was reflected by a sharp decrease in the nitrate reductase activity of the leaves. A. brasilense was found in association with the root populations at 8 × 10⁷ cells per gram of dry weight. An increase in shoot growth occurred at this time, but no significant increase in total plant nitrogen could be demonstrated. ¹⁵N₂ enrichment experiments confirmed that fixation was occurring, but only about 5% of the nitrogen fixed by A. brasilense was incorporated into the plants within 3 weeks. There was thus no evidence of direct bacterium-to-plant transport of fixed nitrogen, but rather a slow transfer suggesting the gradual death of bacteria and subsequent mineralization of their nitrogen, at least under growth-room conditions.     

Effects of single and co-inoculation with native phosphate solubilising strain Pantoea sp J49 and the symbiotic nitrogen fixing bacterium Bradyrhizobium sp SEMIA 6144 on peanut (Arachis hypogaea L.) growth

In the present study, attempts were made to analyze the effect of co-inoculation with an efficient phosphate solubilising native isolate Pantoea sp J49 and the symbiotic nitrogen fixing Bradyrhizobium sp SEMIA 6144 strain on Arachis hypogaea L. plants growth. Single and co-inoculation of peanut plants growing in plastic pots containing soil with low P content were developed. Plants were harvested at R1 and R4 growth stages and were analyzed in different growth parameters. Survival of strain Pantoea sp J49 was analyzed in soil samples and in root tissues. Plants inoculated only with Pantoea sp J49 showed the highest shoot and root weight in both reproductive growth stages evaluated. Plants co-inoculated with this strain and Bradyrhizobium sp SEMIA 6144 showed increase in aerial dry weight at R1 stage. Survival assays demonstrated that Pantoea sp J49 survives not only in the peanut rhizosphere but also inside plant tissues, including nodules formed when it was co-inoculated with Bradyrhizobium sp SEMIA 6144. Results obtained in this study confirm the great potential of the native Pantoea sp J49 isolate in the promotion of peanut plant growth, probably related with its capacity to solubilise phosphate.     

Differential responses of the sunn4 and rdn1-1 super-nodulation mutants of Medicago truncatula to elevated atmospheric CO2

Background and AimsNitrogen fixation in legumes requires tight control of carbon and nitrogen balance. Thus, legumes control nodule numbers via an autoregulation mechanism. ‘Autoregulation of nodulation’ mutants super-nodulate are thought to be carbon-limited due to the high carbon-sink strength of excessive nodules. This study aimed to examine the effect of increasing carbon supply on the performance of super-nodulation mutants.MethodsWe compared the responses of Medicago truncatula super-nodulation mutants (sunn-4 and rdn1-1) and wild type to five CO₂ levels (300–850 μmol mol⁻¹). Nodule formation and nitrogen fixation were assessed in soil-grown plants at 18 and 42 d after sowing.Key ResultsShoot and root biomass, nodule number and biomass, nitrogenase activity and fixed nitrogen per plant of all genotypes increased with increasing CO₂ concentration and reached a maximum at 700 μmol mol⁻¹. While the sunn-4 mutant showed strong growth retardation compared with wild-type plants, elevated CO₂ increased shoot biomass and total nitrogen content of the rdn1-1 mutant up to 2-fold. This was accompanied by a 4-fold increase in nitrogen fixation capacity in the rdn1-1 mutant.ConclusionsThese results suggest that the super-nodulation phenotype per se did not limit growth. The additional nitrogen fixation capacity of the rdn1-1 mutant may enhance the benefit of elevated CO₂ for plant growth and N₂ fixation.