Effect of nitrogen levels and Rhizobium strains on symbiotic N2 fixation and grain yield of Phaseolus vulgaris L. genotypes in normal and saline-sodic soils

Summary Following screening, selection, characterization, and symbiotic N₂ fixation with 12,5, 25.0, and 40.0 mg N kg^–1 in normal and saline-sodic soils, only two Phaseolus vulgaris genotypes (HUR 137 and VL 63) and two Rhizobium spp. strains (ND 1 and ND 2) produced maximum nodulation, nitrogenase activity, plant N contents, and grain yields in saline-sodic soil, with 12.5 mg N kg^–1, compared with the other strains. However, interactions between strains (USDA 2689, USDA 2674, and ND 5) and genotypes (PDR 14, HUR 15, and HUR 138) were significant and resulted in more nodulation, and greater plant N contents, nitrogenase activity, and grain yields in normal soils with 12.5 mg N kg^–1 compared with salt-tolerant strains. Higher levels of N inhibited nodulation and nitrogenase activity without affecting grain yields. To achieve high crop yields from saline-sodic and normal soils in the plains area, simultaneous selection of favourably interacting symbionts is necessary for N economy, so that bean yields can be increased by the application of an active symbiotic system.     

Effects of Rhizobium and vesicular-arbuscular mycorrhiza inoculations on nodulation,symbiotic nitrogen fixation and soybean yield in subtropical-tropical fields

Summary Field experiments were carried out to determine the effects of single and mixed inoculations with Rhizobium and vesicular-arbuscular mycorrhiza (VAM) on nodulation, symbiotic N₂ fixation and yield of soybeans in six Taiwan subtropical-tropical sites. Inoculation with Rhizobium alone significantly increased nodulation, nodule weight and nitrogenase activity of nodules in three out of six experimental fields, and affected soybean yields in the range –13% to + 134%. Inoculation with VAM fungi alone did not have a significant effect on nodulation and nitrogenase activity. Mycorrhiza inoculation affected soybean yields in the range –13% to + 65%, but only the yield increases at one out of six sites with N application were statistically significant. Mixed inoculation with Rhizobium and mycorrhiza affected yields in the range –8% to + 145% A synergistic effect from mixed inoculation of Rhizobium-mycorrhiza on soybean yields was found in one out of six experimental fields. The yield response to N application (40 kg N ha^–1) in these six paddy-field trials was not significant. These results suggest that single or mixed inoculation of rhizobia can greatly assist soybean grain yields and can replace N fertilizers.     

Isolation,selection and evaluation of Rhizobium under controlled conditions

Abstract

Rhizobia were screened in growth chamber and greenhouse tests with the host plants: bean (Phaseolus vulgaris L.), lentil (Lens esculenta Moench.), cowpea (Vigna unguiculata (L.) Walp) and peanut (Arachis hypogaea L.). Rhizobial isolates varied in effectiveness, time to nodulation, and host plant specifities. Initial screening procedures in plastic growth pouches allowed selection of infective strains of Rhizobium. These tests enabled the selection of a small number of highly effective isolates for more critical evaluation. Highly significant correlations (p=0.01) were obtained between shoot dry weight and total nitrogen, suggesting that the technique was applying sufficient selection pressure to differentiate variation in N₂ fixation among strains of Rhizobium. Results indicate that highly effective rhizobia can be efficiently selected under controlled conditions following a step‐wise procedure.     

Lentil (Lens culinaris L.) growth promoting rhizobacteria and their effect on nodulation in coinoculation with rhizobia

Lentil is cultivated in Chilean Mediterranean drylands, in areas with soils that are nutrient depleted and eroded. Inoculation of lentil with rhizobia in co-inoculation with growth promoting rhizobacteria would allow higher biomass and an opportunity for early nodulation and increased nitrogen fixation. The objective of this research was to select rhizosferic bacteria (PGPR) from lentils and to evaluate their effect on lentil nodulation in co-inoculation with rhizobia. Sixty three lentil rhizobacteria isolates where obtained from nine soils in the mediterranean area. These were fingerprinted through BOX-PCR reducing the number to 57 distinct strains. The strains were evaluated for ACCdeaminase activity, IAA production and compatibility with rhizobia. Seventeen strains showed ACC-deaminase activity, all of them synthesized IAA and 38 were compatible with the rhizobia. Ten selected strains were identified as Pseudomonas spp. through 16S rRNA sequencing. The strains were inoculated in lentil seedlings growing on seed germination pouches, to evaluate nodule formation. The strain LY50a increased early nodulation in 85% in comparison to the control inoculated with rhizobia (AG-84) only. In conclusion, bacteria from the rhizosphere from Mediterranean soils of Chile can be used as nodulation promoters in lentils.     

Mycorrhizal etiology of favorable proteoid rhizogenesis,nodulation, and nitrogenase of lupines

Lupine (Lupinus albus L.) has been cultivated as a food‐grain legume for more than 3,000 years. Persistent productivity occurs on low fertility soils within regions of drought and unfavorable temperature extremes. Distinctive rhizosphere characteristics of Proteaceae genera include development of determinate rootlet clusters termed proteoids. This unique morphology contributes to adaptive tolerance with soil‐climate duress prohibitive to most food Leguminosae. Objectives of this study were to determine tripartite components for mycorrhizal colonization and effective Rhizobium symbiosis with proteoid nodulation governing productivity, nitrogenase activity and subsequent nitrogen (N) fixation of Lupine. Highest nitrogenase activity levels with largest top growth, nodulation and seed yield resulted with mycorrhizal colonization plus calcium (Ca) treatments. Mycorrhizal colonization without soil fertility amendments resulted in higher yields and nodulation than all phosphorus (P), Ca, and potassium (K) soil fertility treatment combinations without mycorrhizae. Phosphorus+Ca soil additions were greater than either plant nutrient used separately without mycorrhizal colonization. Nodule histological determinants were highly correlated with governing cytosol enzyme activity levels. Nitrate reductase (NR)was significantly lower and phosphoenol‐pyruvate carboxylase (PEPC) was significantly higher with mycorrhizal colonization. Differences were not significant for cytosol components of amine‐amide N, aspartate transaminase (AST), glutamate dehydrogenase (GDH), glutamine syn‐thetase (GS) and glutamate oxoglutarate transaminase (GOGAT).     

Nitrous oxide emissions associated with nitrogen fixation by grain legumes

Nitrous oxide (N₂O) emissions and biological nitrogen (N₂) fixation by grain legumes are two major processes of N transformation in agroecosystems. However, the relationship between these two processes is not well understood. The objective of this study was to quantify N₂O emissions associated with N₂ fixation by grain legumes under controlled conditions. The denitrifying capability of two Rhizobium leguminosarum biovar viciae strains, 99A1 and RGP2, was tested in pure culture in the presence of nitrate and in symbiosis with lentil (Lens esculenta Moench) and pea (Pisum sativum L.), respectively, in sterile Leonard jars. Lentil and pea, either inoculated or N-fertilized, were grown in soil boxes under controlled conditions. Profile N₂O concentration and surface N₂O emissions were measured from soil–crop systems, and were compared with that of a cereal – spring wheat (Triticum aestivum L. ac. Barrie). Results indicated that: 1) neither R. leguminosarum strain, 99A1 or RGP2 was capable of denitrification in pure culture, nor in symbiosis with lentil and pea in sterile Leonard jars, suggesting that introducing these Rhizobium into soils through rhizobial inoculation onto lentil and pea will not increase denitrification or N₂O emissions; 2) soil-emitted N₂O from well-nodulated lentil and pea crops grown under controlled conditions was not significantly different than that from the check treatments, indicating that biological N₂ fixation by lentil and pea was not a direct source of N₂O emissions.     

Nitrogenase divarication with histological attributes distinctive for glycosylated aerial stem nodules and nitrosylated root nodules of sesbania

Exceptional symbiotic nitrogen fixation with Sesbania has provided high soil fertility for many past centuries of paddy rice production. Unique stem nodulation results in high nitrogenase activity levels of S. rostrata, Brem, during rapid growth in continuously flooded rice fields that greatly disfavor legume root nodulation and this functional development. The objective of this study was to determine plant nutrient interactions that influence contrasting root and aerial stem nodule histology governing effective nitrogenase activity levels and nitrogen fixation. Top growth, nodulation, and nitrogenase activity levels were significantly increased with increased available soil P. Response to K levels and Ca additions resulted only when soil P was adequate in all treatment combinations. However, there was no significant correlation between fresh nodule weight, nitrogenase activity, and nodules plant^‐1 for both root nodules and aerial stem nodules. Nodule histology was highly contrastive with nodule type and Rhizobium morphology, cytosol composition, and governing enzyme activity levels. Distinctive nonpleomorphic cocci bacteroids of functional aerial stem nodules have tentative designation as Azorhizobium caulinodans gen. nov. sp. nov.     

Conditions for successful Rhizobium-legume symbiosis in saline environments

Summary The Rhizobium-legume symbiosis in arid ecosystems is particularly important for locations where the area of saline soils is increasing and becoming a threat to plant productivity. Legumes, which are usually present in arid ecosystems, may be adapted to fix more N₂ under saline conditions than legumes grown in other habitats.Legumes are known to be either sensitive or moderately resistant to salinity. The salt sensitivity can be attributed to toxic ion accumulations in different plant tissues, which disturb some enzyme activities.Among the basic selection criteria for salt-tolerant legumes and rhizobia are genetic variability within species with respect to salt tolerance, correlation between accumulations of organic solutes (e. g., glycine betaine, proline betaine, and proline) and salt tolerance, and good relationships between ion distribution and compartmentation, and structural adaptations in the legumes.Salt stress reduces the nodulation of legumes by inhibiting the very early symbiotic events. Levels of salinity that inhibit the symbiosis between legumes and rhizobia are different from those that inhibit the growth of the individual symbionts. The poor symbiotic performance of some legumes under saline conditions is not due to salt limitations on the growth of rhizobia.Prerequisites for a successful Rhizobium-legume symbiosis in saline environments include rhizobial colonization and invasion of the rhizosphere, root-hair infection, and the formation of effective salt-tolerant nodules.The possibility of exploring the Rhizobium-legume symbiosis to improve the productivity of saline soils is reviewed in this paper.     

Effect of sewage sludge on nodulation and N2−fixation in alfalfa grown on calcareous loamy soils

The influence of different rates of sludge applications to calcareous loamy soils of Saudi Arabia, on nodulation and symbiotic N₂?fixation in alfalfa plants (Medicago sativa L.) was studied in a pot experiment. The effect of heavy metals accumulation in soil due to continuous irrigation of the test soil with sewage water was also investigated. Application of up to 80 g sludge per pot enhanced nodulation, nitrogenase activity, dry matter yield and N-contents of alfalfa plants growing in loamy soils either previously irrigated with sewage water or well water. However, sludge applied at the rate of 160–200 g pot^?1 inhibited the nodulation, N accumulation and dry matter yield of alfalfa. The response of alfalfa to sludge was dependent on the rhizobial strain used. Our results also showed that accumulation of heavy metals due to continuous irrigation of a calcareous sandy loam soil with sewage water, for more than 10 years, didn't inhibit N₂?fixation in alfalfa plants, but enhanced it. Microelements in alfalfa plants increased with increase in the rate of sludge application. Although high rates of sludge application affected nodulation and N₂?fixation of alfalfa, dry matter and the nitrogen contents of the plants were not highly affected. Therefore, the inhibitory effect of high rates of sludge was most probably due to the toxic effect of heavy metals on the microsymbiont rather than on the plants.     

Nodulation and nitrogen fixation of Lupinus species with Bradyrhizobium (lupin) strains in iron-deficient soil

 The effect of six Bradyrhizobium sp. (lupin) strains (WPBS 3201D, WPBS 3211D, USDA 3040, USDA 3041, USDA 3042 and CB 2272) and Fe supply on nodulation, N₂-fixation and growth of three lupin species (Lupinus termis, L. albus and L. triticale) grown under Fe deficiency in an alkaline soil, were examined in sterilized and non-sterilized pot experiments. When inoculated with USDA 3040, 3041, 3042 and CB2272 without Fe addition, the three lupin species had a very low nodule number and mass, low shoot and root dry matter accumulation and lower N yield. However, inoculation with WPBS 3201D and 3211D without Fe treatments increased all these parameters substantially. The ability of WPBS 3201D and 3211D to form nodules on the three lupin species under conditions of Fe stress could be attributed to their ability to scavenge Fe from Fe-deficient environments through their siderophore production. Addition of Fe to the other four strains significantly increased nodulation and N₂-fixation of the three lupin species, indicating that the poorer nodulation and N₂-fixation of these strains in the absence of Fe, resulted from a low ability to obtain Fe from alkaline soils. Bradyrhizobium strains WPBS 3201D and 3211D were superior to the other four strains in terms of promoting greater nodulation, N₂-fixation, plant growth and N accumulation of L. termis and L. albus. However, the other four strains were more efficient in symbiotic association with L. triticale. The greater variations in nodule efficiencies (specific nitrogenase activity) under different levels of Fe supply could be attributed to the quantities of bacteroid protein and leghaemoglobin in the nodules. The results suggested that Bradyrhizobium (lupin) strains differ greatly in their ability to obtain Fe from alkaline soils, and that the selection of bradyrhizobial strains which are tolerant of Fe deficient soils could complement plant breeding for the selection of legume crops for Fe-deficient soils. Received: 5 January 1998     

Single and Multistrain Rhizobial Inocula for Pinto and Black Bean Cultivars

ABSTRACT

Common bean (Phaseolus vulgaris L.) is relatively poor in dinitrogen (N₂) fixation, so selecting compatible host cultivar and Rhizobium strain combinations may offer an improvement. The effectiveness of six rhizobial strains was evaluated using five bean cultivars of bean (three pinto and two black bean) in a growth-room experiment. We subsequently selected the three best strains to assess whether multi-strain inoculation had advantages over single-strain inoculation in growth-room and field experiments. In the first-growth-room experiment, Rhizobium strains UMR 1899, RCR 3618, and USDA 2676 were selected for high nodulation, plant dry weight, shoot nitrogen (N), and N₂ fixation. In a second growth-room experiment, the individual strains and a mixture of the three strains generally did not differ in the parameters evaluated. Total shoot N accumulated ranged from 172.9 to 162.8 mg plant^?1, of which 32.1% to 33.6% (equivalent to 54.0 to 59.2 mg plant^{? 1}) was fixed. In field experiments, plant biomass and seed N₂ fixed did not differ among the inoculants at any site. These results suggest that the three strains were equally effective and that the multi-strain inoculant offered no consistent advantage over the single-strain inoculants.     

Phosphorus-Mobilizing Rhizobacterial Strain Bacillus cereus GS6 Improves Symbiotic Efficiency of Soybean on an Aridisol Amended with Phosphorus-Enriched Compost

Legume plants are an essential component of sustainable farming systems. Phosphorus (P) deficiency is a significant constraint for legume production, especially in nutrient-poor soils of arid and semi-arid regions. In the present study, we conducted a pot experiment to evaluate the effects of a phosphorus-mobilizing plant-growth promoting rhizobacterial strain Bacillus cereus GS6, either alone or combined with phosphate-enriched compost (PEC) on the symbiotic (nodulation-N₂ fixation) performance of soybean (Glycine max (L.) Merr.) on an Aridisol. The PEC was produced by composting food waste with addition of single super phosphate. The bacterial strain B. cereus GS6 showed considerable potential for P solubilization and mobilization by releasing carboxylates in insoluble P (rock phosphate)-enriched medium. Inoculation of B. cereus GS6 in combination with PEC application significantly improved nodulation and nodule N₂ fixation efficiency. Compared to the control (without B. cereus GS6 and PEC), the combined application of B. cereus GS6 with PEC resulted in significantly higher accumulation of nitrogen (N), P, and potassium (K) in grain, shoot, and nodule. The N:P and P:K ratios in nodules were significantly altered by the application of PEC and B. cereus GS6, which reflected the important roles of P and K in symbiotic performance of soybean. The combined application of PEC and B. cereus GS6 also significantly increased the soil dehydrogenase and phosphomonoesterase activities, as well as the soil available N, P, and K contents. Significant positive relationships were found between soil organic carbon (C) content, dehydrogenase and phosphomonoesterase activities, and available N, P, and K contents. This study suggests that inoculation of P-mobilizing rhizobacteria, such as B. cereus GS6, in combination with PEC application might enhance legume productivity by improving nodulation and nodule N₂ fixation efficiency.     

Variation in nodulation and N2 fixation by the Gliricidia sepium/Rhizobium spp. symbiosis in a calcareous soil

Summary Variation in nodulation and N₂ fixation by the Gliricidia sepium/Rhizobium spp. symbiosis was studied in two greenhouse experiments. The first included 25 provenances of G. sepium inoculated with a mixture of three strains of Rhizobium spp. N₂ fixation was measured using the ¹⁵N isotope dilution method 12 weeks after planting. On average, G. sepium derived 45% of its total N from atmospheric N₂. Significant differences in fixation were observed between provenances. The percentage of N derived from atmospheric N₂ ranged from 26 to 68% (equivalent to 18–62 mg N plant^-1) and was correlated with total N in the plant (r=0.70; P=0.05). The second experiment included six strains of Rhizobium spp. and two methods of inoculation and the plants were harvested 14,35 and 53 weeks after planting. In the first harvest significant differences were found between the number of nodules and the percentage and amount of N₂ fixed. There was also a significant correlation between the number of nodules and the amount of N₂ fixed (r=0.92; P=0.05). In the final harvest no correlation was observed, although there were significant differences between the number of nodules and the percentage of N derived from the atmosphere. The amount of N₂ fixed increased with time (from an average of 27% at the first harvest to 58% at the final harvest) and was influenced by the Rhizobium spp. strain and the method of inoculation. It ranged from 36% for Rhizobium sp. strain SP 14 to 71% for Rhizobium SP 44 at the last harvest. Values for the percentage of atmosphere derived N₂ obtained by soil inoculation were slightly higher than those obtained by seed inoculation.     

Efficiency of N2 fixation in Vicia faba L. in combination with different Rhizobium leguminosarum strains

The efficiency of symbiotic dinitrogen (N₂) fixation in Vicia faba L. in combination with 3 different Rhizobium leguminosarum strains was studied in a pot experiment during vegetative and reproductive growth. The objective of the experiments was to assess variability among Rhizobium strains inoculated on single legume species and determine possible reasons for observed variations. Dry matter formation, N₂ fixation and the carbon (C) costs of N₂ fixation were determined in comparison with nodule free plants grown with urea. Nodule number and the capacity of different respiratory chains in the nodules were also measured. The plants inoculated with the Rhizobium strain A 37 formed less dry matter and fixed less N compared to the other two Rhizobium strains (Vic 1 and A 150). This coincided with a lower number of nodules and higher C costs of N₂ fixation. The C costs for N₂ fixation were in all cases significantly lower during reproductive growth compared to vegetative growth. Neither the latter nor the differences in C expenditure for N₂ fixation between the Rhizobium strains could be explained in terms of differences or shifts in the capacity of different respiratory chains in the nodules.     

Evaluation of 15N-isotope dilution for measurement of nitrogen fixation in chickpea (Cicer arietinum L.)

Summary N accumulation, nodulation, and acetylene reduction activity were measured at frequent intervals during the growth of two chickpea genotypes, and N₂ fixation was estimated by an isotope-dilution method, using safflower as a non-N₂-fixing reference. Safflower was more efficient at N uptake than both the chickpea genotypes for at least the first 50 days and thus could not be used as an accurate reference control. We recommend that further work should employ non-nodulatiog genotypes of chickpea as reference plants and use slow-release forms of 15N fertilizer. Direct genotype comparison by isotope dilution estimated that genotype K 850 fixed 16–18 kg ha^–1 more N than G 130, and this difference was supported by the greater nodule mass and acetylene reduction activity in the K 850 cultivar. Inoculation with an ineffective chickpea Rhizobium sp. led to 69% nodulation on cultivar K 850 but only 33% on G 130. While nodule weight, N uptake, and acetylene reduction activity decreased with inoculation in K 850, the isotope dilutions were similar for both inoculation treatments. The lack of a significant effect on N₂ fixation was ascribed to the partial success of inoculant establishment.Published as Journal Article No. JA 692 of the International Crops Research Institute for the Semi Arid Tropics, Patancheru, A.P. 502324, India     

Foliar application of molybdenum in common beans. I. Nitrogenase and reductase activities in a soil of high fertility

Although common bean (Phaseolus vulgaris L.) has a good potential for N₂ fixation, poor nodulation following inoculation, principally under field conditions, has led to increased nitrogen (N) fertilizer use in this crop. In the face of the negative environmental effects of N fertilizer, alternative methods have been studied to minimize the amount to be applied. In this sense, foliar application of molybdenum (Mo) has been cited as a promising method. Several papers show that high bean yields (1,500–2,500 kg ha^‐1), may be obtained in the southeasten region of Brazil, when there is an application of N as side dressing or Mo spray 25 days after plant emergence. A field experiment was carried out to verify the effect of Mo foliar application on nitrogenase and nitrate reductase activities and on bean yield. Treatments included Rhizobium inoculation (with and without), foliar application of Mo (0 and 40 g ha^‐1), N at planting (0 and 20 kg ha^‐1) and N applied as side dressing (0 and 30 kg ha^‐1). Molybdenum and N as side dressing were used 25 days after plant emergence. Molybdenum increased greatly the nitrogenase activity and extended the period of high nitrate reductase activity, with a consequent increase in total shoot N. Increase of nitrogenase activity did not depend on inoculation, showing that soil native rhizobia may increase in effectiveness when appropriately handled. Bean yield did not differ significantly when fertilized with either Mo or N as side dressing.     

Rhizobiophage effects on nodulation,nitrogen fixation,and yield of field-grown soybeans (Glycine max L. Merr.)

Summary Previous laboratory and greenhouse studies have shown that phages significantly reduce soil populations of homologous rhizobia. Reductions in nodulation and N₂ fixation have also been observed. The purpose of the current study was to examine the effect of a phage specific ofBradyrhizobium japonicum USDA 117 on nodulation, nodule occupancy, N₂ fixation and soybean growth and yield under field conditions. The phage was inoculated in combination withB. japonicum USDA 117 and/orB. japonicum USDA 110 (resistant strain) into a rhizobia-free sandy loam soil and planted toGlycine max (L.) Merr. Williams. When the phage was applied to soil inoculated withB. japonicum USDA 117 alone, significant reductions in nodule weight and number, shoot weight, foliar N, nitrogenase activity, and seed index were observed. When, however, the soil also contained the non-homologous strain,B. japonicum USDA 110, no significant effects on any of these parameters were found. Nodule occupancy by competing strains ofB. japonicum USDA 110 and USDA 117 was also affected by the phage. In soil which did not contain the phage, 46% and 44% of the identified nodules were occupied by USDA 110 and 117, respectively. When the phage was present in the soil, nodule occupancy byB. japonicum USDA 117 was reduced to 23%, while occupancy byB. japonicum USDA 110 was increased to 71%. These results suggest that nodulation by selected strains of rhizobia can be restricted and nodulation by more effective, inoculated strains can be increased through the introduction of a homologous phage to soils.     

Soil fertility effects with tripartite symbiosis for growth,nodulation and nitrogenase activity of vicia faba L

Fababean (Vicia faba L.) is one of the oldest known important grain legume food crops grown within the temperate and subtropical regions of the world. This species is adapted for both forage and food grain production as a cool season annual crop on a wide range of soil and climatic conditions with effective tripartite symbiosis. Both Rhizobium and endophyte mycorrhiza are essential for high levels of production and symbiotic N₂ fixation. The objective of these greenhouse and field studies was to determine effects of Glomus fasciculatum colonization with soil fertility treatments to a Psammentic Paleustalf (Eufaula series) on growth, nodulation, nitrogenase activity and nodule composition for V. faba, var. Major, fuh Rumi (Nile) inoculated with R. leguminosarum Frank.

Top growth and nodule mass were increased approximately 10 fold and nitrogenase activity about 7 fold with the highly significant effect of mycorrhiza and response to low soluble Ca₃(PO₄)₂ fertilization in greenhouse studies. With both effective mycorrhiza and Rhizobium inoculation in the field experiments, seed yields were correlated with top growth (r = 0.841). Phosphorus and Ca fertilization resulted in highly significant increases in seed yields. Nitrogenase activity was correlated with nodule wt. (r = 0.958) and highly significant increases resulted with P and Ca soil amendment. Plant nutrient element composition of nodules increased with the fertilization treatments for P, Ca and increased K levels. Sodium content decreased significantly with increased K fertilization (r = ‐0.846). Potassium composition increased significantly with P content (r = 0.523). Enhanced N₂‐fixation along with increased high protein forage and food grain production with Fababeans have much potential. However, soil fertility and management techniques for improved production include effective mycor‐rhizal colonization.     

Response of the pigeonpea-Rhizobium symbiosis to salinity stress: variation among Rhizobium strains in symbiotic ability

Summary There were significant differences among pigeonpea [Cajanus cajan (L.) Millsp] Rhizobium sp. strains (IC 3506, IC 3484, IC 3195, and IC 3087) in their ability to nodulate and fix N₂ under saline conditions. Pigeonpea plants inoculated with IC 3087 and IC 3506 were less affected in growth by salinity levels of 6 and 8 dS m^-1 than plants inoculated with the other strains. For IC 3506, IC 3484, and IC 3195, there was a decrease in the number of nodules with increasing salinity, while the average nodule dry weight and the specific nitrogenase activity remained unaffected. However, in IC 3087, the number of nodules increased slightly with increasing salinity. Leaf-P concentrations increased with salinity in the inoculated plants irrespective of the Rhizobium sp. strain, and leaf-N concentrations decreased with increasing salinity in IC 3484 and IC 3195 only. Shoot-Na and-Cl levels were further increased in these salt-sensitive strains only at 8 dS m^-1. Therefore there may be scope for selecting pigeonpea Rhizobium sp. symbioses better adapted to saline conditions. The Rhizobium sp. strains best able to form effective symbioses at high salinity levels are not necessarily derived from saline soils.Submitted as JA No. 919 by the International Crops Research Institute for the Semi-Arid Tropics (ICRISAT)     

Salinity tolerance of Rhizobium mutants: Growth and relative efficiency of symbiotic nitrogen fixation

A salinity-tolerant strains of Rhizohium able to grow and fix nitrogen in symbiosis with lentil (Lens esculenta) in saline soil was derived frorn effective Rhizobium strain RL 5. A forced mutation with the mutagen nitrosoguanidine resulted in the isolation of five different mutant strains. The salinity tolerance, streptomycin resistance, growth, nodulation behaviour and relative efficiency of symbiotic N₂-fixation of these strains were studied. Among the five mutants and parent, LM 4 and LM 1 successfully tolerated 200 μ g ml^?1 streptomycin and 1.5%NaCl. These two mutants also significantly increased number and dry weight of nodules per plant, dry matter yield of the crop and N₂-fixation. Between the two, LM 4 seemed generally the better.