Total Nodule Mass Research Articles

Clade-dependent effects of drought on nitrogen fixation and its components – Number, size, and activity of nodules in legumes

Drought affects the growth of legumes directly, and indirectly, by reducing total nitrogen fixation . Here, we compiled published data to compare the sensitivity to water deficit on plant growth and total nitrogen fixation traits, i.e., the number of nodules per plant, average nodule mass, and nitrogen fixation per unit nodule mass. Hierarchies of phenotypic plasticity have been established for seeds and organelles , whereby variation in number associates with conserved size. By analogy, our first hypothesis is that there is a hierarchy of plasticities between nitrogen fixation traits. Our second hypothesis is that determinate nodules are more sensitive to water deficit than their indeterminate counterparts, because the latter can reactivate meristems when water becomes available. In our sample, onset of stress treatment averaged 28 d after sowing; median duration of stress was 12 d; and intensity of stress (ratio of shoot biomass between stressed and control) averaged 0.65. These drought conditions (i) reduced total nitrogen fixation and average nodule mass more severely than plant shoot mass, (ii) elicited a hierarchy of plasticities whereby number of nodules per plant varied substantially, and average nodule mass and nitrogen fixation per unit nodule mass were relatively conserved, and (iii) affected more severely Milletioids (determinate, ureide exporting nodules) than their IRLC counterparts (indeterminate, amide exporting nodules). • Total N fixation was partitioned as nodule number, average nodule mass, and nitrogen fixation per unit nodule mass. • Early-season drought reduced nodule number more than average nodule mass and N fixation per unit nodule mass. • Drought effect were more severe in Milletioid (determinate, ureide) than in IRLC (indeterminate, amide) nodules.

Rhizobium leguminosarum strain combination effects on nodulation and biological nitrogen fixation with Vicia villosa

Biological nitrogen fixation (BNF) resulting from symbiosis between legumes and rhizobia helps improve soil N fertility. Inoculated soils with specific strains of rhizobia may increase potential BNF in legumes, but the efficacy of these rhizobia in promoting BNF may be limited by competition from resident rhizobia already present in soils. We evaluated the ability of four strains to nodulate and increase BNF in Vicia villosa (hairy vetch) as individual and combined inoculants in a laboratory experiment. Plants were inoculated with a single rhizobia strain or equal ratios of 2–4 strains and grown under controlled conditions. After 46 d, entire plant shoots were harvested and analyzed for biomass, N content, nodule number, and nodule mass. Nodule occupancy was assessed using DNA fingerprinting of characteristic rhizobia genes. Negative-N and positive-N control plants averaged 6.3 and 83.2 mg total shoot N, respectively. Average total shoot N of inoculated vetch treatments was between that of uninoculated control treatments. Nodule number, total nodule mass, and BNF efficiency (total shoot N mass per total nodule mass) did not significantly differ between individual strains. Neither BNF efficiency nor nodule number were indicative of competitive ability for nodule occupancy. The rhizobia did not display any consistent synergistic or antagonistic effect on BNF when combined in inoculants and nodule occupancy did not favor any specific strain. Because vetch inoculated with local Rlv strains produced similar amounts of N as uninoculated plants fertilized with N, our work suggests that rhizobia sourced from local soils may produce sufficient BNF with vetch.

Enhanced rhizobial symbiotic capacity in an allopolyploid species of Glycine (Leguminosae).

Previous studies have shown that polyploidy can alter biotic interactions, and it has been suggested that these effects may contribute to the increased ability for colonization of new habitats shown by many allopolyploids. Little is known, however, about the effects of allopolyploidy, which combines hybridity and genome doubling, on symbiotic interactions with rhizobial bacteria. We examined interactions of the allopolyploid Glycine dolichocarpa (designated T2) with novel rhizobial partners, such as might occur in a context of colonization, and compared these with the responses of its diploid progenitors, G. tomentella (D3) and G. syndetika (D4). We assessed root hair response, nodule formation, nodule mass, nodule number, and plant biomass. The allopolyploid (T2) showed a greater root hair deformation response when exposed to rhizobia, compared with either diploid. T2 had a greater probability of forming nodules with NGR234 compared with diploid D4, and greater total nodule mass per nodulated plant compared with diploid D3. T2 also had greater plant biomass responses to nitrogen and when exposed to NGR234. The allopolyploid is characterized by transgressive responses to rhizobia for some variables, while also combining certain parental diploid responses such that its capacity for interactions with rhizobia appears to be greater than for either diploid progenitor. This overall enhanced nodulation capacity and the ability to make greater gains from exposure to both rhizobia and additional nitrogen indicate a greater potential of the allopolyploid to benefit from these factors both generally and in a context of colonization.

Nodule growth and nitrogen fixation of Calopogonium mucunoides L. show low sensitivity to nitrate

It is well established that nitrate is a potent inhibitor of nodulation and nitrogen fixation in legumes. The objective of this study was to demonstrate the relative insensitivity of these processes to nitrate with Calopogonium mucunoides, a tropical South American perennial legume, native to the cerrado (savannah) region. It was found that nodule number was reduced by about half in the presence of high levels of nitrate (15 mM) but nodule growth (total nodule mass per plant) and nitrogen fixation (acetylene reduction activity and xylem sap ureide levels) were not affected. Other sources of N (ammonium and urea) were also without effect at these concentrations. At even higher concentrations (30 mM), nitrate did promote significant inhibition (ca. 50%) of acetylene reduction activity, but no significant reduction in xylem sap ureides was found. The extraordinary insensitivity of nodulation and N2 fixation of C. mucunoides to nitrate suggests that this species should be useful in studies aimed at elucidating the mechanisms of nitrate inhibition of these processes.

Nitrogen Fixation by White Lupin under Phosphorus Deficiency

White lupin is highly adapted to growth in a low-P environment. The objective of the present study was to evaluate whether white lupin grown under P-stress has adaptations in nodulation and N2 fixation that facilitate continued functioning. Nodulated plants were grown in silica sand supplied with N-free nutrient solution containing 0 to 0.5 mm P. At 21 and 37 d after inoculation (DAI) growth, nodulation, P and N concentration, N2 fixation (15N2 uptake and H2 evolution), root/nodule net CO2 evolution and CO2 fixation (14CO2 uptake) were measured. Furthermore, at 21 DAI in-vitro activities and transcript abundance of key enzymes of the C and N metabolism in nodules were determined. Moreover, nodulation in cluster root zones was evaluated. Treatment without P led to a lower P concentration in shoots, roots, and nodules. In both treatments, with or without P, the P concentration in nodules was greater than that in the other organs. At 21 DAI nitrogen fixation rates did not differ between treatments and the plants displayed no symptoms of P or N deficiency on their shoots. Although nodule number at 21 DAI increased in response to P-deficiency, total nodule mass remained constant. Increased nodule number in P-deficient plants was associated with cluster root formation. A higher root/nodule CO2 fixation in the treatment without P led to a lower net CO2 release per unit fixed N, although the total CO2 released per unit fixed N was higher in the treatment without P. The higher CO2 fixation was correlated with increased transcript abundance and enzyme activities of phosphoenolpyruvate carboxylase and malate dehydrogenase in nodules. Between 21 and 37 DAI, shoots of plants grown without P developed symptoms of N- and P-deficiency. By 37 DAI the P concentration had decreased in all organs of the plants treated with no P. At 37 DAI, nitrogen fixation in the treatment without P had almost ceased. Enhanced nodulation in cluster root zones and increased potential for organic acid production in root nodules appear to contribute to white lupin's resilience to P-deficiency.

Root nodulation and nitrogen accumulation and partitioning in legume crops as affected by soil solarization

Soil solarization is a preplanting technique used in hot climates to control weeds and soilborne pathogens consisting of mulching the soil surface with polyethylene sheets. The increase in temperature associated with solarized soil could affect nitrogen availability for grain legume crops through effects on nitrogen fixing soil microorganisms or other mechanisms. To examine the effects of solarization on natural root nodulation and nitrogen accumulation and partitioning in the plant, two solarization field experiments were carried out over two planting seasons, involving genotypes of both faba bean (Vicia faba) and chickpea (Cicer arietinum). The effect of sowing date was also studied in the first season. Solarization increased the maximum soil temperature by 9–10 °C in the first, and by 13–15 °C in the second season. At 5 cm below the solarized soil surface, a temperature of over 46 °C prevailed for 146 and 280 h over the two respective seasons, while this temperature was not attained in unmulched soil. Solarization delayed the initiation of nodulation and consistently reduced the nodule number per host plant, but generated an approximate doubling of mean nodule weight. The total nodule mass per plant was not affected by the treatment in the first season, but was reduced in the second season. Solarization significantly increased the concentrations of NO3− -N, Na+, Zn2+, Ca2+ and K+ in the soil extract, and the total nitrogen accumulated in the whole plant. This latter increase was due to both higher plant growth and a greater plant nitrogen concentration. The increased nitrogen level in the plant was not uniform with respect to plant component, varying from 57% in the roots to 198% in the pods and seeds. The plants grown in non-solarized soil accumulated about 31% of their total N content in the shoots of the parasitic weed Orobanche crenata. Solarization dramatically improved grain yield by 300–900% in both seasons and in all genotypes studied, due to increased N availability in soil, N accumulation in plants, improved plant growth, and complete control of the parasite weed O. crenata. On the basis of these beneficial effects, soil solarization, which avoids site contamination and is suited to organic farming, should be a good opportunity in Mediterranean areas where the level and stability of grain yields are low, and the infestation of O. crenata is high.

Effects of UV-B radiation on plant growth, symbiotic function and concentration of metabolites in three tropical grain legumes.

Vigna unguiculata (L.) Walp. (cowpea), Glycine max (L.) Merr (soybean) and Phaseolus vulgaris (L.) (common bean) plants were exposed to UV-B radiation at above- and below-ambient levels, and their effects on growth, symbiotic performance and root concentration of metabolites were assessed. Moderately and highly elevated UV-B exposures averaging 32 and 62% above ambient had no effect on plant total dry matter, nodule number, nodule mass, nodule size, N fixed or root concentration of flavonoids, anthocyanins, soluble sugars and starch in the three species studied. However, N concentrations were markedly reduced in roots of G. max and P.vulgaris, and in leaves of P. vulgaris, which contrasted with the significant increase in stems and leaves of V.unguiculata. Below-ambient UV-B exposures averaging 22% of ambient also altered growth and metabolism of these legumes. Total plant dry matter, nodule number, nodule dry mass, N fixed and root starch concentrations in V.unguiculata decreased relative to both visible and UV-A radiation controls, whereas in G. max and P. vulgaris, these parameters were not altered. Root concentrations of flavonoids and anthocyanins in all species tested were also unchanged with below-ambient UV-B exposures. Taken together, growth and symbiotic function of these species remained unaltered with exposure to above-ambient UV-B, but differed in their response to below-ambient UV-B radiation.

Physiological and growth responses of two African species, Acacia karroo and Themeda triandra, to combined increases in CO2 and UV‐B radiation

The interactive effects of increased carbon dioxide (CO2) concentration and ultraviolet‐B (UV‐B, 280–320 nm) radiation on Acacia karroo Hayne, a C3 tree, and Themeda triandra Forsk., a C4 grass, were investigated. We tested the hypothesis that A. karroo would show greater CO2‐induced growth stimulation than T. triandra, which would partially explain current encroachment of A. karroo into C4 grasslands, but that increased UV‐B could mitigate this advantage. Seedlings were grown in open‐top chambers in a greenhouse in ambient (360 μmol mol‐1) and elevated (650 μmol mol‐1) CO2, combined with ambient (1.56 to 8.66 kJ m‐2 day‐1) or increased (2.22 to 11.93 kJ m‐2 day‐1) biologically effective (weighted) UV‐B irradiances. After 30 weeks, elevated CO2 had no effect on biomass of A. karroo, despite increased net CO2 assimilation rates. Interaction between UV‐B and CO2 on stomatal conductance was found, with conductances decreasing only where elevated CO2 and UV‐B were supplied separately. Increases in water use efficiencies, foliar starch concentrations, root nodule numbers and total nodule mass were measured in elevated CO2. Elevated UV‐B caused only an increase in foliar carbon concentrations. In T. triandra, net CO2 assimilation rates were unaffected in elevated CO2, but stomatal conductances and foliar nitrogen concentrations decreased, and water use efficiencies increased. Biomass of all vegetative fractions, particularly leaf sheaths, was increased in elevated CO2. and was accompanied by increased leaf blade lengths and individual leaf and leaf sheath masses. However, tiller numbers were reduced in elevated CO2. Significantly moderating effects of elevated UV‐B were apparent only in individual masses of leaf blades and sheaths, and in total sheath and shoot biomass. The direct CO2‐induced growth responses of the species therefore do not support the hypothesis of CO2‐driven woody encroachment of C4 grasslands. Rather, differential changes in resource use efficiency between grass and woody species, or morphological responses of grass species, could alter the competitive balance. Increased UV‐B radiation is unlikely to substantially alter the CO2 response of these species.

Symbiotic Effectivity of Four Phaseolus vulgaris Genotypes after Inoculation with Different Strains of Rhizobium under Controlled Conditions

Abstract Four varieties of P. vulgaris L. were tested for their symbiotic nitrogen fixation effectivity in combination with nine different strains of Rhizobium leguminosarum bv. phaseoli and Rhizobium tropici. Plants were grown under controlled conditions and harvested 23 days after planting. Acetylene reducing activity, total N-content and dry weight of individual plant components were determined. Significant differences due to plant x bacterium interaction were assessed by ANOVA, especially for the total nodule mass per plant and the acetylene reducing activity per nodule dry weight. Data for acetylene reducing activity per plant correlated highly with the corresponding data for the total N-content. The comparison of the total N-content in symbiotically grown plants, lacking supply of mineral N, with plants luxuriously supplied with mineral N (relative N-accumulation rate) revealed high values (between 60% and 70% of maximal N-uptake) for some symbiotically active plant/bacterium combinations for this early developmental stage of the symbiosis N2 fixation potential for such symbioses.

Ureide content and growth of cowpea [ Vigna unguiculata (L.) walp. var tuy] plants as affected Bradyrhizobium strains and inoculum position

The effect of three effective Bradyrhizobium sp. cowpea strains and inoculum position on biomass production, nodulation and ureide contents were determined in 45-day-old cowpea plants. Treatments were: single inoculations, where each one of the Bradyrhizobium strains (I-125,I-38 or I-700) was placed at 2 or 10cm from the surface of the sterilized sand; double inoculations, in which the same strain was located at 2 and 10cm; and dual-strain inoculations, where I-125 was placed at 2 or 10cm together with I-700 or I-38 either at 10 or 2 cm. Distance between inocula was enough to ensure a root zone free of nodules between inoculum sites. Analysis of nodulation profiles in plants exposed to different treatments indicated that the strain I-125 showed a greater motility and formed a larger nodule mass than I-38 and I-700. In single inoculations I-38 and I-700 produced an equal nodule mass regardless of inoculum position, in contrast to I-125 which showed reduced nodulation at 10 cm. In plants doubly or dual-strain inoculated with I-125 and I-700 about 73% of the total nodule mass was found in the upper root segments. However, due to a dry weight reduction in both nodule masses the total nodule mass per plant resembled that of single inoculations. An inhibition of delayed nodulation was only detected when I-38 was included in the double, or dual-strain inoculations with I-125. Thus, in cowpea the expression of the autoregulatory mechanism upon subsequent nodulation is strain dependent. In plants harbouring the strains I-38 or I-125, nodule mass position did not affect the ureide contents in nodules or in the petiole of the first trifoliolate leaf. An effect of inoculum position on nodule ureide contents was found in plants inoculated with I-700, which in turn contained up to three times more ureide than plants receiving other inoculation treatments. The increased concentrations of ureide recorded in I-700 inoculated plants were not translated into higher dry matter yields, pointing toward a threshold level effect. For all treatments combinations, the position of nodules on the roots had no effect on plant dry matter production, although plants inoculated with I-38 showed the highest biomass. Correlation analyses carried out between variables indicated sink-source relationships between nodule or petiole ureide contents and plant biomass. A significant positive correlation was detected between nodules and petiole ureide content, while non-significant correlations were found between nodule mass and plant biomass or between total nodule mass and nodule ureide contents.

Shoot apex removal does not alter autoregulation of Modulation in soybean

ABSTRACTThe suppression of new nodule development in soybean (Glycine max (L.) Merr.) has been previously demonstrated to involve the shoot through reciprocal grafts between the wild‐type cultivar Bragg and its supernodulating mutant nts382. Using the same grafting technique, but modified through the excision of the shoot apex region and emerging lateral shoots, we show here that autoregulation of nodule number still existed despite apex removal. This radical treatment lowered total nodule number per plant as well as root, shoot and nodule dry weight. Bragg shoots grafted onto nts382 roots gave wild‐type nodulation (26 nodules, 15mg total nodule mass) as compared to nts382 shoots grafted onto Bragg roots (340 nodules, 277 mg total nodule mass). Specific nodule mass differed between supernodulating (about 0·5‐1·0mg per nodule) and wild‐type nodulating (2·3 mg per nodule) plants. In contrast to other growth characteristics, apex removal did not affect specific nodule size, except in plants with wild‐type shoots and nts382 (supernodulation) roots. Apex removal only slightly affected the percentage of nodule weight per total root weight in nts382, but had a severe effect in wild type. Growth reductions varied between the normal and supernodulating plants. The fact that autoregulation of nodulation still functions in plants devoid of functional shoot apices suggests that the autoregulation signal may not be derived from the apex regions and that the leaf may be a likely source.

Nodulation and growth of subterranean clover ( Trifolium subterraneum L.) in a drying acid soil

Subterranean clover (cv. Mt Barker) was grown in undisturbed acid soil (pH 4.3), in cultivated acid soil (pH 4.3) and in lime-amended (pH 5.5) soil and after 12 weeks of growth was either watered to field capacity or basally irrigated with the equivalent of half the amount of water transpired by the control plants. Basally-irrigated plants were able to exploit subsoil moisture and maintain adequate plant-water relations whilst the soil at the top of the soil profile dried slowly. Most nodules were confined to the less acidic uppermost and basal regions of the undisturbed acid soil profile and no new nodules were produced on the root system once basal irrigation commenced. The permanent wilting point of the compact, undisturbed acid soil (−1.5 MPa) corresponded with a soil moisture content of 7% w/w and was attained 3 weeks after commencement of basal irrigation. Nitrogen accumulation by the subterranean clover plants grown in this treatment ceased when the top 9 cm of the acid soil profile reached the permanent wilting point. The permanent wilting point of the top 9 cm of the limed and unlimed cultivated soil occurred at a soil moisture content of 5% w/w and was reached 5 weeks after commencement of basal irrigation. Nodules occurred on roots at depth in the limed and unlimed cultivated soil and nodulation continued after commencement of basal irrigation compensating for lower nodule numbers on the upper crown roots that were in contact with the drying soil. Nitrogen continued to be accumulated by these plants throughout the experiment. Nodule size varied in accordance with soil cultivation treatment. In particular, larger nodules were produced on the roots of plants grown in undisturbed acid soil whereas smaller, more numerous nodules occurred on the roots of plants grown in lime-amended soil and to a lesser extent on the roots of plants grown in cultivated acid soil. However, total nodule mass was equivalent on plants grown in each of the soil treatments. Continued nodulation at depth on the roots of subterranean clover due to cultivation and incorporation of lime enabled nitrogen fixation to continue when nodules on roots near the soil surface became desiccated and senescent due to the localized moisture deficit.

Consequences of soil acidity and the effect of lime on the nodulation of Trifolium subterraneum L. Growing in an acid soil

Nodulation of subterranean clover roots growing in a very acid soil was influenced by soil pH. Restricted nodulation occurred in regions with very low soil pH and this consequently affected the distribution of nodules on roots throughout the entire soil profile. Nodulation was improved in all profile regions where lime was incorporated into the soil; this resulted in a more uniform distribution of nodules. Surface application of lime influenced nodulation only at the very top of the soil profile. Restrictions to nodulation at low pH were largely compensated for by the plant, either by a shift in the distribution of nodules to less acidic regions of the profile, or by an increase in the mass of individual nodules. Total nodule mass per plant was therefore generally the same in all pasture treatments despite large differences in the total number of nodules. The major determinants of restricted nodulation of subterranean clover roots in the acid soil appeared to be low numbers of Rhizobium trifolii or poor colonization of the more acidic regions of the soil profile. Inoculation and lime coating of seed did not overcome poor nodulation associated with the very acidic regions within the soil profile.

Importance of bacteroid number in nodules and effective nodule mass to dinitrogen fixation by cowpeas

Biological nitrogen fixation is considered an important trait of cowpeas (Vigna unguiculata (L.) Walp. var. ‘California Blackeye’ No. 5) for economical production yet the process does not alone provide the quantity of nitrogen required by the plant for maximum productivity. Two experiments were undertaken to determine the potential of an increase in nodule mass and number of bacteroids resulting in increased nitrogen fixation. Cowpeas were grown in a glasshouse for 7 weeks under conditions forcing near total dependence on biological nitrogen fixation for growth. Nodule mass on the roots was varied by inoculating seeds with various ratios of effective and ineffective rhizobia that could be identified serologically and by the color of nodule formed. The results of both experiments demonstrated a linear relationship between total nodule mass formed by the effective rhizobia and quantity of nitrogen fixed. The regression coefficients were high in both experiments (r=0.99** and 0.91**). The relationship between total nitrogen fixed and total number of bacteroids of the effective strain was not consistent. In one experiment the regression coefficient was 0.93** but in the other experiment it was 0.65**. From these results it appears that there is good potential for increasing nitrogen fixation in cowpeas by increasing nodule mass. An increase in nodule mass would also result in an increase in the number of bacteroids.

Effects of Water and Salt Stress on Growth and Acetylene Reduction in Alfalfa

AbstractThe interactive effects of three irrigation water salinities (1.0, 3.0, and 9.0 dS/m) and three quantities of water applied per irrigation (2.3, 4.7, and 7.0 cm) on plant growth and N fixation by alfalfa (Medicago sativa L. cv. Resistador) were assessed in the greenhouse. Salt and water treatments were started after nodulation had been established in young plants and samples were harvested at 10, 30, and 50 d thereafter. Alfalfa top growth was limited by both salt and water stresses. Irrigation water salinity had a greater effect on top growth than root growth while root distribution was unaffected by either the quantity of water applied or by water salinity. The effects of salinity on plant growth were masked by water stress. Specific nodule activity (µmol C2H4 s−1 kg−1) was enhanced by increasing quantity of water applied at each irrigation and was adversely affected by increased irrigation water salinity. In contrast, both nodulation and total nodule mass were insensitive to salt stress and sensitive only to severe water stress. Alfalfa plants continued to reduce acetylene at the third sampling even under severe water and salt stress. These data indicate that the Resistador cultivar of alfalfa apparently continues to fix N2 during periods of substantial environmental stress.

Compensation against Ineffective Nodulation in Soybean1

Two cultivars of soybean [Glycine max (L.) Merr.] were inoculated with nine ratios of ineffective (SM‐5) and effective (CC709) strains Rhizobium japonicum. Plants were grown in the greenhouse and harvested after 33 days. Nitrogenase activity, total shoot N, nodule number, nodule mass, and the proportions of nodules formed by each strain were determined. Nodules were identified serologically by the fluorescent antibody technique. Significant increases in total N in the shoot occurred as both the proportion and number of nodules formed by effective strain CC709 increased. Shoot N was highly correlated with nitrogenase activity and nodule mass. The relationship between percent effective nodules and shoot N was nonlinear so that 95% of maximum N accumulation was obtained when only 75% of the nodules (on a nodule number basis) were effective. The average size an effective nodule was at least 2.5 times larger than an ineffective nodule and their average weight increased from 2.38 to 4.88 mg/ nodule as the proportion of effective nodules declined from 100 to 18.8%. The proportion of total nodule tissue that was effective was greater than the corresponding percent effective nodules. This compensatory mechanism tended to keep the amount of effective nodule tissue constant up to a point even though the proportion of effective nodules declined. There was a deleterious effect on N2 fixation only when infection by the ineffective strain was great enough to reduce total nodule mass.

Variations in Ability of Rhizobium japonicum Strains To Nodulate Soybeans and Maintain Fixation in the Presence of Nitrate

This study investigated differences in sensitivity to nitrate of soybean (Glycine max cv. Davis) symbioses with 16 different Rhizobium japonicum strains. When nitrate (20 mM) was added to established symbioses, there were no significant differences in the degree of inhibition of acetylene reduction for any of the 16 strains. When nitrate was present during the establishment of nodules, high levels of nitrate (10 mM) were equally inhibitory on all symbioses, whereas specific strain effects appeared at low (0.5 mM) to medium (2.0 mM) levels of nitrate. At 1.5 mM nitrate in solution culture, the days to emergence of nodules varied from less than 10 (CB:1809, Nit61A118) to more than 16 (11 of 16 strains). In a clay-pot trial maintained at the low nitrate level (0.5 mM), symbioses with CB:1809 increased total nodule mass by 30% relative to nitrate-free controls. In the presence of 2.0 mM nitrate, CB:1809 maintained total nodule mass. For the remaining 6 strains tested, total nodule mass decreased to below the levels of the nitrate-free controls. In a separate clay pot trial, CB:1809 increased its competitive ability relative to USDA:110 when nitrate was added. If no nitrate was added, CB:1809 occupied 0.97 times as many nodules as USDA:110; when 10 mM nitrate was added, CB:1809 occupied 1.75 times as many nodules as USDA:110. Attempts to select nitrogen-adapted substrains of R. japonicum through sequential isolation and infection of plants grown on nitrate were not successful.

Symbiotic specificity in nodulation and nitrogen fixation between winged bean and Rhizobium

Four selections of winged bean (syn. Indies Goa bean, Psophocarpus tetragonolobus) and 1 selection of the closely related species P. palustris were tested with 3 Rhizobium strains, in a glasshouse with simulated tropical conditions, to study their symbiotic specificities in terms of nodulation and N 2 fixation. There were highly significant differences in nodulation factors such as nodule distribution within root-zone, nodule morphology and nodule mass, and in N 2 fixation factors such as nitrogenase activity and the amount of effective nodule tissues between the plant selections, Rhizobium strains and their interactions at sixth and ninth week after seedling emergence. The strain NGR 258 was highly effective on winged bean and highly ineffective on P. palustris in terms of nodulation and N 2 fixation. The strain SRI 1 was highly ineffective and could not be used in field practice, while strain MAR 655 showed a good symbiotic association with some of the winged bean genotypes, formed a good amount of effective nodule tissues equal to that of NGR 258, but performed poorly compared to NGR 258 in terms of total nodule mass and N 2 fixation. The winged bean selection LBNC 1 was the best genotype in terms of nodulation and N 2 fixation. The symbiotic specificities observed in the host and Rhizobium combination showed that winged bean selections LBNC 1 and TPT 2 with strain NGR 258 were excellent symbiotic systems while P. palustris and NGR 258 should not be used in field practice. P. palustris could be used in hybridization programs with winged bean without the nodulation and N 2 fixation, although care should be taken in selecting the strain of Rhizobium inoculum for such lines. P. palustris was equally good as winged bean selections as a potential mulch/cover crop in terms of N 2 fixation.

Soybean Nodulation as Affected by Wheel Traffic1

AbstractA significant quantity of nitrogen is made potentially available for plant uptake through symbiotic fixation in the nodules of legume roots. Several aspects of soil environment that can affect the formation and development of these nodules can be altered by soil being compacted by farm vehicular wheel traffic. Field experiments were conducted on a Nicollet silty clay loam (Aquic Hapludoll) to define the extent to which wheel traffic can affect nodulation on roots of soybeans [Glycine max (L.) Merr. ‘Corsoy’]. Wheel traffic of tractors and implements during normal field operations for soybean culture in the northern Corn Belt was controlled to produce soybean rows with no wheel traffic on either side, and wheel traffic on only one or both sides. Measurements of nodule number and mass from undisturbed soil monoliths showed that rows with wheel traffic on both sides had from 20 to 30% fewer nodules and about 36% less total nodule mass than rows with wheel traffic on only one side. Wheel traffic increased the specific nodule mass in both the tracked interrow area and the adjacent nontracked row area. Nontracked interrows had a greater percentage of nodule mass in the upper 30 cm than did wheel tracked interrows. Thus, by controlling the wheel traffic of normal farming operations, the quantity and distribution of nodules can be altered in ways which have implications for increasing nitrogen fixation.

Effect of Inoculum Rate on Competitive Nodulation of Glycine max L. Merrill. II. Field Studies1

AbstractTo provide a scientific basis for predicting successful competitive nodulation of soybeans (Glycine max L. Merr.) by Rhizobium japonicum, the quantitative relationship between soil and inoculum rhizobia, in nodule formation, was determined in the field. Soybean seeds were planted at a rate of 1 seed/2.5 cm of row and were inoculated with up to 3.3 ✕ 1038 R. japonicum cells/2.5 cm of row. Field plots were grown on six soils in Iowa having a range of soil rhizobia populations of 1.2 ✕ 101 to 2.3 ✕ 105/g of soil. Numbers of nodules on tap and lateral roots of soybeans grown on soils containing fewer than 12 rhizobia/g were increased when inoculation rates exceeded 3.3 ✕ 104 cells/seed. Neither numbers of nodules nor total nodule mass was increased by inoculation of soybeans grown on soils containing more than 1 ✕ 103 rhizobia/g. When an inoculation rate of 3.3 ✕ 106 rhizobia/seed was used on soybeans grown in soils with fewer than 12 rhizobia/g and in soils with approximately 1 ✕ 103 rhizobia/g, the inoculum strain produced, respectively, 65% and 35% of the nodules. From our results it may be predicted that if the inoculum rhizobia are to form 50% or more of the nodules an inoculum rate of at least 1,000 times the soil population (per g soil) must be used. Commercial inoculants in the United States probably are not supplying adequate numbers of rhizobia for successful competitive nodulation of soybeans grown on land previously cropped to soybeans.