Tap Root Nodules Research Articles

Using machine learning enabled phenotyping to characterize nodulation in three early vegetative stages in soybean.

The symbiotic relationship between soybean [Glycine max L. (Merr.)] roots and bacteria (Bradyrhizobium japonicum) lead to the development of nodules, important legume root structures where atmospheric nitrogen (N2) is fixed into bio-available ammonia (NH3) for plant growth and development. With the recent development of the Soybean Nodule Acquisition Pipeline (SNAP), nodules can more easily be quantified and evaluated for genetic diversity and growth patterns across unique soybean root system architectures. We explored six diverse soybean genotypes across three field year combinations in three early vegetative stages of development and report the unique relationships between soybean nodules in the taproot and non-taproot growth zones of diverse root system architectures of these genotypes. We found unique growth patterns in the nodules of taproots showing genotypic differences in how nodules grew in count, size, and total nodule area per genotype compared to non-taproot nodules. We propose that nodulation should be defined as a function of both nodule count and individual nodule area resulting in a total nodule area per root or growth regions of the root. We also report on the relationships between the nodules and total nitrogen in the seed at maturity, finding a strong correlation between the taproot nodules and final seed nitrogen at maturity. The applications of these findings could lead to an enhanced understanding of the plant-Bradyrhizobium relationship and exploring these relationships could lead to leveraging greater nitrogen use efficiency and nodulation carbon to nitrogen production efficiency across the soybean germplasm.

Root Distribution and Nitrogen Fixation Activity of Tropical Forage Legume American Jointvetch (Aeschynomene americanaL.) cv. Glenn under Waterlogging Conditions

We investigated the root distribution and nitrogen fixation activity of American jointvetch (Aeschynomene americanaL.) cv. Glenn, under waterlogging treatment. The plants were grown in pots under three different treatments: no waterlogging (control), 30 days of waterlogging (experiment 1), and 40 days of waterlogging (experiment 2). The plants were subjected to the treatments on day 14 after germination. Root dry matter (DM) weight distribution of waterlogged plants was shallower than controls after day 20 of waterlogging. Throughout the study period, the total root DM weight in waterlogged plants was similar to that in the controls. Enhanced rooting (adventitious roots) and nodule formation at the stem base were observed in waterlogged plants after day 20 of waterlogging. The average DM weight of individual nodules on the region of the stem between the soil surface and water surface of waterlogged plants was similar to that of individual taproot nodules in the controls. Waterlogged plants had slightly greater plant DM weight than the controls after 40 days of treatment. The total nitrogenase activity (TNA) of nodules and nodule DM weight were higher in waterlogged plants than in the controls. Waterlogged American jointvetch had roots with nodules both around the soil surface and in the area between the soil surface and water surface after 20 days of waterlogging, and they maintained high nitrogenase activity and net assimilation rate that resulted in an increased growth rate.

Aerenchyma formation in crop species: A review

Flooding is a major problem in many crop areas around the world. However, many wetland plant species can expand their roots into flooded soils because of the presence of longitudinal aerenchyma channels that facilitate oxygen diffusion from the shoots to the root tips. Aerenchyma also forms in rice roots, allowing rice plants to grow well in flooded paddy fields. Aerenchyma formation therefore helps plants to survive flooding. “Primary aerenchyma” forms in the roots of some cereal crops such as rice, maize, barley and wheat. “Secondary aerenchyma” forms in the stem, hypocotyl, tap root, adventitious roots, and root nodules of some legume crops such as soybean. This paper reviews the recent progress in the study of aerenchyma formation, and highlights the role that primary aerenchyma in cereal crops and secondary aerenchyma in soybean can play in improving their tolerance to flooding.

Formation and function of secondary aerenchyma in hypocotyl, roots and nodules of soybean (Glycine max) under flooded conditions

Flooding is a major problem in many areas of the world and soybean is susceptible to the stress. Understanding the morphological mechanisms of flooding tolerance is important for developing flood-tolerant genotypes. We investigated secondary aerenchyma formation and function in soybean (Glycine max) seedlings grown under flooded conditions. Secondary aerenchyma, a white and spongy tissue, was formed in the hypocotyl, tap root, adventitious roots and root nodules after 3 weeks of flooding. Under irrigated conditions aerenchyma development was either absent or rare and phellem was formed in the hypocotyl, tap root, adventitious roots and root nodules. Secondary meristem partially appeared at the outer parts of the interfascicular cambium and girdled the stele, and then cells differentiated to construct secondary aerenchyma in the flooded hypocotyl. These morphological changes proceeded for 4 days after the initiation of the flooding. After 14 days of treatment, porosity exceeded 30% in flooded hypocotyl with well-developed secondary aerenchyma, while it was below 10% in hypocotyl of irrigated plants that had no aerenchyma. When Vaseline was applied to the hypocotyl of plants from a flooded treatment to prevent the entry of atmospheric oxygen into secondary aerenchyma, plant growth, especially that of roots, was sharply inhibited. Thus secondary aerenchyma might be an adaptive response to flooding.

Altered exopolysaccharides of Bradyrhizobium japonicum mutants correlate with impaired soybean lectin binding, but not with effective nodule formation.

The exact mechanism(s) of infection and symbiotic development between rhizobia and legumes is not yet known, but changes in rhizobial exopolysaccharides (EPSs) affect both infection and nodule development of the legume host. Early events in the symbiotic process between Bradyrhizobium japonicum and soybean (Glycine max [L.] Merr.) were studied using two mutants, defective in soybean lectin (SBL) binding, which had been generated from B. japonicum 2143 (USDA 3I-1b-143 derivative) by Tn5 mutagenesis. In addition to their SBL-binding deficiency, these mutants produced less EPS than the parental strain. The composition of EPS varied with the genotype and with the carbon source used for growth. When grown on arabinose, gluconate, or mannitol, the wild-type parental strain, B. japonicum 2143, produced EPS typical of DNA homology group I Bradyrhizobium, designated EPS I. When grown on malate, strain 2143 produced a different EPS composed only of galactose and its acetylated derivative and designated EPS II. Mutant 1252 produced EPS II when grown on arabinose or malate, but when grown on gluconate or mannitol, mutant 1252 produced a different EPS comprised of glucose, galactose, xylose and glucuronic acid (1:5:1:1) and designated EPS III. Mutant 1251, grown on any of these carbon sources, produced EPS III. The EPS of strain 2143 and mutant 1252 contained SBL-binding polysaccharide. The amount of the SBL-binding polysaccharide produced by mutant 1252 varied with the carbon source used for growth. The capsular polysaccharide (CPS) produced by strain 2143 during growth on arabinose, gluconate or mannitol, showed a high level of SBL binding, whereas CPS produced during growth of strain 2143 on malate showed a low level of SBL binding. However, the change in EPS composition and SBL binding of strain 2143 grown on malate did not affect the wild-type nodulation and nitrogen fixation phenotype of 2143. Mutant 1251, which produced EPS III, nodulated 2 d later than parental strain 2143, but formed effective, nitrogen-fixing tap root nodules. Mutant 1252, which produced either EPS II or III, however nodulated 5-6 d later and formed few and ineffective tap root nodules. Restoration of EPS I production in mutant 1252 correlated with restored SBL binding, but not with wild-type nodulation and nitrogen fixation.

Growth and N 2-fixation of soybeans inoculated with strains of Bradyrhizobium japonicum differing in energetic efficiency and PHB utilization

The hypothesis, that accumulation of poly-hydroxybutyrate (PHB) in bacteroids of soybean root nodules and utilization of this polymer during pod-filling, are factors in N 2 fixation during seed development, was tested in a glasshouse experiment. Lincoln soybeans, inoculated with strains of B. japonicum of different energetic efficiency (Hup + or Hup −), were grown under bacteriologically-controlled conditions in open pots of combined-N-free medium, until the pod-filling stage. Nitrogen fixation was measured as accumulation of total N in shoots (including seeds). Bacteroids, prepared from tap-root nodules at intervals during growth, were assessed for PHB and protein content. During vegetative growth, all of the strains were of similar symbiotic effectiveness but during pod-filling, plants inoculated with the three Hup + strains fixed N 2 and grew at about twice the rates of plants inoculated with the three Hup − strains. Bacteroids of all three Hup + strains accumulated PHB (from 40 to >50% dry wt) during vegetative plant growth and utilized it during pod-filling. Two of three Hup − strains accumulated little PHB in the bacteroids and all three utilized little or none of it during pod-filling. These results are discussed in relation to a supply of energy-yielding substrates to nodules during pod-filling, to the influence of energetic efficiency of the bacteroids on PHB accumulation and to the role of PHB utilization in sustaining nodule O 2-demand during seed development.

Field response of the Glycine-bradyrhizobium symbiosis to modified early-nodule occupancy

A 2 yr field study was conducted to monitor the response of the Glycine-Bradyrhizobium symbiosis to high initial levels of nodulation by selected strains of B. japonicum when grown in soil containing indigenous soybean bradyrhizobia. Nodulated soybean seedlings were prepared under greenhouse conditions and transplanted to 15N-amended field microplots when ca 23 days old. The population of indigenous bradyrhizobia at the study site was ca 7 × 10 5g −1 soil. Inoculation treatments included strains USDA 122 (a superior N 2 fixer), USDA 94 (a strong rhizobitoxine producer), and a soil suspension containing a mixture of strains indigenous to the study site. Occupancy of root nodules was monitored serologically with an ELISA and morphologically by colony type on yeast extract-mannitol agar. The frequencies of serogroups 122 and 94 in early-formed (tap root) nodules of plants receiving USDA 122 and 94 as inocula dropped from 99.5 and 94.5% during vegetative growth to 55.0 and 28.5% during seedfill, respectively. The incidence of B. japonicum DNA homology Group II increased during host development for those plants initially treated with the soil suspension. Relative to the soil control treatment, inoculation with USDA 122 significantly increased soybean seed yield by 32%, N 2 fixation by 29% and total shoot N content by 30%. Conversely, USDA 94 decreased the magnitude of these variables by 39, 44 and 42%, respectively. These results demonstrate that the successful introduction of contrasting strains of B. japonicum into early-formed nodules can modify soybean productivity despite subsequent nodulation by indigenous soybean bradyrhizobia.

Competitiveness of indigenous populations of Bradyrhizobium japonicum serocluster 123 as determined using a root-tip marking procedure in growth pouches

Soil Bradyrhizobium populations limit nodule occupancy of soybean by symbiotically-superior inoculant strains throughout much of the American midwest. In this study, the competitiveness of indigenous populations of B. japonicum serocluster 123 from Waukegan and Webster soils was evaluated in growth pouches using a root-tip marking procedure. The native rhizobia were from soils incubated 0–8 h in soybean root exudate (SRE) or plant nutrient solution (PNS) prior to inoculation. Populations of serocluster 123 strains in soil and nodule occupancy by these strains were assessed using fluorescent antibodies prepared against B. japonicum USDA 123. There were no significant differences in populations that came from SRE or PNS incubated soils: both populations increased in number over the incubation period. Nodule occupancy by both populations in growth pouches was similar to that previously encountered in field studies with these two soils. With the Waukegan soil, the serocluster 123 population dominated nodulation forming 69 and 62% of taproot nodules above and below the root tip mark, respectively. However, for the more alkaline Webster soil, serocluster 123 strains were much less competitive, producing only 9 and 13%, respectively, of the nodules formed above and below the root tip mark. In growth pouches, soil populations of bradyrhizobia from the Webster soil produced significantly more nodules than those from the Waukegan soil, but both strains and a pure culture of USDA 110 had a similar distribution of nodules.

Exclusion of inefficient Bradyrhizobium japonicum serogroups by soybean genotypes

Soybean [Glycine max (L.) Merr.] forms a symbiosis with serogroups of Bradyrhizobium japonicum that differ in their dinitrogen fixing abilities. The objectives of this study were to identify soybean genotypes that would restrict nodulation by relatively inefficient serogroups indigenous to a large portion of the southeastern USA, and then characterize the nodulation responses of selected genotypes with specific bradyrhizobial strains under controlled conditions. From field screening trials followed by controlled single and competitive inoculations of serogroups USDA 31, 76 and 110, twelve soybean genotypes out of 382 tested were identified with varying levels of exclusion abilities. Soybean nodule occupancies and nodulation characteristics were influenced by plant genotype, environment (i.e. field or greenhouse), bradyrhizobial serogroup, and location of nodules (i.e. tap or lateral root). The cultivar Centennial sustains high seed yields even though it nodulates to a high degree with the inefficient serogroup USDA 31. In contrast, data from the released cultivars Braxton, Centennial and Coker 368 indicate that they may have been selected to exclude the inefficient serogroup USDA 76 from their tap root nodules, possibly contributing to high seed yield.

Symbiotic interactions between soybean and competing strains ofBradyrhizobium japonicum

Selected symbiotic characteristics of fiveBradyrhizobium japonicum strains were assessed in association with ‘Ransom’ soybean plants (Glycine max [L.] Merr.). In the first of two greenhouse experiments, relative nodulation competitiveness of the strains was examined. Strains were grouped into pairs, and corresponding cells were applied to surface-disinfected seeds so as to provide seven ratios of cell numbers between the two strains. Tap root nodules were harvested 28 days after sowing and serotyped by means of an enzyme-linked immunosorbent assay. Strains differed considerably in nodulation competitiveness, and these differences were successfully quantified using relationships previously proposed in the literature. A second experiment involved assessment of the reproducibility of this technique and characterization of the symbiotic response to single- and double-strain inocula. Differences in relative nodulating abilities of strains were apparent between experiments and were possibly related to observed variations in greenhouse temperatures. Plant shoot weight and total N content were not significantly correlated with nodule number or weight when evaluated across inoculation treatments, but these correlations were often significant within inoculation treatments. Certain double-strain inocula produced either positive or negative effects on shoot weight, N content, and nodulation, when compared with values predicted from corresponding controls receiving single-strain inocula.

Nodulation of Glycine max by Six Bradyrhizobium japonicum Strains with Different Competitive Abilities

The root nodule locations of six Bradyrhizobium japonicum strains were examined to determine if there were any differences which might explain their varying competitiveness for nodule occupancy on Glycine max. When five strains were added to soybeans in plastic growth pouches in equal proportions with a reference strain (U.S. Department of Agriculture, strain 110), North Carolina strain 1028 and strain 110 were the most competitive for nodule occupancy, followed by U.S. Department of Agriculture strains 122, 76, and 31 and Brazil strain 587. Among all strains, nodule double occupancy was 17% at a high inoculum level (10 CFU pouch) and 2% at a low inoculum level (10 CFU pouch). The less competitive strains increased their nodule representation by an increase in the doubly occupied nodules at the high inoculum level. Among all strains, the number of taproot and lateral root nodules was inversely related at both the high and low inoculum levels (r = -0.62 and -0.69, respectively; P = 0.0001). This inverse relationship appeared to be a result of the plant host control of bacterial infection. Among each of the six strains, greater than 95% of the taproot nodules formed at the high inoculum density were located on 25% of the taproot length, the nodules centering on the position of the root tip at the time of inoculation. No differences among the six strains were observed in nodule initiation rates as measured by taproot nodule position. Taproot nodules were formed in the symbiosis before lateral root nodules. One of the poorly competitive strains (strain 76) occupied three times as many taproot nodules as lateral root nodules when competing with strain 110 (nodules were harvested from 4-week-old plants). Among these six wild-type strains of B. japonicum, competitive ability evidently is not related to nodule initiation rates.

Nodulation competition among Bradyrhizobium japonicum strains as influenced by rhizosphere bacteria and iron availability

Bacteria isolated from the root zones of field-grown soybean plants [Glycine max (L.) Merr.] were examined in a series of glasshouse experiments for an ability to affect nodulation competition among three strains of Bradyrhizobium japonicum (USDA 31, USDA 110, and USDA 123). Inocula applied at planting contained competing strains of B. japonicum with or without one of eleven isolates of rhizosphere bacteria. Tap-root nodules were harvested 28 days after planting, and nodule occupancies were determined for the bradyrhizobia strains originally applied. Under conditions of low iron availability, five isolates (four Pseudomonas spp. plus one Serratia sp.) caused significant changes in nodule occupancy relative to the corresponding control which was not inoculated with rhizosphere bacteria. During subsequent glasshouse experiments designed to verify and further characterize these effects, three fluorescent Pseudomonas spp. consistently altered nodulation competition among certain combinations of bradyrhizobia strains when the rooting medium did not contain added iron. This alteration typically reflected enhanced nodulation by USDA 110. Two of these isolates produced similar, although less pronounced, effects when ferric hydroxide was added to the rooting medium. The results suggest that certain rhizosphere bacteria, particularly fluorescent Pseudomonas spp., can affect nodulation competition among strains of R. japonicum. An additional implication is that iron availability may be an important factor modifying interactions involving the soybean plant, B. japonicum, and associated microorganisms in the host rhizosphere.

Response of Rhizobium leguminosarum isolates to different forms of inorganic nitrogen during nodule development in pea ( Pisum sativum L.)

Combined nitrogen inhibits nodule development and nitrogen fixation in Rhizobium -legume symbioses. Isolates of R. leguminosarum which had been shown to vary in symbiotic effectiveness in the presence of NN 4NO 3 were used as inoculum for peas grown with NH 4NO 3 KNO 3 or NH 4Cl and harvested between 14 and 24 days after planting. Plants grown in 2m m NH 4NO 3 showed inhibition of nodule development and function 17 days after seeding but there were no significant differences (SDs) between isolates of high and low effectiveness. In the presence of 5 m M NH 4NO 3 nodule development and C 2H 2 reduction were severely inhibited from 14 days onwards. Plants inoculated with isolates of high effectiveness had more tap-root nodules, higher C 2H 2 reduction rates and higher leghemoglobin content than those inoculated with isolates of low effectiveness. In plants grown with 5 m m KNO 3 or 5 m m NN 4Cl, NO 3 − had the major inhibitory effect on nodulation and nodule development, but NH 4 + began to show inhibitory effects on nodule development after 17 days. There were no SDs between isolates of low and high effectiveness at these concentrations of NO 3 − and NH 4 +. Differences in symbiotic response of R. leguminosarum to combined nitrogen are small and appear during nodule development rather than at nodule initiation.

Asparagine Biosynthesis in Soybean Nodules

Asparagine biosynthesis in soybean (Glycine max [L.] Merr.) nodules has been difficult to demonstrate due to the poor conversion of suspected immediate precursors to asparagine and the instability of the key enzyme asparagine synthetase. The present study was designed to explore the effects of two ammonium assimilation inhibitors on the metabolism of (14)CO(2) to [(14)C]asparagine and to demonstrate the existence in nodules of the enzyme asparagine synthetase. When detached nodules were incubated in (14)CO(2), radioactivity in asparagine (as a percentage of amino acid cpm) increased 10-fold over 4 hours. Vacuum infiltration of 10 mm methionine sulfoximine or 10 mm azaserine prior to (14)CO(2) incubations decreased both the rate of dark fixation and the radioactivity in the amino acid fraction. These inhibitors also decreased the recovery of label in aspartate and asparagine. These results, plus the sequence of labeling of metabolites from (14)CO(2), are consistent with a glutamine-dependent synthesis of asparagine from aspartate with oxalacetate as a precursor to aspartate.An enzyme catalyzing the ATP- and glutamine-dependent amidation of aspartic acid to form asparagine was isolated from soybean nodules. High levels of sulfhydryl protectants were required and the inclusion of glycerol and substrates in the extraction buffer helped to stabilize the enzyme. Enzyme activity in taproot nodules increased between 38 and 41 days after planting and peaked soon after flower initiation (45 days). The activity then declined to basal levels by 70 days. On a total enzyme activity basis, there was 170-fold more asparagine synthetase activity in the infected zone of the nodule than in the cortex, and 205-fold more activity in the cytosol than the bacteroid fraction. The enzyme has a broad pH maximum around pH 8.25, and the apparent K(m) values for the substrates aspartate, MgATP, and glutamine are 1.24 mm, 0.076 mm, and 0.16 mm, respectively. Ammonium ion can replace glutamine as the nitrogen donor, but the K(m) value of the enzyme for ammonium ion is 40-fold higher than that for glutamine.

Título en español

Condiciones de alta temperatura y baja humedad del suelo durante los siete días siguientes a la siembra, fueron utilizadas para evaluar tipos y dosis de inóculo en soya (Glycine max (L.) Merr.) cultivada en un suelo libre de Rhizobium japonicum en Puerto Rico. La mitad del experimento, sin riego durante los primeros siete días, tuvo un contenido de humedad inferior al punto de marchitez y temperatura máxima del suelo entre 38 y 40° C a 2.5 cm de profundidad. La parte regada tuvo un contenido de humedad cercano a la capacidad de campo y temperaturas máximas del suelo entre 31 y 35° C a 2.5 cm de profundidad. El inóculo granular aplicado al suelo fue significativamente mejor que el inóculo líquido colocado en el surco a profundidades de 2.5, 5.0 y 7.5 cm debajo de la semilla y que el inóculo de turba aplicado a la semilla. Solamente el inóculo granular aplicado al suelo produjo un aumento significativo en la nodulación con una dosis diez veces la recomendada, al comparar cada tratamiento con su dosis respectiva normal. El número de nódulos en la raíz primaria, el número total de nódulos y el peso de la materia seca de los nódulos por planta fueron significativamente mayores en la sección regada que en la no regada.

Soybean Nodulation as Influenced by Seedling Vigor1

abstractNodulation in soybeans [Glycine max (L.) Merr.] is influenced by environmental, rhizobial, and host symbiont factors. This investigation was conducted to determine the effects of seedling vigor on early soybean nodulation. Seed quality of three soybean cultivars was adjusted using the accelerated aging technique in order to provide seed lots with a high degree of variation in seedling vigor. Seedling vigor was defined as the number of days from planting to emergence. Aged seeds planted in both a sandbench and the field were uniformly inoculated in the furrow with a Rhizobium japonicum liquid inoculum. Days to seedling emergence was negatively correlated with plant top weight, number of tap root nodules, total number of nodules, and nodule dry weight per plant. Significant decreases in the number of tap root nodules and total number of nodules were observed in a sandbench trial between each of the three earliest emerging treatments (4, 5, and 6 days after planting). In all cases, the first plants to emerge had significantly more tap root nodules than the last plants to emerge. Significance in number of nodules/plant and nodule weight/plant between these two treatments was observed in six of the eight instances.The results suggest that soybean nodulation may be influenced by seedling vigor and that seedlings with uniform vigor should be utilized when conducting nodulation evaluations.

Physiological and biochemical studies on senescing tap root nodules of soybeans.

Senescence of soybean (Glycine max L. Merr.) tap root nodules was investigated by comparing changes in various physiological and biochemical activities with changes in capacity to fix nitrogen. Field-grown Beeson and Calland varieties of soybeans of various ages were sources of tap root nodules. With both varieties, the number of tap root nodules per plant remained constant between 56 and 86 days after planting but fresh weight, dry weight, and mass of tap root nodules increased duing this period. Nitrogen (C2H2)fixation by attached tap root nodules was maximum on a fresh weight, dry weight, or nitrogen basis about 56 days after planting for either variety. Metabolic activities of bacteroids as measured by carbon dioxide evolution from glucose and succinate did not appear to vary among nodules of different ages. There was also no indication of mobilization or deposition or deposition of iron, molybdenum, calcium, zinc, and nitrate in aging tap root nodules. Nitrate levels in the aerial portion of the plants decreased significantly after the initial decline in acetylene reduction. Nicotinamide deamidase activity in the cytosol and in extracts of bacteroids did not change significantly as tap root nodules aged. However, significant and consistent changes were observed in initial pH values of nodule breis and the initial decline occurred before (Calland) or concurrently (Beeson) with the initial decline of nitrogen fixation.

Compatibility of Rhizobium japonicum with Chemical Seed Protectants1

AbstractThe literature on the compatibility of Rhizobium sp. with seed protectant chemicals is controversial because of variation in methods and the lack of quantitative data. The present study was planned to develop a reliable quantitative method of measuring compatibility of Rhizobium japonicum with seed‐applied fungicides. The effect of four seed protectant chemicals on survival of R. japonicum on soybean (Glycine max L. Merr. cv. ‘Hark’) seed was studied in laboratory growth chamber experiments. The chemicals tested and levels used were tetramethyl‐thiuram‐disulfide (thiram), 0.6 g/kg seed; N‐trichloromethylthio‐4‐cyclohexene‐l,2‐dicarboximide (captan), 0.8 g/kg seed; 5,6‐Dihydro‐2‐methyl‐l,4‐oxathiin‐3‐carboxanilide (carboxin), 1.1 g/kg seed; and pentachloronitrobenzene (PCNB), 0.9 g/kg seed. Quantitative measurements for compatibility included plate counts for survival of R. japonicum on inoculated chemically treated seed and counts of taproot nodules on plants at 2 weeks age. R. japonicum were compatible with Thiram and Carboxin but not with PCNB and Captan. PCNB reduced Rhizobium survival on seed and decreased taproot nodulation. Captan was less toxic to rhizobia than was PCNB, but taproot nodulation of plants was greatly reduced by captan. Carboxin had little effect on viable rhizobia or taproot nodulation when seeds were planted within 4 hours of inoculation. Nodulation was poor when these seeds were planted 24 hours after inoculation despite the presence of what would normally be considered adequate viable rhizobia. Thiram had no adverse effect on viable rhizobia or taproot nodulation even when seeds were held 24 hours before planting. The results also indicate that compatibility must be measured quantitatively in relation to time and that nodulating ability of the surviving bacteria is the prime factor in determining compatibility.

Studies on Soybean Nodule Senescence

Soybean Glycine max. L. Merr. nodule senescence was studied using the loss of acetylene reduction by intact tap root nodules as its indication. Tap root nodules from two varieties (Calland and Beeson) of field-grown soybeans were used. The specific activities of nitrogenase (micromoles/minute gram fresh weight of nodules) as measured by the acetylene reduction assay decreased abruptly between 58 to 65 and 68 to 75 days after planting the Beeson and Calland soybeans, respectively. Major changes were not detected in dry weight, total nitrogen, and leghemoglobin levels during the period when in vivo nitrogenase activity declined. Ammonium levels in the cytosol of nodules and poly-beta-hydroxybutyrate increased moderately just prior to or coincidental with the loss of nitrogenase activity. Neither enzymes that have been postulated to be involved in ammonium assimilation nor NADP(+)-specific isocitrate dehydrogenase exhibited any large changes in specific activities during the initial period when nitrogenase activity declined.

Effect of Inoculum Size on Nodulation of Glycine max (L.) Merril, Variety Ford1

AbstractA criterion for determining adequacy of inoculation of Glycine max by Rhizobium japonicum is needed for testing of commercial inoculants and for determining adequacy of nodulation. This study was undertaken to determine the inoculum level needed for maximum nitrogen fixation of Glycine max and to find a qualitative nodulation pattern that would be useful in recognizing good nodulation. To accomplish this end, soybeans were inoculated and grown in pots containing sand or soil in the greenhouse. Inoculation levels up to 2.4 ✕ 105 and 1.4 ✕ 107 Rhizobium japonicum cells per seed were used for soil‐ and sand‐grown plants respectively. Taproot nodulation was still increasing at the highest inoculum level in soil and sand, but lateral root nodulation had leveled off. Maximum total nitrogen fixed in plant tops was achieved at inoculation levels of 2 ✕ 103 and 1 ✕ 105 cells per seed for soil‐ and sand‐grown plants, respectively. At these inoculation levels at least four taproot nodules were formed. Results suggest that taproot nodulation of soybeans is a qualitative characteristic that may be useful in determining the adequacy of nodulation and may indicate abundance of rhizobia for a soybean variety.