Trifluralin-induced chlorosis in soybeans (Glycine max (L.) Merr.) grown on clayey,high pH soils

Summary The cause of leaf chlorosis, frequently observed on soybeans (Glycine max (L.) Merr.) grown on high pH soils of the Mississippi Blackland Prairie, is thought to be low Fe availability and restricted rooting. Three greenhouse experiments were conducted using two soils, Sumter, a Rendollic Eutrocrept and Okolona, a Typic Chromudert; nine soybean cultivars differing in Feefficiency; and trifluralin (α-α-α-trifluoro-2,6-dinitro-N, N-di-propyl-p-toludine). Trifluralin at rates greater than 0.56 kg/ha caused chlorosis which was more severe on the Sumter, a soil low in available Fe. Fe-efficient cultivars were more resistant to the chlorosis induced by trifluralin than the Fe-inefficient cultivars. It was concluded that the chlorosis is an Fe deficiency caused by reduced uptake. The herbicide-induced chlorosis can be avoided by proper dosage and placement of the herbicide.     

Effects of N-serve on soybeans and soil nitrogen transformations

Summary Plant dry weight, total N, and total Ca was increased at 0.1 and 1 ppm N-serve. At greater 10 ppm the plants showed visual symptoms of a stunted growth, stem elongation, flowers, and pods failed to form or were aborted, young leaves were curled, and roots were club shaped with many branches. These symptoms were increasingly evident with increasing N-serve application rates. The reason was attributed to an auxin effect. Dry wt and total N in the plant was less than the control at the higher N-serve applications. There was little effect on nitrogenase activity at less than 10 ppm N-serve. Nodulation tended to increase at 0.1 and 1 ppm N-serve.Nitrification was inhibited up to 104 days at 20 ppm N-serve. The soil pH of the high N-serve rates was decreased at 104 days probably due to nitrification. Generally there were little detectable differences among treatments in soil organic N. The average soil organic N from 0 to 104 days decreased by 0.01%. Average increase in total N within each pot at harvest was equivalent to about 138 kg N/ha.     

The inheritance of a urease-null trait in soybeans

Summary Four soybean seed urease nulls (lacking both the activity and antigen of the embryo-specific urease) were intermated and the F₁ and F₂ seed examined for urease activity. Both generations were without urease activity, and the nulls were therefore considered noncomplementing. In crosses of each null line to cultivars homozygous for the allelic, codominantly inherited urease slow or fast isozyme, the F₁ seed expressed the embryo-specific urease isozyme of the urease-expressing parent. A 3 1 segregation for presence and absence of urease was observed in progeny from F₁ and heterozygous F₂ plants. The F₂ and F₃ from fastXnull combinations revealed that urease-positive seed were all phenotypically urease fast, while the same seed from slowXnull combinations showed a segregation of one seed containing a fast urease, either exclusively or in a heterozygous state with the slow isozyme, for every 69 phenotypic slows. Data pooled from F₂ plants which segregate for both the presence (Sun) and absence (Sun) of urease and for the fast (Eu1-b) or slow (Eu1-a) urease allele indicate that the null lesion (Sun) is linked to Eu1 by approximately one map unit. The evidence is consistent with two models: (1) sun is an allele at the embryo-specific urease isozyme locus (Eu1) and that a high degree of exchange (and/or conversion) within the locus results in a 1% recombination frequency between the null trait and urease allozyme; (2) sun is at a distinct locus which is separated by one map unit from the embryo-specific urease isozyme locus (Eu1) upon which it acts in the cis position. Polyadenylated embryo RNA from one of the null lines, PI 229324, exhibited no urease template activity in vitro. Thus, the lack of urease antigen is due to lack of accumulation of translatable urease mRNA. The availability of soybeans lacking seed urease should be extremely useful to breeders as a trait for linkage studies and to geneticists as a transformation marker.Portions of this work were funded by the Illinois and Missouri Agricultural Experiment Stations, the SOHIO-University of Illinois Center of Excellence in Crop Molecular Genetics and Genetic Engineering and by grants PCM-8219652 from the National Science Foundation and USDA/SEA-CRCR-1-1374 from the USDA Competitive Grants Office     

Conditional lethality involving nuclear and cytoplasmic chlorophyll mutants in soybeans

Summary A conditionally lethal phenotype occurred when a nuclear chlorophyll mutant (y ₂₀-k ₂) was present with a cytoplasmic chlorophyll mutant (cyt-Y ₂) in soybean (Glycine max [L.] Merr.). Nuclear mutant y ₂₀-k ₂, Genetic Type Collection Number T253, has yellow foliage, tan-saddle-pattern seed and is viable. The y ₂₀-k ₂ mutant cannot be separated by classical genetic tests into two separate components, y ₂₀ (yellow foliage) and k ₂ (tan-saddle-pattern seed). Mutant cyt-Y ₂, T275, is inherited cytoplasmically, has yellow foliage, and is viable. The genotype cyt-Y ₂ y ₂₀-k ₂/ y ₂₀-k ₂ is a conditional lethal; the genotype is lethal under field conditions, but plants survive under greenhouse conditions. This interaction is unique to y ₂₀-k ₂. This conditionally lethal genotype may be useful in molecular studies on the interaction between nuclear and plastid genomes.This is a joint contribution of North Central Region, USDA ARS, and Journal Paper No. J-11429 of the Iowa Agriculture and Home Economics Experiment Station, Ames, Iowa, and the Agriculture Experiment Station, Univ. of Puerto Rico, Mayaguez Campus, Mayaguez, Puerto Rico 00708. Projects 2471 and 2475. The research was supported in part by the Iowa Soybean Promotion Board.     

The influence of temperature and nitrate on vegetative growth and nitrogen accumulation by nodulated soybeans

Summary Inoculated soybeans [Glycine max (L.) Merrill] were grown in controlled environments to evaluate the relationship between temperature and applied NO₃−N on growth rates, N accumulation, and acetylene reduction activity during the vegetative growth stage. Soybeans were grown at day/night temperatures of 22/18 and 26/22°C in sand culture with daily applications of 21.4 mM (high) and 2.1 mM (low) NO₃−N in a complete nutrient solution for durations of 14, 21, and 42 days after emergence and with an N-free solution. Dry matter and N accumulation were greater at 26/22 than 22/18°C. In general, both increased as the level and duration of applied NO₃−N was increased. These increases were attributable to an abbreviation in the interval between emergence and onset of rapid growth. The presence and assimilation of NO₃−N, even at the high level, did not inhibit development of functional nodules. Neither mass nor acetylene reduction activity of nodules was reduced by high NO₃−N; however, the root mass was increased by NO₃−N more than the nodule mass. There was an interaction between temperature and NO₃−N on specific nodule activity as measured by acetylene reduction. The specific nodule activity was unaffected by NO₃−N at 22/18°C, but at 26/22°C the specific activity was lower in the absence of NO₃−N than when NO₃−N was present. Apparently, rapid early growth at 26/22°C depleted cotyledonary reserves of N before nodules became active and, thereafter, the plants were unable to develop adequate leaf area to support nodule development and functioning. This result has implications in N fertilization of late-planted soybeans. Paper number 6637 of the Journal Series of the North Carolina Agricultural Research Service, Raleigh, North Carolina, 27650. The research was supported in part by a grant from the North Carolina Soybean Producers Association and by USDA-SEA-CR grant 701-15-26.     

Manganese and iron interactions on their uptake and distribution in soybean (Glycine max (L.) Merr.)

Summary The uptake and distribution of iron and manganese were studied in a manganese-sensitive soybean cultivar (‘Bragg’) grown over a range of supply levels of these nutrients in solution culture. At high (90 and 275 μM) manganese levels, increasing the iron concentration in solution from 2 to 100 μM partially overcame the effects of manganese toxicity. Interactions between manganese and iron occurred for dry matter yields, rate of Mn absorption by the roots, and the proportions of manganese and iron transported to the tops. No interaction was observed for the rate of root absorption of iron. The percentage distribution of manganese in the plant top increased with increasing iron, despite a reduced rate of Mn uptake. On the other hand, iron uptake was independent of solution Mn concentration and increased with increasing solution Fe. Also more iron was retained in the roots at high Mn and/or Fe levels in solution. Concentrations of manganese and iron in roots, stems and individual leaves were affected independently by the manganese and iron supplyi.e. without any interaction occurring between the two elements. In general, the concentration in a plant part was related directly to the solution concentration. Symptoms resembling iron deficiency correlated poorly with leaf Fe concentrations whereas high levels of manganese were found in leaves displaying Mn toxicity symptoms.     

Dynamics of Seed Protein Biosynthesis in Two Soybean Genotypes Differing in Drought Susceptibility

The dynamics of seed storage protein biosynthesis was studied under field conditions during two vegetative seasons. Two soybean (Glycine max L. Merr.) genotypes were examined: BOSA (drought tolerant) and L 121 (drought susceptible). Seed samples were taken from plants at three stages of seed maturation (50 and 70 d after flowering, and at full maturity). The earlier synthesis of the -subunit of the 7S protein occurred in the drought susceptible cultivar. We have not found such differences in the synthesis of the - and -subunits of the 7S protein. Our results did not confirm significant genotypic differences in protein composition of the mature seeds between the cultivars studied, but have pointed out to the differences in the dynamics of protein biosynthesis during seed maturation and desiccation.     

Nitrogen accumulation and redistribution in soybean genotypes with variation in seed protein concentration

Breeding for high seed protein concentration in soybean [Glycine max (L.) Merrill] often results in lower yield, but the basis for this negative relationship is not well understood. To address this question, we evaluated the N acquisition characteristics of three high protein and three normal soybean genotypes in the field for 3 years. Plants were grown in 0.76 m rows following conventional cultural practices and water stress was minimized with sprinkler irrigation. We determined the mass and N concentration of leaves, petioles and stems at the beginning of seed filling (growth stage R5) and of stems at maturity. The N concentration of abscised leaves and petioles was also determined. There was significant variation among genotypes in total seed N (g m⁻²) at maturity (range from 14.7 to 24.4 g N m⁻²) as a result of variation in seed N concentration and yield. There was no evidence that the larger amounts of mature seed N were associated with a larger vegetative N reservoir at growth stage R5 as determined by vegetative mass at R5 or the concentration of N in vegetative tissues. Increasing seed N at maturity did not lower the N concentration in abscised leaves and petioles, or in the stems at maturity. The rate and timing of leaf senescence (loss of chlorophyll) was essentially the same for all genotypes. With no increase in the contribution from redistributed N, increases in N uptake or fixation during seed filling must have been responsible for the higher levels of seed N at maturity in high-protein genotypes. These data suggest that increasing total seed N at maturity by selecting for higher seed protein concentration or higher yield in soybean does not require, as some models suggest, a larger vegetative N reservoir at the beginning of seed filling or more rapid senescence.     

Effect of soil pH and sewage sludge on VA mycorrhizal infection of soybeans

Summary Small plots were amended in 1976 or 1978 with four kinds of sewage sludge. The sludges represented samples considered to be relatively free of heavy metals as well as sludges highly contaminated with heavy metals. Sludges were added to a silt loam soil at rates of 224 or 448 Mgha⁻¹. The soils were maintained at a high or low pH regime. In 1984, soybeans (Glycine max L. Merril. var. ‘Clark’) were planted and grown to the R4 stage. After harvest, roots were removed from the soil, washed, and examined for VA mycorrhizal infection. It was found that the heavy metal content of the sludge alone was generally not related to determining the extent of mycorrhizal infection. A heat treated sludge, high in heavy metals, exhibited the highest degree of mycorrhizal infection when the soil was maintained at a pH of 6.2. With this treatment, 52% of the root segments examined were infected by mycorrhiza. When the same sludge was added to a soil with a slightly lower pH (5.7) none of the roots examined were infected by mycorrhiza. When soybean roots were examined from soils that received no sludge and were maintained at either a low (5.6) or high (6.2) pH, there was no significant difference in mycorrhizal infection between the pH regimes. These results therefore indicate that sewage sludge may inhibit mycorrhizal infection if the sludge contains a high concentration of heavy metals and the sludge is applied to the soil with a low pH. Scientific Article No. A-4093 and Contribution No 7078 of the Maryland Agric. Exp. Stn., Dept. of Agronomy, University of Maryland, College Park, MD 20742.     

Agronomic evaluation of tissue-culture-derived soybean plants

Summary Genetic alterations of regenerated plants based on the tissue culture process (somaclonal variation) have become common for many plant species including soybean [Glycine max (L.) Merr.]. The objective of this study was to test for the presence of tissue-culture-derived genetic variation in eight agronomic traits in homozygous progeny regenerated by organogenesis using the commercially important cultivar Asgrow A3127. A total of 86 lines derived by repeated self-pollination of nine regenerated plants was grown in two locations for 2 years. When compared to the unregenerated parent, statistically significant variation (P<0.05) was found for maturity, lodging, height, seed protein and oil, but not for seed quality, seed weight, or seed yield. All of the variation noted was beneficial and did not involve decreased yield. Since the differences were not large, the results indicate that the tissue culture process is not necessarily detrimental to plant performance, which is an important consideration since tissue culture techniques are used in many genetic engineering methods.     

Iron-deficiency chlorosis evaluation of soybean with tissue culture

Summary The objectives of this study were: (i) to develop a tissue culture technique for the evaluation of Fe efficiency in soybean, and (ii) to compare the laboratory technique with field Fe chlorosis scores. Nineteen genotypes that had low and high levels of Fe efficiency were evaluated in the laboratory and at five field locations. Friable callus was induced from epicotyl sections, weighed, and placed on two different modified Murashige and Skoog media; one low in -naphthaleneacetic acid and the other low in Fe. Callus growth was rated as lack of growth compared to respective controls. As an example, Fe-inefficient cultivars (Asgrow A3205 and Pride B216) had significantly reduced growth compared to Fe-efficient germ plasm lines (All and A14). Correlation between the laboratory and field chlorosis rating was highest for the low auxin medium (r ² = 0.78), although correlation for the low Fe medium was also significant (r ² = 0.72). These results show that in vitro evaluation for Fe efficiency can be a useful tool for plant breeders.     

Phosphorus fertilization and tillage effect on dinitrogen fixation in soybeans

Summary Tillage has been shown to affect the uptake of phosphorus (P) and yield of soybeans, [Glycine max (L.) Merr.], but there is little information concerning the effects of P fertilization on nitrogen (N₂) fixation in soybeans under no-tillage. Two field experiments were conducted in 1980 and 1981 to determine the effects of soil P on N₂ fixation under no-tillage and to study the interaction of P fertilization and tillage of N₂ fixation, nutrient uptake, and yield of soybeans. In Exp. I, P was applied in 1977 at five rates up to 384 kg P ha⁻¹ and the effects of residual soil P were evaluated in 1980 and 1981 under no-tillage management. Nitrogen fixation rates, as measured by acetylene reduction assay, were significantly affected by soil P in Exp. I, but the assay proved to be a poor technique for estimating total plant N in these tests. Acetylene reduction rates and plant P increased rapidly as soil P increased from 2 to 20 mg kg⁻¹, with little additional increase above 20 mg P kg⁻¹. In Exp. II, rates (0, 32, 64, and 128 kg P ha⁻¹) and time (fall, spring and fall plus spring) of P application were compared under conventional tillage and no tillage. However, plant P increased with increasing levels of applied P. Applied P had no affect on acetylene reduction rates but rates were greater for no-tillage than conventional tillage at the V9 and R5 stages of growth in 1981. Plant uptake of P was more efficient under no-tillage than under conventional tillage in 1980 and 1981. Application of 64 kg P ha⁻¹ under no-tillage resulted in equivalent plant P levels as the 128 kg P ha⁻¹ applied under conventional tillage.     

The role of active Bradyrhizobium japonicum in iron stress response of soybeans

The objective of this study was to identify the sites of H-ion exudation and Fe(III) reduction along both inoculated and non-inoculated roots of A7 and T203 soybeans. A split-root system was used in which half the roots of each plant were inoculated and actively fixing nitrogen and the other half were not. Expectedly, the Fe-stress response was strong on both sides of the split-root system in the +N-Fe treatment of variety A7 (inactive nodules) but not of variety T203. The Fe-stress response of A7 was enhanced by the presence of active nodules. Variety T203 is Fe inefficient and normally fails to produce any Fe-stress response, but in the absence of nitrogen and iron (–N–Fe), inoculated roots responded to Fe stress with exudation of both H-ions and reductants. Intact split-root systems were embedded in agar to determine the location of H-ion exudation and Fe(III) reduction. On the inoculated side of the –N–Fe and –N+Fe treatments (active nodules) of both soybean varieties, H-ion production was associated mainly with the active nodules. However, quantities of H-ion release were much greater under Fe stress (–N–Fe) than with adequate Fe (–N+Fe). Reduction of Fe(III) to Fe(II) was found only on the nodulated side with T203, but on both sides with A7. In variety T203 the Fe reduction was associated with younger roots located just below the nodule clusters on the inoculated side of the –N treatments. Active nodules appear to play a key role in the Fe-deficiency stress response of T203 soybean.     

Establishment of early instar Heliothis zea on soybeans

Early instar Heliothis zea (Boddie) (Lepidoptera: Noctuidae) established on all stages of soybean (Glycine max [L.] Merrill) tested when larvae were confined to plants, although establishment decreased after podset stage and as seeds matured. In tests where larvae were allowed to disperse freely on plants, establishment also decreased as maturity increased, but was much lower in each stage than when confined to plants. Neonate larvae settled more quickly on terminals and expanding trifoliates than on mature trifoliates based on initiation of feeding and movement away from initial sites of placement. The ability of neonate larvae placed a short distance from soybean plants to reach a host was affected by high soil and ambient temperature. The results of these experiments suggest that larval host establishment and spin-down behavior play a major role in the late instar population distribution among soybean fields and that maturity of the host strongly influences that behavior.

---

Résumé Des expériences ont été réalisées dans deux types de conditions (avec ou sans possibilité de dispersion) pour déterminer si le taux d'installation des chenilles de premier stade de H. zea Bod. dépendait d'une mortalité liée au stade développement de Glycine max L. L'installation était considérée comme réussie quand les chenilles avaient atteint le second stade. Bien qu'il fût possible de distinguer les variétés de soja tant par le taux d'installation des chenilles que par les dégâts provoqués, les plantes les plus jeunes étaient plus colonisées par des chenilles que les témoins plus âgés. Toutes les variétés de soja à n'importe quel stade étaient acceptées lorsque la dispersion des chenilles était impossible. Avec possibilité de dispersion, les chenilles ont aussi accepté tous les stades phénologiques, mais en moins grands nombres que lorsque la dispersion était impossible. Le comportement de dispersion était beaucoup plus fréquent sur les plantes à port indéterminé que sur celles à port déterminé. Les possibilités de réinstallation après dispersion des chenilles de premier stade ont été limitées.

     

Mapping of putative quantitative trait loci controlling the total oligosaccharide and sucrose content of Glycine max seeds

Although oligosaccharides and sucrose are very important nutritional components of soybean seeds, little information is available about inheritance of oligosaccharide and sucrose content. The objective of this study was to identify quantitative trait loci (QTLs) that determine the oligosaccharide and sucrose content of soybean. The 117 F_2:10 recombinant inbred lines developed from a cross of “Keunolkong” and “Shinpaldalkong” were used. Narrow-sense heritability estimates, on a plot mean basis, of oligosaccharide and sucrose content were 79.07 and 74.84%, respectively. Four QTLs for oligosaccharide content were located on linkage groups (LG) C2, H, J, and L. Sucrose content was related with two QTLs located on LG H and J. Total oligosaccharide and sucrose content have two common QTLs on LG H and J.     

Cytological studies of triploids and their progeny from male-sterile (ms1) soybean

Summary Triploids (2n=3X=60) were obtained from genetic male-sterile (ms1 ms1) soybean [Glycine max (L.) Merr.] plants. Meiosis, pollen fertility, and chromosome number of their progeny were studied. Studies of meiosis in fertile and sterile triploids revealed no distinguishable differences in chromosome associations. Male-sterile plants formed coenocytic microspores characteristic of the ms1 mutant. Restitution of some dyad and tetrad nuclei were observed in male-sterile plants. Chromosomes of the triploids tended to occur in trivalents during diakinesis and metaphase I (MI), but multivalents, bivalents, and univalents also were observed. Average types and frequencies of chromosome associations per cell in diakinesis and MI from 542 pollen mother cells were 0.004 IX + 0.06 VI + 0.002 V + 0.005 IV + 16.99 III + 1.79 II + 5.03 I. Some secondary associations, nonhomologous pairing, and aberrant nucleolar distributions occasionally were observed. Such behavior support the hypothesis of duplicated genomes and the polyploid origin of soybean. Pollen fertility in male-fertile triploid plants (Ms1 ms1 ms1) varied from 57% to 82%, with an average of about 71%. Chromosome numbers of progenies obtained from these fertile triploids varied from 2n=40 to 2n=71, and exhibited a near-random distribution, with the majority (about 60%) being between 56 and 65. Progenies of the fertile triploids gave segregation ratios for the ms1 allele, which confirmed the Ms1 ms1 ms1 genotype.Joint contribution: Agricultural Research Service, U.S. Department of Agriculture, and Journal Paper No. J-11672 of the Iowa Agriculture and Home Economics Experiment Station, Ames, IA 50011, USA, Project 2471     

Inheritance of two independent isozyme variants in soybean plants derived from tissue culture

Summary Soybean [Glycine max (L.) Merr.] plants were regenerated via somatic embryogenesis from nine soybean cultivars. Our objective was to identify and characterize genetically novel mutations that would further our understanding of the soybean genome. Variant isozyme patterns were observed in two independent tissue culturederived lines. Genetic analyses were conducted on these two isozyme variants, and they were heritable. No variant isozyme patterns were evident in control (parental) soybean lines. In the cultivar BSR 101, a mutation of Aco2-b (aconitase) to a null allele was detected. The Aco2-bn mutant, Genetic Type T318, had not been previously observed in soybean. In the Chinese cultivar Jilin 3 (PI 427.099), a chlorophyll-deficient plant was identified that also lacked two mitochondrial malate-dehydrogenase (Mdh null) isozyme bands. These two mutant phenotypes, chlorophyll-deficient and Mdh null, were found to cosegregate. The Jilin 3 mutant, Mdh1-n (Ames 1) y20 (Ames 1) Genetic Type T317, was allelic to three chlorophyll-deficient, Mdh1 null mutants [Mdh1-n (Ames 2) y20 (Ames 2) (T323), Mdh1-n (Ames 3) y20 (Ames 3) (T324), and Mdh1-n (Ames 4) y20 (Ames 4) (T325)] previously identified from a transposon-containing soybean population, and to a chlorophyll-deficient, Mdh1 null mutant [Mdh1-n (Urbana) y20 (Urbana) k2, Genetic Type T253] which occurred spontaneously in soybean. The recovery of two isozyme variants from progeny of 185 soybean plants regenerated from somatic embryogenesis indicates the feasibility of selection for molecular variants.     

Evaluation of soybean resistance to Phialophora gregata culture filtrate in tissue culture

Summary Resistance to the fungal pathogen, Phialophora gregata (Allington and Chamberlain) W. Gams, the cause of brown stem rot (BSR) in soybean [Glycine max (L.) Merr.], is an important trait for cultivars grown in the northern USA. A novel tissue culture method was developed where ten soybean cultivars were differentiated on the ability of their excised cotyledons to remain green and initiate callus in a tissue culture medium containing P. gregata culture filtrate. Cultivar BSR classifications by the cotyledon method corresponded to greenhouse root-dip assay classifications in 80%, 100%, and 90% of the three P. gregata isolate treatments. Another method, employing pieces of somatic callus exposed to the culture filtrate, had a 70% average correspondence to the greenhouse results. Physiologic specialization was demonstrated in parallel in vivo/in vitro assays for the first time. These data suggest that the cotyledon method would accurately identify soybean lines resistant to certain aberrant or wild-type P. gregata isolates.     

Genetic variation for quantitative traits in soybean lines derived from tissue culture

Tissue culture may generate useful genetic variation for quantitative traits. The objective of this study was to analyze genetic variation for ten quantitative traits of soybean [Glycine max (L.) Merr.] among lines derived from the tissue culture of three cultivars. The three cultivars used to obtain R0 plants from tissue culture were BSR 101, Hodgson 78, and Jilin 3. A total of 63 R0-derived lines of BSR 101, eight of Hodgson 78, and 42 of Jilin 3 was planted with the untreated controls in row plots in a randomized complete-block design with three replications at two locations for each of 2 years. The traits evaluated were days to beginning bloom (R1), beginning seed (R5), beginning maturity (R7), full maturity (R8), height, lodging, seed yield, seed weight, protein content, and oil content. Significant (P < 0.05) variation was observed among lines for each of the ten quantitative traits. There was 57.1% of the BSR 101 lines, 87.5% of the Hodgson 78 lines, and 76.2% of the Jilin 3 lines that were significantly different from the controls for at least one trait. The percentages of lines that were significantly different from the control for an individual trait ranged from 2.7% for oil content to 25.7% for R7. The magnitude of the changes was relatively small. Although this genetic variation may be useful for cultivar development, greater variability at less expense would be expected with conventional artificial hybridization.Journal Paper No. J-14958 of the Iowa Agriculture and Home Economics Experiment Station, Ames, IOWA, USA Project No. 2475.     

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.