RHIZOBIUM COMPETITION IN ANTIGUAN SOILS: STRAIN ISOLATION

The enzyme-linked immunosorbent assay was used to determine the competitive ability of three Rhizobium strains introduced into Antiguan soil. Strain-specific antisera were prepared against each strain. Field experiments were conducted in Antigua using Rhizobium strains USDA 3384, USDA 3473, and USDA 3474 as a peat-base inoculant and pigeon pea as the test crop. Nodules from the respective treatments were removed and prepared for ELISA studies. There was cross reactivity between the antisera, but it was greatly reduced or eliminated by repeat adsorption with the cells of the cross-reacting strains. Nodule occupancy by plants treated with Rhizobium 3384, 3473, and 3384 was 70%, 90%, and 100%, respectively. Nodules from 3384 and 3474 treated plants contained cells with no antigenic homology to the three antisera. We concluded that these nodules were developed from indigenous Rhizobium strains found in Antiguan soils.

Host plant effect on competition among strains of Rhizobium leguminosarum

Analysis of plasmid profiles was used to type Rhizobium leguminosarum biovar viciae strains isolated from nodules of peas, lentils and faba beans grown in two different soils. One soil was from a native pasture with no previous history of cultivation, the other was from a plot in a rotation study which included lentils every 2 years. The results indicated a strong preference of both peas and faba beans for strains having certain specific plasmid profiles. Strains belonging to one plasmid profile group (group 2) formed over half the nodules on peas grown in soil from the rotation plot but were never found on faba beans grown in the same soil, while strains from another group (group 5) formed nearly all of the nodules on faba beans grown in soil from the rotation plot, but no nodules on peas. Competitiveness for pea nodulation was correlated with an ability to catabolize homoserine, an amino acid found in large quantities in pea root exudate. Strains having plasmid profiles corresponding to those of strains that have been used in commercial inoculants over the last few years were isolated only rarely, regardless of the soil and host plant studied. Key words: Rhizobium, competition, plasmids, legumes, nodulation.

Populations of Rhizobium leguminosarum biovars phaseoli and viceae in fields after bean or pea in rotation with nonlegumes

Populations of Rhizobium leguminosarum bv. phaesoli and bv. viceae in southern Alberta soils were measured over a period of 4 years using a most probable number method. Five fields cropped to bean (Phaseolus vulgaris L.), five fields cropped to pea (Pisum sativum L.), and two fields cropped to wheat were used as test sites. Legume crops had received appropriate legume inoculants. Fields were sampled in the fall of the crop year and in the spring of the following 3 years during which fields were cropped to nonlegumes or left fallow. Numbers of R. leguminosarum bv. phaseoli were 100 to 1000 times higher in fields that had been planted to bean than in fields that had been planted to pea or wheat. Fields that had been planted to pea maintained populations of R. leguminosarum bv. viceae 10 to 100 times higher than fields that had been planted to bean or wheat. Wheat fields, which had never had legumes grown in them, contained between 1 and 100 rhizobia per gram of soil of both biovars of R. leguminosarum, indicating that both biovars are native to southern Alberta soils. The numbers of rhizobia did not decrease in proportion to the population of other bacteria in the soil over the duration of the experiment. Plasmid profiles of soil Rhizobium isolates obtained in the last year of the experiment showed that none of the isolates had plasmid profiles similar to those of strains added as inoculants in the 1st year of the experiment. These results show that fields cropped to legumes and receiving rhizobial inoculants in this study maintained high populations of rhizobia for several years after harvest of the legume crop.Key words: Rhizobium leguminosarum bv. phaseoli, Rhizobium leguminosarum bv. viceae, nodule, plasmid profiles, inoculum potential, rhizobium competition.

Dynamics of Rhizobium competition for nodulation of Pisum sativum cv. Afghanistan

Nodulation of Pisum sativum cv. Afghanistan by Rhizobium leguminosarum strain Tom can be blocked by the nonnodulating R. leguminosarum strain PF2. This system was used to study the dynamics of rhizobial binding to seedling root surfaces. Appreciable numbers of 35S-labeled rhizobia bound to the root surface within 2 h after inoculation. PF2 colonized the root surfaces earlier and in greater numbers that did Tom. Radioactive extracellular polysaccharides and lipopolysaccharides from either organism failed to bind to the seedling roots in measurable quantities.

MINIMAL ANTIGENIC CONSTITUTION OF 28 STRAINS OF RHIZOBIUM JAPONICUM

Somatic antigen groups and minimal somatic antigen constitution of 28 strains of Rhizobium japonicum were determined on the basis of their somatic cross-reactions and agglutinin absorptions. A total of 17 serological groups and 24 antigens was proposed to account for the reactions obtained. Some groups consisted of a complex of three or four strains while others comprised only one strain. Still others were composed of strains which were common to two serological groups. The antigenic complexity of rhizobial cells and the possibility of using "key-antisera" and polyvalent mixtures of "key-antisera" are discussed in relation to the classification of large numbers of field or greenhouse nodule isolates in Rhizobium competition studies.