SUMMARY: Phage types representative of the population of indigenous Rhizobium meliloti at each of two sites were evaluated for plasmid content by agarose gel electrophoresis and for symbiotic effectiveness with Medicago sativa cv. Saranac. Relative to four strains used commercially, 55 and 65 phage types representing these populations showed a high average level of symbiotic effectiveness; only a single type from one site was relatively ineffective in symbiosis. On the basis of plasmid number and molecular mass, 160 isolates comprising 45 and 48 types from both sites were placed in 22 different groups with 17 and 13 groups from the respective sites. The number of plasmids varied between one and five per isolate with molecular masses ranging from 5 MDa to considerably greater than 267 MDa. Only five isolates lacked a plasmid with mobility in agarose gels corresponding to that of a reference megaplasmid but instead showed a band of lesser mobility and therefore greater molecular mass. Phage types, which were divided into plasmid groups solely on the basis of differences between isolates from each site, may reflect adaptation of R. meliloti to their respective sites. Differences between isolates within certain phage types due to the presence or absence of a single plasmid may have resulted from genetic interchange between indigenous R. meliloti. There was no significant correlation between plasmid number or mass and symbiotic effectiveness or phage sensitivity of the phage types from either site.
SUMMARY: The ability of two strains of Rhizobium leguminosarum biovar trifolii (formerly R. trifolii) to compete for nodule sites in soil with indigenous R. l. biovar trifolii was studied using ELISA. In pot experiments with soil containing effective indigenous strains, one of the inoculated strains, 285, was able to form nodules on red clover (Trifolium pratense L.), not only on the tap root but also on later-formed roots. Under field conditions, also in a soil containing effective indigenous R. l. biovar trifolii strains, both inoculated strains were detectable in minor proportions in the nodules of the established red clover. The inoculated strains survived in the soil during the field season. As the indigenous strains were effective, no increase in dry matter production was obtained in this experiment. However, in pot experiments in a soil with ineffective indigenous red clover bacteria, the other inoculated strain, 7612, known to be an effective nitrogen-fixing strain, was less successful and was only detectable incidentally. Although the nodule occupancy of the successful competitor strain, 285, only ranged between 7% and 27% on the upper tap root and was merely occasional on the other parts of the root in the ‘ineffective’ soil, increases in dry matter production were obtained, indicating that the nodules on a specific plant have varying significance for the apparent nitrogen fixation.
SUMMARY: Horizontal movement of Azospirillum brasilense Cd in soil and its vertical movement in the plant rhizosphere were studied. No movement was detected in the absence of living plants. In a controlled environment the bacteria moved horizontally at least 30 cm from the inoculation point to the first growing plant. Once the first root system was colonized, all the neighbouring plants became inhabited. Horizontal movement under field conditions was at least 160 cm, and depended on the presence of live plant roots. Several weeds that grew in the passes between plots acted as efficient vectors. The numbers of A. brasilense Cd decreased with increasing distance from the inoculated plot. Vertical movement in soil columns in a controlled environment was up to 40 cm. Under field conditions, bacteria were detected as deep as 50 cm in the root systems of wheat plants in various types of soil. During the growing season bacteria were mostly found on and in young roots at a depth of 20–50 cm and near the soil surface. A map of depth distribution of A. brasilense Cd showed an uneven colonization pattern within the same root system or between adjacent plants. It was concluded that A. brasilense Cd moved horizontally and vertically in various soil types and that this movement was mainly dependent on the presence of plants.