UBC Theses and Dissertations
Plant/bacteria coadaptation in a grass/legume pasture Chanway, Christopher Peter
The relationship between plants and rhizosphere bacteria collected from a 45 year old permanent pasture was investigated. Several methods of strain identification within Rhizobium trifolii were evaluated. Separation of bacterial isolates based on differences in intrinsic antibiotic resistance was not appropriate because strains developed hybrid resistance patterns when grown in a common broth. Serological analyses of bacterial antigens using polyclonal antiserum yielded two reliable methods for identifying R. trifolii isolates. Agglutination and immunofluorescence procedures were not useful in distinguishing these strains but immunodiffusion and the enzyme-linked immunosorbent assay (ELISA) were highly suitable. Adaptation of the ELISA allowed isolates to be identified directly from individual root nodules without first subculturing the bacteria. A strain of Bacillus polymyxa isolated from the same pasture was shown to stimulate growth of crested wheatgrass (Agropyron cristatum L.) and white clover (Trifolium repens L.). The primary manifestation of the effect was increased root weight (P < 0.05), but shoot responses were also observed. Perennial ryegrass (Lolium perenne L.) generally reacted negatively to inoculation with this bacterium. Further stimulation of growth was noted when ramets of the white clover genotype homologous to (sharing a common origin) B. polymyxa were inoculated in pure stands (P < 0.05). Clones of the homologous perennial ryegrass genotype also showed a yield increase from slightly below control levels to slightly above them when tested in a similar manner. Detailed analysis of the crested wheatgrass response to inoculation revealed that bacterial production of indole acetic acid was the most likely cause of the growth stimulation. Other bacterial characteristics such as the ability to fix atmospheric nitrogen or to solubilize organic phosphorus were concluded to be unrelated to the growth response. Co-adaptive compatibility between genotypes of L. perenne and T. repens was not apparent when the effect of R. trifolii was ignored. However, when clones of pasture plants that had been neighbours in the field were inoculated with R. trifolii isolated from root nodules of the "parental" clover genotype, biotic specialization between the pasture plants became evident. The magnitude of the effect, which was characterized by superior white clover yields (P < 0.05), could be largely accounted for by the presence of the adapted L. perenne/R. trifolii combinations, regardless of the white clover genotype. Since T. repens was the dominant component in the species mixture, these trends were also apparent when total forage biomass was analyzed (P < 0.05). However, ecological combining ability was found to be lowest in these associations (P < 0.05). Similar experimentation with isolates of B. polymyxa (or B. polymyxa-like organisms) was performed. Again the grass/bacteria combination was shown to be influential in the growth response as the presence of homologous L. perenne/B. polymyxa combinations resulted in superior white clover and perennial ryegrass performance (P < 0.05). When T. repens was inoculated with a mixture of R. trifolii strains, unrelated isolates formed more root nodules than did homologous ones (P < 0.05). The presence of perennial ryegrass did not mitigate this effect. However, when homologous R. trifolii was administered as a single strain inoculum, yield advantages in white clover were observed (P < 0.05). If B. pol ymyxa was present, homologous strains of R. trifolii tended to form most of the root nodules regardless of the T. repens or L. perenne genotypes. The significance of the yield advantages observed in various two and three-way plant/microbe genotype combinations is discussed with respect to above ground plant performance.
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