Graduate Thesis Or Dissertation
 

Microbial changes and interactions in the rhizosphere of vesicular-arbuscular mycorrhizal plants

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  • The influence of vesicular-arbuscular mycorrhizae (VAM) on populations of general taxonomic and functional groups of naturally-occurring rhizosphere bacteria and actinomycetes associated with roots of sweet corn (Zea mays var. rugosa) and subterranean clover (Trifolium subterraneum L.) was assayed using selective media. Sporangial production by Phytophthora cinnamomi was also used as a bioassay to detect qualitative changes in rhizosphere soil microorganisms stimulatory or inhibitory to sporangium production. Establishment of VAM increased total bacterial populations and numbers of fluorescent pseudomonads on the root surface (rhizoplane) of sweet corn and clover, but did not affect numbers of actinomycetes. Total numbers of bacteria and actinomycetes in the rhizosphere soil was not affected by VAM, but the presence of VAM did affect populations of specific groups of bacteria and actinomycetes in both the rhizosphere and rhizoplane. Facultative anaerobes increased, fluorescent pseudomonads, Streptomyces spp and chitinase-producing microorganisms all decreased in the VAM rhizosphere. Fewer sporangia were formed by Phytophthora cinnamomi in leachates from VAM rhizosphere soil than in leachates from nonmycorrhizal root rhizosphere soil, and fewer zoospores were released. The data suggest that rhizosphere microbial populations associated with VAM are qualitatively different from those associated with normycorrhizal roots. The significance of changes in microbial populations or activities due to VAM are discussed. In addition, plant growth and nodulation of subterranean clover were studied in nonsterile soil inoculated with a plant growth-promoting rhizobacterium (PGPR) isolate of Pseudomonas putida and indigenous VA mycorrhizal fungi. Although inoculation with either PGPR or VAM alone increased plant growth after 12 weeks, a significant increase in root dry weight, compared to uninoculated controls, was observed only when both PGPR and VAM were present. Shoot dry weight of plants inoculated with both PGPR and VAM was significantly greater than with PGPR alone, VAM alone or uninoculated controls. Nodulation was enhanced significantly by either PGPR or VAM alone (50% increase over controls at 12 weeks), but was significantly greater (103% increase over controls) when both PGPR and VAM were present. Inoculation with PGPR increased VAM infection from 7% to 23% of the root system infected at 6 weeks, but VAM infection levels were similar (ca 50%) at 12 weeks. Populations of PGPR increased similarly in the rhizosphere of both mycorrhizal and nonmycorrhizal plants. Concentrations (ppm) of Fe, Cu, Al, Zn, Co and Ni were considerably greater in the shoots of plants inoculated with both PGPR and VAM than in plants inoculated with either PGPR or VAM alone. Possible mechanisms for these interactions are discussed.
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