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The effect of other rhizosphere microorganisms on the ability of Paenibacillus spp. to promote the growth of lodgepole pine (Pinus contorta var. latifolia (Dougl. engelm.)) Bent, Elizabeth

Abstract

The interaction between lodgepole pine and plant growth-promoting rhizobacteria (PGPR) strains Paenibacillus polymyxa L6 and Pw-2 was examined. Experiments were conducted to determine whether the extent of PGPR-mediated growth promotion seen under gnotobiotic conditions is influenced by the presence of co-inoculated rhizobacteria (Curtobacterium flaccumfaciens PF322 or Pseudomonas fluorescens M20) or a co-inoculated ectomycorrhizal fungus (Rhizopogon subcularescens Smith 11616), whether a change in the extent of PGPR-mediated plant growth promotion is related to (1) a quantitative change in the extent of PGPR population density on or within the root, (2) a qualitative change in PGPR colonization of the root surface (determined using immunofluorescence techniques and confocal laser scanning microscopy), (3) a change in levels of auxins and/or cytokinins within root tissue, and finally whether P. polymyxa L6 and Pw-2 vary in their ability to form spores in the rhizosphere. It was concluded that the extent of P. polymyxa-mediated growth promotion of lodgepole pine can be reduced by the presence of a single rhizobacterial co-inoculant. The decreases in growth promotion were not related to reductions in rhizospheric population densities of PGPR, and were not linked to qualitative changes in the ability of PGPR to colonize specific root surface microsites. Rhizobacteria had strain-specific effects on root hormone levels, and the possibility that decreases in growth promotion are related to the effects of combinations of bacteria on root hormones should be investigated. Pw-2 was found to produce significantly more spores than L6 under the experimental conditions, and the possiblity that Pw-2, but not L6, was identifed as an endophyte on chemically surface-sterilized root tissues due to this fact should be considered. Initial attempts to identify Pw-2 colonization sites in the root interior via confocal microscopy were not successful, and further attempts should be made to verify the endophytic status of Pw-2. Co-inoculating seedlings with P. polymyxa and R. subcularescens did not result in additive growth effects on pines. However, further work is needed to determine if additive growth improvements from P. polymyxa and ectomycorrhizal fungus co-inoculation will occur under different growing conditions, and/or in the presence of different ectomycorrhizal fungi.

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