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UBC Theses and Dissertations

Plant growth promoting rhizobacteria (PGPR) for interior spruce (Picea engelmannii x P. glauca) seedlings Shishido, Masahiro


The relationship between interior spruce (Picea engelmannii Parry x P. glauca (Moench) Voss) and plant growth promoting rhizobacteria (PGPR) was studied under controlled environments and in the field. Large, statistically significant biomass increases were detected in spruce seedlings after PGPR inoculation, but seedling growth responses were variable. Coadaptation involving host plants, PGPR and forest floor soils did not appear to explain such variability. Synergistic effects of PGPR and mycorrhizal fungi on seedling growth were detected, but growth promotion also occurred in the absence of mycorrhizae, which suggests that bacteria x mycorrbizae interactions were unrelated to seedling growth response variability. PGPR colonization of seedling tissues was assessed using immunofluorescent microscopy and dilution plate counts. When three Bacillus and three Pseudomonas strains were inoculated onto spruce seedling roots under gnotobiotic conditions, only Bacillus Pw2R and Pseudomonas Sm3RN were detected inside stem vascular tissues four months later. A field experiment was performed to evaluate differences in root colonization and seedling growth promotion between these endophytic and non-endophytic PGPR. Relative growth rates of spruce seedlings in the field suggested that once induced in the greenhouse, seedling growth promotion persisted under field conditions for at least four months. However, endophytic PGPR offered no apparent advantage over non-endophytes as growth promoters. Mechanistic studies of plant growth promotion in sterile microcosms suggested that strains L6-16R, Pw2R, S20R, Sm3RN and Sw5RN did not depend on the presence of other deleterious microorganisms to promote seedling growth, but that strain Ss2RN might. However, addition of sterilized forest soil extracts facilitated seedling growth promotion by most PGPR. These results suggest that abiotic soil compounds may act as precursor substrates for PGPR production of plant growth stimulating substances, possibly, but not restricted to, phytohormones. Patchy distribution of such precursors in soil could cause seedling growth variability in response to PGPR inoculation. Finally, PGPR were observed to change soil microbial community population sizes and carbon substrate utilization patterns. Seedlings buffered these effects in only one of two forest soils evaluated, indicating that the origin of the soil microbial community is important in determining microfloral responses to PGPR introduction.

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