| Summary: | 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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