Colonization and growth promotion of outplanted spruce seedlings pre-inoculated with plant growth-promoting rhizobacteria in the greenhouse

Seeds of two hybrid spruce (Picea glauca (Moench) Voss x Picea engelmannii Parry ex Engelm.) ecotypes were inoculated with one of six plant growth-pro moting rhizobacteria (PGPR) strains previously shown to be able to stimulat e spruce growth in controlled environments. The resulting seedlings were gr own in the greenhouse for 17 weeks before outplanting at four reforestation sites. Inoculation with five of the six strains caused significant seedlin g growth promotion in the greenhouse, which necessitated analysis of relati ve growth rates (RGR) to evaluate seedling performance in the field. Four m onths after outplanting, most strains enhanced spruce shoot or root RGRs in the field, but seedling growth responses were strain specific. For example , Pseudomonas strain Ss2-RN significantly increased both shoot and root RGR s by 10-234% at all sites, but increases of 28-70% were most common. In con trast, Bacillus strain S20-R was ineffective at all outplanting sites. In a ddition, seedlings inoculated with four of the six strains had significantl y less shoot injury than control seedlings at all sites. Evaluation of root colonization by PGPR indicated that bacterial population declines were not related to spruce growth response variability in the field. Our results in dicate that once plant growth promotion is induced in the greenhouse, seedl ing RGR can increase by more than 100% during the first growing season in t he field. However RGR increases of 21-47% were more common and may be more representative of the magnitude of biomass increases that can result from P GPR inoculation.