Variable effects of emergence-promoting rhizobacteria on conifer seedling growth under nursery conditions

Summary Inoculation of white spruce (Picea glauca Voss.) seed with Bacillus polymyxa strain L5 under nursery conditions significantly increased the number of seedlings that emerged after sowing. No significant effects on seedling emergence were detected when white spruce seed was inoculated with Bacillus polymyxa strain L6, or when Douglas-fir Pseudotsuga menziesii (Mirb.) Franco] seed was inoculated with either strain L5 or L6. However, white spruce seedlings originating from L5-inoculated seed had significantly lower root dry weights when measured 13 weeks after sowing, and reduced shoot dry weights 26 weeks after sowing compared with uninoculated controls. Inoculation of white spruce seed with strain L6 also resulted in seedlings with decreased root dry weights compared with uninoculated controls 13 weeks after sowing, but the significant inhibition of root growth was not apparent 26 weeks after sowing. Douglasfir seedlings originating from L5-inoculated seed had significantly lower root and shoot dry weights compared with uninoculated controls 13 but not 26 weeks after sowing. Inoculation of Douglas-fir seed with strain L6 resulted in seedlings with decreased root collar diameters and shoot dry weights 13 weeks after sowing, and lower root dry weights 26 weeks after sowing compared with uninoculated controls. These results demonstrate that the effects of bacterial inoculation on seedling emergence and on plant growth are independent, and that emergence-stimulating bacteria may inhibit subsequent seedling growth.