Effect of Plant Growth PromotingBacillusStrains on Pine and Spruce Seedling Growth and Mycorrhizal Infection

Rifamycin-resistant derivatives of plant growth promoting Bacillus polymyxa strains L6, Pw-2, and S20 were used to evaluate the interaction of bacterial–mycorrhizal co-inoculation on pine and spruce seedling growth. We were particularly interested in determining if the mechanism by which bacteria stimulated seedling growth depended on the presence of ectomycorrhizae. Mycorrhizal inoculum was introduced by adding 2ml of one of six forest floor soil types originating from different spruce and pine stands to seedling containers. Mycorrhizal roots developed in 34% of pine and 27% of spruce seedlings treated with forest soil, but no differences between forest soils were detected. Most mycorrhizae were formed by Wilcoxina sp. (E-strain) (98% for spruce and 67% for pine); small numbers of Amphinema -like, Mycelium radicis atrovirens, Suillus -like, Thelephora -like, and Tuber -like mycorrhizae were also found on pine (27% in total). Thelephora- like fungi comprised 2% of spruce mycorrhize. In the absence of bacterial inoculum, spruce seedling biomass was positively correlated with the number of mycorrhizal root tips, but this trend was not detected in spruce inoculated with bacteria or in pine. Bacterial inoculation did not influence the mycorrhizal status of seedlings, but all three Bacillus strains stimulated growth of both conifer species. Root biomass, in particular, was significantly enhanced by up to 18% compared with uninoculated controls. Mycorrhizal fungi improved the growth of spruce seedlings, but plant growth promotion by Bacillus was similar for mycorrizal and non-mycorrhizal seedlings of both species. Our results suggest that Bacillus strains L6-16R, Pw-2R, and S20-R enhance conifer seedling growth through a mechanism unrelated to mycorrhizal fungi.