EFFECT OF PLANT-GROWTH PROMOTING BACILLUS STRAINS ON PINE AND SPRUCE SEEDLING GROWTH AND MYCORRHIZAL INFECTION

Rifamycin-resistant derivatives of plant growth promoting Bacillus pol ymyxa strains L6, Pw-2, and S20 were used to evaluate the interaction of bacterial-mycorrhizal co-inoculation on pine and spruce seedling gr owth. 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 2 mi of one of six forest floor soil types originating From different spru ce 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 mycorrhiza e were formed by Wilcoxina sp. (E-strain) (98% for spruce and 67% for pine); small numbers of Amphinema-like, Mycelium radicis atrovirens, S uillus-like, Thelephora-like, and Tuber-like mycorrhizae were also fou nd on pine (27% in total). Thelephora-like fungi comprised 2% of spruc e mycorrhize. In the absence of bacterial inoculum, spruce seedling bi omass was positively correlated with the number of mycorrhizal root li ps, but this trend was not detected in spruce inoculated with bacteria or in pine. Bacterial inoculation did not influence the mycorrhizal s tatus of seedlings, but all three Bacillus strains stimulated growth o f 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 promo tion by Bacillus was similar for mycorrizal and non-mycorrhizal seedli ngs 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. (C) 1996 Annals of Botany Company