Loblolly pine seed were inoculated at sowing with one of four plant growth-promoting rhizobacteria (PGPR) strains at four different rates both in the nursery and in the greenhouse. Seedlings were harvested at 4, 8, and 12 wk after sowing in the greenhouse and at 5, 10, and 15 wk after sowing in the nursery. Ectomycorrhizae colonization, estimated as the total number ectomycorrhizal roots per seedling and averaging by gram root biomass, were not evident on any treatment at either 5 wk in the nursery or 4 wk in the greenhouse. However, by 8 wk after sowing, many treatments significantly (P < 0.0001) inhibited ectomycorrhizal root formation. Generally, increasing the rate of all bacteria applied in the greenhouse decreased ectomycorrhizal root formation. Conversely, strain LS212 at 1011 cfu resulted in an increase in ectomycorrhizal roots in the nursery. Increases in seedling root growth were also dependent on bacterial strain and rate, whereas shoot biomass was not effected. Decreases in ectomycorrhizae colonization accounted for 61% of the variation in increases in root biomass, suggesting high metabolic costs of mycorrhizal maintenance in the presence of some rhizobacteria. However, differences in shoot height and survival after outplanting could not be attributed to the mycorrhizae colonization effect. These studies demonstrate that mycorrhizae may serve as a carbon sink early in seedling development and indicate a complex interaction between soil microflora and bacterium in nursery and greenhouse settings. FOR. SCI. 46(3): 437–441.
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