Differential effects of rhizobacterial strain and dose on the ectomycorrhizal colonization of loblolly pine seedlings

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 ectomy corrhizal roots per seedling and averaging by gram root biomass, were not e vident on any treatment at either 5 wk in the nursery or 4 wk in the greenh ouse. However, by 8 wk after sowing, many treatments significantly (P < 0.0 001) inhibited ectomycorrhizal root formation. Generally, increasing the ra te of all bacteria applied in the greenhouse decreased ectomycorrhizal root formation. Conversely, strain LS212 at 10(11) 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 co sts of mycorrhizal maintenance in the presence of some rhizobacteria. Howev er, differences in shoot height and survival after outplanting could not be attributed to the mycorrhizae colonization effect. These studies demonstra te that mycorrhizae may serve as a carbon sink early in seedling developmen t and indicate a complex interaction between soil microflora and bacterium in nursery and greenhouse settings.