Many mycorrhizosphere microorganisms can produce plant growth regulating compounds in vitro. Plant growth regulating compounds are known to mediate root growth and development. In reforestation practices, conifer tree seedling growth and survival are linked to rapid root growth and development after outplanting. Multifactorial experiments were conducted to document variations in conifer seedling growth, survival and endogenous root indole acetic acid (IAA) content as influenced by exogenously applied plant growth regulators and inoculation with ectomycorrhizal fungi capable of differential in vitro IAA and ethylene production. Capacity for in vitro IAA and ethylene production by mycorrhizal fungal isolates was partially correlated to levels of endogenous root IAA and root growth in inoculated seedlings as well as field transplanted nursery-grown seedlings. Although degree of correlation usually varied with fungal isolate as well as conifer species, IAA-mediated changes in root growth and seedling survival could indirectly be attributed to in vitro fungal plant growth regulator production capability. Exogenous application of plant growth regulating substances at different times during seedling growth and development resulted in significant changes in growth, survival and endogenous root IAA content of both experimental seedlings and field transplanted nursery-grown seedlings. Although effects varied with conifer species and stock types, IAA mediated increases in root growth and survival are projected to be cost beneficial. These results are not only indicative of the complexity of interactions which can occur between symbionts in the mycorrhizosphere but also suggest potential methods for manipulating plant growth regulator mediation of conifer root growth for practical purposes in reforestation. / Graduation date: 1994