INOCULATION OF TREE ROOTS WITH PLANT-GROWTH PROMOTING SOIL BACTERIA -AN EMERGING TECHNOLOGY FOR REFORESTATION

Seedling root systems are colonized heavily by asymbiotic soil bacteri a, many of which have the potential to influence plant growth signific antly. A heterogeneous group of these microorganisms is well known for their ability to colonize roots and stimulate growth of agricultural plant species, sometimes doubling seedling biomass accumulation only a few weeks after inoculation, but more usually resulting in less spect acular biomass gains (e.g., 15%-30% greater than uninoculated controls within a growing season). Plant growth promoting soil bacteria may ex ert such effects through a variety of mechanisms, and include microorg anisms that stimulate seedling emergence or infection by symbiotic fun gi and bacteria. Other plant beneficial soil bacteria possess biologic al control activity or are capable of transforming plants genetically. Inoculation of tree seedlings with such bacteria before outplanting w ould be an inexpensive, environmentally benign, and easily applied nur sery treatment, but com paratively little work has been performed with these microorganisms in forestry. Recent results with various tree sp ecies, however, indicate that seedling performance can be significantl y enhanced through bacterial inoculation of root systems: pine and spr uce biomass increased 32%-49% 1 yr after inoculation and outplanting a t a reforestation site. In addition, infection by desired species of e ctomycorrhizal fungi can also be enhanced by inoculation with certain strains of root colonizing bacteria. Results from studies performed wi th beneficial asymbiotic tree root associated bacteria are reviewed in this article in relation to the possible uses of such microorganisms for artificial forest regeneration.