The importance of Frankia as a nitrogen fixer in soils, and the future of Frankia in the area of silviculture is of ecological significance. This proposal will focus on the saprophytic growth and the influence of the rhizosphere on metabolic activity of Frankia. A conceptual model distinguishing physiological classes of Frankia in soil will guide testing of the hypothesis that Frankia can grow saprophytically in soil. Methods that quantify changes in the Frankia population (biomass) as well as infective units in soil, will include use of DNA probes and restriction fragment length polymorphism (RFLP) analysis to identify and trace a Frankia strain inoculated at low levels into a non-sterile soil system. Polymerase chain reactions as a means to increase the sensitivity of such methods will be used. The experiment to test for Frankia saprophytic competence involves inoculation of non-sterile soil at low levels with a pure culture of a spore-negative type of Frankia in the alder compatability group. Changes in its population biomass (DNA content) and numbers of infective units (MPN bioassay) with time will be measured. Variables include rhizosphere vs. non- rhizosphere soil for three plant species: red alder, yellow birch and Douglas-fir, to test if rhizosphere soil of host or non-host plants promote saprophytic growth of Frankia. The role of spores as infective units will be examined by experiments to test whether rhizosphere stimulates spore germination.
