The interaction between Frankia and ectomycorrhizae on a shared Alnus rubra host

Most studies examining tripartite mutualisms examine whether adding a second endosymbiont increases host plant benefit. My thesis uses two experiments to examine the relationship between Frankia and ectomycorrhizae on a shared Alnus rubra host. The first involved manipulating both mutualisms with fertilisers of varying phosphorus and nitrogen concentrations. Alnus rubra plants were inoculated with Frankia, Paxillus involutus, or both Frankia and P. involutus and fertilised with one of five fertilisers. Increasing nitrogen concentrations had a positive effect on growth while increasing phosphorus concentrations had no effect. Frankia increased plant growth while mycorrhizae had no effect on plant growth due to a lack of mycorrhizal development. There was a non-significant trend for mycorrhizae to increase the specific nitrogenase activity of the nodules; however, this trend did not translate into an increase in plant growth. It is possible that either the Frankia or the mycorrhizae used the nitrogen. In my experiment, P. involutus has a beneficial effect on Frankia but the effect of Frankia on P. involutus is still unclear.; The second experiment was designed to examine the interaction between the two endosymbionts on endosymbiont colonisation. Alnus rubra plants were inoculated with various combinations of spore positive Frankia, spore negative Frankia, Paxillus involutus, and Hebeloma crustuliniforme. (Abstract shortened by UMI.)...