Mutualism is the relationship between two species where both species benefit as a result of that relationship. However, partners may not benefit each other at all densities; at high densities, a partner may be more costly than beneficial to the other. In the mutualism between arbuscular mycorrhizal fungi and plants, plants give carbon in exchange for phosphorous and other nutrients. Mycorrhizal fungi are obligate mutualists in the interaction, depending on plants for all of their carbon. We investigated the mutualism between one species of plant, Plantago aristata, and one species of mycorrhizal fungus, Archaeospora trappei. In a greenhouse experiment, we inoculated soil with different densities of fungi and grew them with different densities of plants. After one generation, we collected and stained roots from the plants.

Results/Conclusions

We determined how the initial densities of plants affect production of fungi in the next generation. We also investigated whether the density of the fungi within roots changed with increasing density of fungi in the inoculum or whether the density of fungi was regulated within roots. Changes in density of fungi over two generations will help predict the outcome of the interaction over the long term.