| Legumes form symbioses with arbuscular mycorrhizal fungi (AMF) and rhizobia. The presence of one microbial symbiont has been recognized to affect the activity of the other, and the interaction of both can be detected through effects on the host plant. Such an interaction forms the so-called tripartite symbiosis, for which beneficial effects on nodulation and N 2-fixation have been assumed to rely completely on increased supply of phosphorus to the plant through the mycorrhizas. After providing a working model for the establishment of the tripartite symbiosis, the main objective of this thesis was to investigate whether the process is regulated by phosphorus. A multi-year field experiment was conducted with soybean (Glycine max (L.) Merr.), and indigenous AMF treatments were produced by either disrupting or leaving intact the extra-radical mycelium while the potential of bradyrhizobia (Bradyrhizobium japonicum (Kirchner) Jordan) was kept constant. The results indicated that even though nodulation was enhanced when the mycorrhizal colonization was elevated at 10 days after emergence, N2 fixation was not affected at later periods of growth. Nevertheless, the P levels in soil and all plant parts, as well as the soil acid and alkaline phosphatase activity were similar irrespective of AMF treatment, showing that the establishment of the tripartite symbiosis was not regulated by P. Therefore, communication between the symbionts was clearly the next factor to be tested and, for the remainder of the thesis, the objective was to determine which flavonoids play a role in the establishment of the tripartite symbiosis. Patterns of root accumulation of specific flavonoids and the extent to which early effects of the tripartite symbiosis would have at later periods of growth were evaluated over three experiments under controlled environmental conditions. The root accumulation of the flavonoids daidzein, genistein and coumestrol diminished in response to the symbionts and N2 fixation was enhanced by the tripartite symbiosis. Furthermore, AMF with different colonization strategies had different impacts on the early development of the tripartite symbiosis but N2 fixation was identical irrespective of the fungus present. |
