Foraging strategies of the external mycelium of the arbuscular mycorrhizal fungi Glomus intraradices and Scutellospora calospora

The responsiveness of the external mycelium of Glomus intraradices and Scutellospora calospora was tested in a multiple-choice experimental system in which mycelium encountered patches amended with nitrogen (N) or phosphorus (P), either alone or in combination with a host plant. We hypothesised that only AMF mycelium with sufficient supply of photosynthate from an actively growing host would respond to the amendments provided. Mycelium was allowed to grow either 11 or 21 weeks before we analysed hyphal proliferation in amended patches introduced in mesh bags that were not reached by roots but by foraging mycelium only. Hyphal length, the AMF signature fatty acid 16:1w5, and root colonisation in new host plant seedlings were used to measure AMF growth and resource allocation in the patches. Mycelium from both fungal strains was able to colonise new host roots and sand in all patches but S. calospora was overall more responsive to the amendments than G. intraradices. G. intraradices grew equally into all patches, including the unamended control, whereas S. calospora produced significantly more hyphal length in the patch containing a host plant than in the rest of the patches. Both strains showed lower hyphal growth at the second harvest and mycelium of G. intraradices lost almost entirely its capacity to develop new mycelium in all choices presented. Lipid measurements showed this fungus did not use storage lipids to exploit the patches. S. calospora mycelium had reduced growth and colonisation ability but still showed some growth in the patches at the second harvest. A reduction in the content of NLFA 16:1w5 from the first to the second harvest suggested that S. calospora mycelium likely used storage lipids to sustain proliferation in the patches. The results indicated that S. calospora was more active and used more resources for foraging than G. intraradices and that external mycelium foraging was maintained mainly with recently acquired plant carbon (C). This supported in general our hypothesis but showed as well that the two AMF strains had different strategies and resource allocation to forage. The overall low response of both AMF to the choices presented suggested that the responsiveness of mycelium searching freely in the substrate is lower than that observed in experimental systems in which the amendments have been placed in close contact with actively growing mycelium fronts in close vicinity with host roots.