Impact of temperature on the arbuscular mycorrhizal (AM) symbiosis: growth responses of the host plant and its AM fungal partner.

The growth response of the hyphae of mycorrhizal fungi has been determined, both when plant and fungus together and when only the fungus was exposed to a temperature change. Two host plant species, Plantago lanceolata and Holcus lanatus, were grown separately in pots inoculated with the mycorrhizal fungus Glomus mosseae at 20/18 degrees C (day/night); half of the pots were then transferred to 12/10 degrees C. Plant and fungal growth were determined at six sequential destructive harvests. A second experiment investigated the direct effect of temperature on the length of the extra-radical mycelium (ERM) of three mycorrhizal fungal species. Growth boxes were divided in two equal compartments by a 20 micro m mesh, allowing only the ERM and not roots to grow into a fungal compartment, which was either heated (+8 degrees C) or kept at ambient temperature. ERM length (LERM) was determined on five sampling dates. Growth of H. lanatus was little affected by temperature, whereas growth of P. lanceolata increased with temperature, and both specific leaf area (SLA) and specific root length (SRL) increased independently of plant size. Percentage of colonized root (LRC) and LERM were positively correlated with temperature when in symbiosis with P. lanceolata, but differences in LRC were a function of plant biomass. Colonization was very low in H. lanatus roots and there was no significant temperature effect. In the fungal compartment LERM increased over time and was greatest for Glomus mosseae. Heating the fungal compartment significantly increased LERM in two of the three species but did not affect LRC. However, it significantly increased SRL of roots in the plant compartment, suggesting that the fungus plays a regulatory role in the growth dynamics of the symbiosis. These temperature responses have implications for modelling carbon dynamics under global climate change.