The direct response of the external mycelium of arbuscular mycorrhizal fungi to temperature and the implications for nutrient transfer

In this study we investigated the direct effects of temperature on the extra-radical mycelium (ERM) of arbuscular mycorrhizal fungi (AMF) and the resulting impact on the host plant nutrition and biomass production. Plantago lanceolata L. plants colonized by Glomus hoi (experiment 1) and either G. hoi or Glomus intraradices (experiment 2) were grown in compartmented microcosm units. AMF hyphae, but not roots, were permitted access to a second compartment containing a 15N:13C dual-labelled organic patch maintained at different temperature treatments. All plants were maintained at ambient temperature. AMF hyphal growth in the patch compartments was relatively insensitive to temperate but results were variable. G. hoi hyphal length density was 5 times higher at ambient (c. 24 °C) than cooled (c. 11 °C) temperatures but only at the end of the first experiment (105 d after patch addition). In contrast, in the second experiment (86 d after patch addition) AMF hyphal growth was unaffected by temperature in the patch compartment. These differences between experiments are likely due to large variation among replicates in the ERM produced and differences in how the organic patch was applied. In experiment 2, plant biomass and phosphate content differed according to the temperature at which the hyphae of both AMF species grew. Plant biomass was greater when the AMF were grown at c. 18 °C than c. 11 °C but was no different at c. 21 °C. These data show that direct temperature responses by the external hyphae of AMF can independently influence associated host plant growth. However, there were also important differences between the two AMF studied both in the amount of nutrients transferred and the distribution of the nutrients.