Respiration of the external mycelium in the arbuscular mycorrhizal symbiosis shows strong dependence on recent photosynthates and acclimation to temperature

• Although arbuscular mycorrhizal (AM) fungi are a major pathway in the global carbon cycle, their basic biology and, in particular, their respiratory response to temperature remain obscure.

• A pulse label of the stable isotope 13C was applied to Plantago lanceolata, either uninoculated or inoculated with the AM fungus Glomus mosseae. The extra-radical mycelium (ERM) of the fungus was allowed to grow into a separate hyphal compartment excluding roots. We determined the carbon costs of the ERM and tested for a direct temperature effect on its respiration by measuring total carbon and the 13C:12C ratio of respired CO2. With a second pulse we tested for acclimation of ERM respiration after 2 wk of soil warming.

• Root colonization remained unchanged between the two pulses but warming the hyphal compartment increased ERM length. δ13C signals peaked within the first 10 h and were higher in mycorrhizal treatments. The concentration of CO2 in the gas samples fluctuated diurnally and was highest in the mycorrhizal treatments but was unaffected by temperature. Heating increased ERM respiration only after the first pulse and reduced specific ERM respiration rates after the second pulse; however, both pulses strongly depended on radiation flux.

• The results indicate a fast ERM acclimation to temperature, and that light is the key factor controlling carbon allocation to the fungus.