• We conducted an experiment to test whether phosphorus (P) uptake by mycorrhizal hyphae could be enhanced by growing the host plant under [CO2] enrichment and whether any response to [CO2] was dependent on C source-sink relationships.
• Plant C assimilation, mass allocation, growth and P uptake were measured in pea (Pisum sativum) plants inoculated with 0, 1 or 5% of a mixture of three Glomus spp. Intra- and extra-radical mycorrhizal development was followed and hyphal 33P uptake from a root-exclusion compartment was measured.
• Total P and 33P content measurements indicated that root, not hyphal, P uptake was increased by elevated [CO2] in the mycorrhizal treatments and that hyphal P uptake was actually reduced by elevated [CO2] after 57 d. Neither intra- nor extraradical mycorrhizal development was related to this response.
• Plant and fungal measurements suggested positive interactions in plant growth and P uptake only when C source-sink relationships were balanced; high C source (enhanced assimilation at elevated [CO2]) and high C sink (increasing mycorrhizal development). The results also indicated that enhanced plant C supply does not alter growth or function of arbuscular mycorrhizal fungi.