Phosphorus uptake by arbuscular mycorrhizal hyphae does not increase when the host plant grows under atmospheric CO₂ enrichment

Summary • We conducted an experiment to test whether phosphorus (P) uptake by mycorrhizal hyphae could be enhanced by growing the host plant under [CO 2 ] enrichment and whether any response to [CO 2 ] 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 33 P uptake from a root‐exclusion compartment was measured. • Total P and 33 P content measurements indicated that root, not hyphal, P uptake was increased by elevated [CO 2 ] in the mycorrhizal treatments and that hyphal P uptake was actually reduced by elevated [CO 2 ] 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 [CO 2 ]) 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.