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

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 P-33 uptake from a root-exclusion compartment was measured. Total P and P-33 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 extra-radical 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. (C) New Phytologist (2002) 154: 751-760.