Increased allocation to external hyphae of arbuscular mycorrhizal fungi under CO 2 enrichment

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Abstract
Prunella vulgaris was inoculated with different arbuscular mycorrhizal fungi (AMF) and grown at two concentrations of CO_2 (ambient, 350 μl l^−1, and elevated, 600 μl l^−1) to test whether a plants response to elevated CO_2 is dependent on the species of AMF colonizing the roots. Using compartments accessible only to AMF hyphae but not to roots, we also tested whether elevated CO_2 affects the growth of external AMF hyphae. Plant biomass was significantly greater at elevated than at ambient CO_2; the biomass of the root system, for example, increased by a factor of 2. The colonization of AMF inside the root remained constant, indicating that the total AMF inside the root system also increased by a factor of 2. The length of external AMF hyphae at elevated CO_2 was up to 5 times that at ambient CO_2, indicating that elevated CO_2 promoted allocation of AMF biomass to the external hyphae. The concentration and content of phosphorus in the stolons differed significantly between ambient and elevated CO_2 but this resulted in either an increase or a decrease, according to which AMF isolate occupied the roots. We hypothesized that an increase in external hyphal growth at elevated CO_2 would result in increased P acquistion by the plant. To test this we supplied phosphorus, in a compartment only accessible to AMF hyphae. Plants did not acquire more phosphorus at elevated CO_2 when phosphorus was added to this compartment. Large increases in AMF hyphal growth could, however, play a significant role in the movement of fixed carbon to the soil and increase soil aggregation.

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