Increased allocation to external hyphae of arbuscular mycorrhizal fungi under CO2 enrichment

Prunella vulgaris is was inoculated with different arbuscular mycorrhizal f ungi (AMF) and grown at two concentrations of CO2 (ambient, 350 mu l l(-1), and elevated, 600 mu l l(-1)) to test whether a plants response to elevate d CO2 is dependent on the species of AMF colonizing the roots. Using compar tments accessible only to AMF hyphae but not to roots, we also tested wheth er elevated CO2 affects the growth of external AMF hyphae. Plant biomass wa s significantly greater at elevated than at ambient CO2; 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 AM F hyphae at elevated CO2 was up to 5 times that at ambient CO2, indicating that elevated CO2 promoted allocation of AMF biomass to the external hyphae . The concentration and content of phosphorus in the stolons differed signi ficantly between ambient and elevated CO2 but this resulted in either an in crease or a decrease, according to which AMF isolate occupied the roots. We hypothesized that an increase in external hyphal growth at elevated CO2 wo uld 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 CO2 when phosphorus was added to this compartment. Large increases in AMF hyphal growth could, however? play a si gnificant role in the movement of fixed carbon to the soil and increase soi l aggregation.