Plantago lanceolata L. and Trifolium repens L.
were
grown for 16 wk in ambient (360 μmol mol−1) and elevated
(610 μmol mol−1) atmospheric CO2.
Plants were inoculated with the arbuscular mycorrhizal (AM) fungus Glomus
mosseae (Nicol. & Gerd.) Gerdemann & Trappe and given
a phosphorus supply in the form of bonemeal, which
would not be immediately available to the plants. Seven sequential
harvests were taken to determine whether the
effect of elevated CO2 on mycorrhizal colonization was
independent of the effect of CO2 on plant growth. Plant
growth analysis showed that both species grew faster in elevated CO2
and that P. lanceolata had increased carbon
allocation towards the roots. Elevated CO2 did not affect the
percentage of root length colonized (RLC); although
total colonized root length was greater, when plant size was taken into
account this effect disappeared. This finding
was also true for root length colonized by arbuscules. No CO2
effect was found on hyphal density (colonization
intensity) in roots. The P content of plants was increased at elevated
CO2, although both shoot and root tissue P
concentration were unchanged. This was again as a result of bigger
plants at elevated CO2. Phosphorus inflow was
unaffected by CO2 concentrations. It is concluded that there
is no direct permanent effect of elevated CO2 on
mycorrhizal functioning, as internal mycorrhizal development
and the mycorrhizal P uptake mechanism are
unaffected. The importance of sequential harvests in experiments is
discussed. The direction for future research
is highlighted, especially in relation to C storage in the soil.