What was done
The authors grew Plantago lanceolata with and without inoculation by the arbuscular mycorrhizal fungus Glomus mosseae for 100 days in open-top chambers receiving atmospheric CO2 concentrations of 380 and 600 ppm to study the effects of elevated CO2 and mycorrhizal colonization on photosynthesis in this species.

What was learned
Both elevated CO2 and mycorrhizal colonization stimulated light-saturated rates of net photosynthesis, with CO2 having greater influence than mycorrhizal colonization.  Elevated CO2, for example, increased photosynthesis by 81% in mycorrhizal plants, whereas mycorrhizal colonization stimulated photosynthesis by only 65% at elevated CO2.  In addition, after measuring photosynthesis over a range of CO2 concentrations (0-1000 ppm), the authors determined that non-mycorrhizal plants had acclimated to elevated CO2, while mycorrhizal ones had not.

Although mycorrhizal colonization did not significantly effect plant biomass, elevated CO2 did; and by day 100, plants grown with atmospheric CO2 enrichment were 26% larger than those grown at ambient CO2.  It is interesting to note that photosynthetic rates were stimulated much more than plant biomass, implying that large amounts of fixed carbon must have been respired by the symbiotic fungus or the plant roots, as it was not detected in soil carbon as organic matter.

What it means
This study indicates that as the CO2 content of the air rises, Plantago lanceolata will likely display increases in photosynthesis, regardless of its mycorrhizal colonization status.  However, if colonized, plants may not exhibit signs of acclimation to elevated CO2, as far as photosynthetic rates are concerned, and can use additional photosynthate to increase their overall biomass.