What was done
The authors grew young oak seedlings for 10 to 15 weeks in environmental chambers at ambient or elevated CO2 concentrations in the presence or absence of mycorrhizal fungi and water stress to determine their interactive effects on seedling growth.  Because biomass production requires protein synthesis, and since sulphur is incorporated into many proteins, the authors "fed" radiolabelled sulphur to attached leaves and tracked its incorporation into biomass to determine if any of the experimental variables affected internal transport processes.

What was learned
Elevated CO2 significantly increased total plant biomass regardless of treatment combination.  Non-mycorrhizal CO2-enriched treated seedlings, for example, exhibited 92 and 128% more biomass than their ambient controls when well-watered and water-stressed, respectively, while mycorrhizal CO2-enriched seedlings displayed about 60% more biomass than their ambient controls irrespective of water status.

Elevated CO2 enhanced the export of sulphur from leaves regardless of mycorrhizal treatment.  When exposed to drought conditions, transport of sulphur from leaves of seedlings grown in ambient CO2 was completely inhibited regardless of mycorrhizal treatment, whereas leaves of seedlings grown in elevated CO2 continued to export sulphur.  Thus, elevated CO2 allowed seedling transport mechanisms to function properly and continue unabated during drought conditions, whereas ambient CO2 did not.

What it means
As the CO2 content of the air rises, oak seedlings should exhibit substantial increases in biomass regardless of any symbiotic relationships they might have with mycorrhizal fungi.  In addition, seedling biomass increases can also be expected to occur even during times of severe drought, as internal transport processes remain functional and send needed resources to actively growing sinks.