What was done
Spring wheat was planted in rows and grown in a FACE experiment located near Phoenix, AZ, USA, employing atmospheric CO2 concentrations of 370 and 550 ppm and irrigation treatments replacing 50 and 100% of potential evapotranspiration to determine the effects of elevated CO2 and water stress on root growth.

What was learned
Elevated CO2 increased in-row root dry weight by an average of 22% during the growing season under both wet and dry irrigation regimes.  In addition, during the vegetative growth phase, atmospheric CO2 enrichment increased inter-row root dry weight by 70%, indicating that plants grown in elevated CO2 developed greater lateral root systems than those of plants grown at ambient CO2.  Similarly, during the reproductive growth phase, elevated CO2 simulated branching of lateral roots into inter-row areas, but only when water was limiting to growth.

In addition to lateral expansion, the vertical disposition of roots in the soil profile was also enhanced by atmospheric CO2 enrichment; and CO2-enriched plants tended to display greater root dry weights at a given depth than did ambiently-grown plants.

What it means
This study suggests that as the CO2 content of the air continues to rise, spring wheat will likely develop larger and more extensively branching root systems that may help them to better cope with soil water-stress conditions.  Thus, atmospheric CO2 enrichment should allow spring wheat crops to be successfully grown in regions where mild water stress may be incurred.