What was done
Spring wheat (Triticum aestivum L. cv. Yecora Roja) was grown in a FACE experiment located near Phoenix, Arizona, USA.  Plants were grown in 25-m diameter plots that received atmospheric CO2 concentrations of 370 and 550 ppm.  In addition, half of the plants were subjected to a well-watered irrigation regime, while the other half received only 50% as much irrigation waters (water-stressed regime).  Thus, the authors studied the effects of elevated CO2 and water stress on grain weight in this important cereal crop.

What was learned
Elevated CO2 increased final grain weights in the upper and lower sections of the main stems by 10 and 24%, respectively, under water-stressed conditions.  Under well-watered conditions, however, elevated CO2 increased final grain weights only in the lower sections of main stems and only by 14%.  Thus, elevated CO2 had a greater positive impact on final grain weights under less-then-optimal water-stressed field conditions, once again demonstrating that atmospheric CO2 enrichment is often more important to stressed plants than it is to non-stressed plants.

What it means
As the air's CO2 concentration rises, spring wheat plants will likely experience increased photosynthetic rates.  In turn, plants can use this enhanced carbohydrate supply to increase the weight of individual grains, regardless of soil water status.  Indeed, data from the present paper documented the largest CO2-induced increases in grain weights in water-stressed plants, as opposed to well-watered control plants.  Thus, wheat growers can anticipate increased grain weight production under various soil moisture conditions with future increases in the air's CO2 content.