What was done
Soybeans were grown in glasshouses receiving atmospheric CO2 concentrations of 360 and 700 ppm for 52 days, before having various environmental stresses imposed on them for eight days during early seed filling to determine the interactive effects of elevated CO2, high temperature, and soil moisture deficit on growth and yield.  For the eight-day stress period, some plants were subjected to a high temperature treatment that was 15°C higher than the air temperature in which control plants were grown.  And some plants were subjected to a water stress treatment in which their soil moisture contents were maintained at 40% of that experienced by control plants.

What was learned
Averaged across all stress treatments and harvests, elevated CO2 increased total plant biomass by 41%.  Both high temperature and water deficit treatments, singly or in combination, reduced overall biomass by approximately the same degree, regardless of CO2 treatment.  Thus, even when the greatest biomass reductions of 17% occurred in the CO2-enriched and ambiently grown plants, in response to the combined stresses of high temperature and low soil moisture, plants grown in elevated CO2 still exhibited an average biomass that was 24% greater than that displayed by plants grown in ambient CO2.

Averaged across all stress treatments and harvests, elevated CO2 increased seed yield by 32%.  In addition, elevated CO2 tended to ameliorate the negative effects of environmental stresses on seed yield. CO2-enriched plants that were water stressed, for example, had an average seed yield that was 34% greater than that displayed by water-stressed controls grown at ambient CO2, while CO2-enriched plants exposed to high temperatures produced 38% more seed than their respectively stressed counterparts.  In fact, the greatest relative impact of elevated CO2 on seed yield occurred in response to the combined stresses of high temperature and low soil moisture, wherein CO2-enriched plants exhibited a seed yield that was 50% larger than that of similarly stressed plants grown in ambient CO2.

What it means
As the CO2 content of the air rises, soybeans will likely respond by displaying significant increases in growth and yield.  These beneficial effects resulting from greater atmospheric CO2 concentrations will likely persist even if temperatures rise or soil moisture levels decline, regardless of their cause.  Thus, as the concentration of this atmospheric trace gas increases, it should enable soybeans to better deal with the two most common environmental stresses faced by plants of today and possibly those of tomorrow: high temperatures and low soil moisture contents.