What was done
The authors grew perennial ryegrass (Lolium perenne L.) in field plots, as part of a FACE experiment, and within controlled-environment chambers, in order to compare growth and carbon allocation responses of this important forage crop to elevated atmospheric CO2 concentrations under different experimental protocols.  In both regimes, the control and elevated CO2 concentrations were maintained for approximately two months at 350 and 600 ppm, respectively.

What was learned
The overall growth response of perennial ryegrass to elevated CO2 was consistently greater for plants grown in the FACE experiment than for plants grown in the controlled-environment chambers.  Elevated CO2, for example, increased total dry matter production by 65 and 54% in the FACE and controlled-environment chambers, respectively.  In addition, it enhanced root dry weights by 109 and 47% for plants gown in the FACE and controlled-environment chambers, respectively.  As a result, the root-to-shoot dry matter ratio was enhanced by 44% as a consequence of atmospheric CO2 enrichment in the FACE experiment, but it was totally unaffected by atmospheric CO2 enrichment in the controlled-environment chamber experiment.

What it means
As the CO2 content of the air increases, swards of perennial ryegrass will likely exhibit increased rates of photosynthesis and dry matter production.  In addition, elevated CO2 concentrations should favor the growth of belowground organs.  Thus, the carbon sequestering prowess of grasslands dominated by perennial ryegrass is likely to increase with future increases in the air's CO2 content.