What was done
Sugar beet (Beta vulgaris L. cv. Ramonskaya) plants were grown for one month in environmental chambers receiving atmospheric CO2 concentrations of 350 and 700 ppm.  In addition, plants were fertilized with three different levels of nitrate.  Thus, the authors studied the interactive effects of elevated CO2 and soil nitrate on the productivity of this important agricultural plant.

What was learned
Plants grown in CO2-enriched air exhibited rates of net photosynthesis that were approximately 50% greater than those displayed by ambiently-grown plants, regardless of soil nitrate availability.  These increases in photosynthetic carbon uptake contributed to CO2-induced increases in aboveground organ dry weights that were 1.6-, 1.4-, and 1.3-fold greater than dry weights observed in control plants receiving one-half, standard, and three-fold levels of soil nitrate, respectively.  Root weights, however, were less responsive to atmospheric CO2 enrichment, displaying 10 and 30% increases in dry weight at one-half and standard nitrate levels, while no significant response was detected at the high soil nitrate concentration.

What it means
As the CO2 content of the air increases, this agricultural crop will likely respond by displaying increased rates of photosynthesis and biomass production, even under conditions of poor soil nitrogen fertility.  Although belowground growth responses were smaller than aboveground growth responses in this study, it is possible that had the experiment lasted longer, the CO2-responsiveness of belowground organs may well have exceeded that of aboveground organs, due to the strong sink demand that would have developed within the enlarging tap root.