What was done
Sugar beets were grown as an annual crop in controlled-environment chambers with atmospheric CO2 concentrations of 360 and 700 ppm and air temperatures of ambient and ambient plus 3°C for three consecutive years.  In addition to CO2 and temperature combinations, sugar beets were also supplied with solutions of low and high nitrogen content to study the main and interactive effects of elevated CO2, temperature, and nitrogen on yield and biochemical composition.  Of particular interest was how elevated CO2 may affect levels of alpha-amino-N and glycinebetaine (two nitrogen-rich compounds) in the tap root that interfere with commercial sucrose extraction from that organ.

What was learned
No significant interactions between elevated CO2 and temperature were found for any growth parameter.  Averaged across all three years and both growth temperatures, atmospheric CO2 enrichment enhanced total biomass by 13 and 25% at low and high nitrogen regimes, respectively.  In addition, elevated CO2 increased root biomass by 12 and 26% for the same situations. Thus, elevated CO2 elicited the largest growth response in plants subjected to high nitrogen as opposed to low nitrogen supply.

As far as biochemical composition goes, elevated CO2 did not significantly alter tap root sucrose content per unit area; but because tap roots grew larger with atmospheric CO2 enrichment, they contained a greater total amount of sucrose within them.  Elevated CO2 significantly reduced tap root concentrations of alpha-amino-N and glycinebetaine by 24 and 13%, respectively, at high nitrogen levels, and decreased both compounds by 16% at low nitrogen.

What it means
As the air's CO2 content rises, sugar beets will likely increase rates of photosynthesis and use the additional carbohydrates produced to enhance overall size and weight of tap roots, which are the economically important part of this plant.  In addition, with more CO2 in the atmosphere, sugar beets should contain reduced amounts of nitrogen-rich compounds, which is a positive thing, as those compounds lower the efficiency at which sucrose is extracted from tap roots.