Background
Symbiotic nitrogen-fixing bacteria exist within specialized nodules associated with leguminous plant roots.  These bacteria use the enzyme nitrogenase to convert atmospheric nitrogen (N2), which plants cannot directly use, into ammonium (NH4⁺), which is readily utilized by plants.  Nitrogen acquired in this manner is readily incorporated into larger N-containing compounds that are translocated throughout plants to various organs.  In soybeans, ureides comprise a class of nitrogenous compounds that are commonly exported from nodules and delivered to shoots.  Previous studies on this crop have demonstrated the existence of a negative feedback loop, wherein the accumulation of ureides in shoots results in decreased nitrogenase activity in nodules.

What was done
The authors grew soybean (Glycine max L. cv. Braxton) plants hydroponically in a glasshouse fumigated with ambient air for one month.  Then, half of the plants were moved to a glasshouse receiving twice-ambient concentrations of atmospheric CO2.  After five days of acclimation, plants growing at both CO2 concentrations had 0, 5 and 10-mM exogenous ureide concentrations added to their nutrient solutions for an additional five days.  Finally, after the ureide treatment, the authors measured rates of nitrogenase activity and tissue concentrations of ureides in various plant organs to determine if atmospheric CO2 enrichment impacted them in any way.

What was learned
The application of exogenous ureides increased ureide concentrations in nodules, roots and leaves of ambiently-growing plants.  However, they had no impact on ureide concentrations in any organs of CO2-enriched plants.  Nonetheless, exogenous ureide application decreased nitrogenase activity in nodules, regardless of CO2 concentration; but reductions in nitrogenase activity were always less severe in CO2-enriched plants than in ambiently-growing plants.  Indeed, at 5- and 10-mM ureide concentrations, nitrogenase activity was reduced by 44 and 71%, respectively, at ambient CO2 concentrations, while it was only reduced by 41 and 56%, respectively, at elevated CO2 concentrations.

What it means
As the atmospheric CO2 concentration increases, soybeans (and perhaps other leguminous species) will likely display less ureide-induced feedback inhibition of nitrogenase activity in their nodules.  Consequently, greater amounts of nitrogen will likely be available to soybeans from N2-fixation in a future CO2-enriched atmosphere, even if tissue ureide concentrations are high, which often occurs due to water stress and other less-than-optimal environmental growth conditions.