Elevated CO(2) concentration around alfalfa nodules increases N(2) fixation

Nodule CO(2) fixation via PEPC provides malate for bacteroids and oxaloacetate for N assimilation. The process is therefore of central importance for efficient nitrogen fixation. Nodule CO(2) fixation is known to depend on external CO(2) concentration. The hypothesis of the present paper was that nitrogen fixation in alfalfa plants is enhanced when the nodules are exposed to elevated CO(2) concentrations. Therefore nodulated plants of alfalfa were grown in a hydroponic system that allowed separate aeration of the root/nodule compartment that avoided any gas leakage to the shoots. The root/nodule compartments were aerated either with a 2500 μl l(−1) (+CO(2)) or zero μl l(−1) (–CO(2)) CO(2)-containing N(2)/O(2) gas flow (80/20, v/v). Nodule CO(2) fixation, nitrogen fixation, and growth were strongly increased in the +CO(2) treatment in a 3-week experimental period. More intensive CO(2) and nitrogen fixation coincided with higher per plant amounts of amino acids and organic acids in the nodules. Moreover, the concentration of asparagine was increased in both the nodules and the xylem sap. Plants in the +CO(2) treatment tended to develop nodules with higher %N concentration and individual activity. In a parallel experiment on plants with inefficient nodules (fix(–)) the +CO(2) treatment remained without effect. Our data support the thesis that nodule CO(2) fixation is pivotal for efficient nitrogen fixation. It is concluded that strategies which enhance nodule CO(2) fixation will improve nitrogen fixation and nodule formation. Moreover, sufficient CO(2) application to roots and nodules is necessary for growth and efficient nitrogen fixation in hydroponic and aeroponic growth systems.