Nonphotosynthetic CO2 Fixation by Alfalfa (Medicago sativa L.) Roots and Nodules

The dependence of alfalfa (Medicago sativa L.) root and nodule nonphotosynthetic CO(2) fixation on the supply of currently produced photosynthate and nodule nitrogenase activity was examined at various times after phloem-girdling and exposure of nodules to Ar:O(2). Phloemgirdling was effected 20 hours and exposure to Ar:O(2) was effected 2 to 3 hours before initiation of experiments. Nodule and root CO(2) fixation rates of phloem-girdled plants were reduced to 38 and 50%, respectively, of those of control plants. Exposure to Ar:O(2) decreased nodule CO(2) fixation rates to 45%, respiration rates to 55%, and nitrogenase activities to 51% of those of the controls. The products of nodule CO(2) fixation were exported through the xylem to the shoot mainly as amino acids within 30 to 60 minutes after exposure to (14)CO(2). In contrast to nodules, roots exported very little radioactivity, and most of the (14)C was exported as organic acids. The nonphotosynthetic CO(2) fixation rate of roots and nodules averaged 26% of the gross respiration rate, i.e. the sum of net respiration and nonphotosynthetic CO(2) assimilation. Nodules fixed CO(2) at a rate 5.6 times that of roots, but since nodules comprised a small portion of root system mass, roots accounted for 76% of the nodulated root system CO(2) fixation. The results of this study showed that exposure of nodules to Ar:O(2) reduced nodule-specific respiration and nitrogenase activity by similar amounts, and that phloem-girdling significantly reduced nodule CO(2) fixation, nitrogenase activity, nodule-specific respiration, and transport of (14)C photoassimilate to nodules. These results indicate that nodule CO(2) fixation in alfalfa is associated with N assimilation.