Carbon Dioxide Fixation in Soybean Roots and Nodules: I. CHARACTERIZATION AND COMPARISON WITH N(2) FIXATION AND COMPOSITION OF XYLEM EXUDATE DURING EARLY NODULE DEVELOPMENT.

These studies demonstrate that soybean (Merr) roots and nodules possess an active system for fixing CO(2). The maximum rates of CO(2) fixation observed for roots and nodules of intact plants were 120 and 110 nanomoles CO(2) fixed per milligram dry weight per hour, respectively. Results of labeling studies suggest a primary role for phosphoenolpyruvate carboxylase in CO(2) assimilation in these tissues. After pulse-labeling with (14)CO(2) for 2 minutes, 70% of the total radioactivity was lost within 18 minutes via respiration and/or translocation out of nodules. During the vegetative stages of growth of soybeans grown symbiotically, CO(2) fixation in nodules increased at the onset of N(2) fixation but declined to a lower level prior to the decrease in N(2) fixation. This decrease coincided with a decrease in the transport of amino acids, especially asparagine, and an increase in the export of ureides. These findings are consistent with a dual role for CO(2) fixation, providing substrates for energy-yielding metabolism and supplying carbon skeletons for NH(4) (+) assimilation and amino acid biosynthesis.