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Articles

Carbon Dioxide Fixation in Soybean Roots and Nodules

I. CHARACTERIZATION AND COMPARISON WITH N₂ FIXATION AND COMPOSITION OF XYLEM EXUDATE DURING EARLY NODULE DEVELOPMENT ¹

Department of Biochemistry, Michigan State University, East Lansing, Michigan 48824

These studies demonstrate that soybean (Merr) roots and nodules possess an active system for fixing CO₂. The maximum rates of CO₂ fixation observed for roots and nodules of intact plants were 120 and 110 nanomoles CO₂ fixed per milligram dry weight per hour, respectively. Results of labeling studies suggest a primary role for phosphoenolpyruvate carboxylase in CO₂ assimilation in these tissues. After pulse-labeling with ¹⁴CO₂ 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, CO₂ fixation in nodules increased at the onset of N₂ fixation but declined to a lower level prior to the decrease in N₂ 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 CO₂ fixation, providing substrates for energy-yielding metabolism and supplying carbon skeletons for NH₄⁺ assimilation and amino acid biosynthesis.

³ To whom reprint request should be addressed.

¹ This research was supported by Grant PCM-77-24683 from the National Science Foundation, by Grant 5901-0410-9-0248-0 from the Competitive Research Grants, Office of the United States Department of Agriculture, Science and Education Administration and by the Michigan Agricultural Experiment Station. This is journal article 9565 of the Michigan Agricultural Experiment Station. Preliminary results of these investigations were presented as an abstract (4).

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