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

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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.

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