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Articles

Carbon Dioxide Fixation in Roots and Nodules of Alnus glutinosa¹

I. Role of Phosphoenolpyruvate Carboxylase and Carbamyl Phosphate Synthetase in Dark CO₂ Fixation, Citrulline Synthesis, and N₂ Fixation

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

Detached roots and nodules of the N₂-fixing species, Albus glutinosa (European black alder), actively assimilate CO₂. The maximum rates of dark CO₂ fixation observed for detached nodules and roots were 15 and 3 micromoles CO₂ fixed per gram dry weight per hour, respectively. The net incorporation of CO₂ in these tissues was catalyzed by phosphoenolpyruvate carboxylase which produces organic acids, some of which are used in the synthesis of the amino acids, aspartate, glutamate, and citrulline and by carbamyl phosphate synthetase. The latter accounts for approximately 30 to 40% of the CO₂ fixed and provides carbamyl phosphate for the synthesis of citrulline. Results of labeling studies suggest that there are multiple pools of malate present in nodules. The major pool is apparently metabolically inactive and of unknown function while the smaller pool is rapidly utilized in the synthesis of amino acids. Dark CO₂ fixation and N₂ fixation in nodules decreased after treatment of nodulated plants with nitrate while the percentage of the total ¹⁴C incorporated into organic acids increased. Phosphoenolpyruvate carboxylase and carbamyl phosphate synthetase play key roles in the synthesis of amino acids including citrulline and in the metabolism of N₂-fixing nodules and roots of alder.

³ Present address: Department of Biology, Washington University, St. Louis, MO 63130.

⁴ To whom requests for reprints should be sent.

¹ This research was supported by Grant 5901-0410-9-0248-0 from the Competitive Research Grants Office of the United States Department of Agriculture, Science and Education Administration. Preliminary results of these studies have been presented previously (Coker, McClure, Schubert Plant Physiol 67: S-445 [1981]). This is Journal Article 10543 of the Michigan Agricultural Experiment Station.