Carbon Dioxide Fixation in Roots and Nodules of Alnus glutinosa: I. Role of Phosphoenolpyruvate Carboxylase and Carbamyl Phosphate Synthetase in Dark CO(2) Fixation, Citrulline Synthesis, and N(2) Fixation.

Detached roots and nodules of the N(2)-fixing species, Albus glutinosa (European black alder), actively assimilate CO(2). The maximum rates of dark CO(2) fixation observed for detached nodules and roots were 15 and 3 micromoles CO(2) fixed per gram dry weight per hour, respectively. The net incorporation of CO(2) 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 CO(2) 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 CO(2) fixation and N(2) fixation in nodules decreased after treatment of nodulated plants with nitrate while the percentage of the total (14)C 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 N(2)-fixing nodules and roots of alder.