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CO₂ Fixation in Alfalfa and Birdsfoot Trefoil Root Nodules and Partitioning of ¹⁴C to the Plant¹

Nonphotosynthetic CO₂ fixation by root nodules of legumes may provide carbon skeletons for assimilation of symbiotically fixed N₂. However, the products formed by this process and their physiological significance in perennial legumes is poorly understood. Therefore, the objectives of this study were to determine if nonphotosynthetic CO₂ fixation by root nodules contributes carbon for the assimilation of fixed N₂ in alfalfa (Medicago sativa L.) and birdsfoot trefoil (Lotus corniculatus L.) and if assimilation products are partitioned to different plant organs. Effective alfalfa nodules excised from or attached to roots had apparent ¹⁴CO₂ fixation rates of 50 to 80 µg CO₂ kg^-1 s^-1 (dry weight) at 0.0012 to 0.0038 mole fraction CO₂. Nodule CO₂ fixation rates increased six- to seven-fold as ambient CO₂ was raised from 0.0038 to 0.0663 mole fraction. These rates were double to triple those of comparably treated nodules of ineffective alfalfa and those of birdsfoot trefoil. Respiration rates of nodules (3 to 4 mg CO₂ kg^-1 s^-1) were 10 to 100-fold higher than ¹⁴CO₂ fixation rates of nodules. This finding suggests that attached nodules normally function at saturating CO₂ levels and may fix up to 1600 µg CO₂ kg^-1 s^-1 when concurrent ¹⁴C export to roots and shoots is accounted for. Pulse chase experiments with ¹⁴CO₂ combined with nodule and xylem sap analysis demonstrated the initial products of root and nodule CO₂ fixation were organic acids. However, the export of fixed ¹⁴C from effective nodules was primarily in the form of amino acids. In contrast, nodule and/or root fixed ¹⁴C in ineffectively nodulated alfalfa and denodulated effective alfalfa and birdsfoot trefoil was transported primarily as organic acids. Aspartate, asparagine, alanine, glutamate, and glutamine were the most heavily labeled compounds in the amino acid fraction of both effective alfalfa and birdsfoot trefoil nodules exposed to ¹⁴CO₂. By contrast, asparate, asparagine, and glutamine were the predominantly labeled amino acids in xylem sap collected from nodulated effective roots exposed to ¹⁴CO₂. The occurrence of nodule CO₂ fixation in alfalfa and birdsfoot trefoil and the export of fixed carbon as asparagine and aspartate to roots and shoots is consistent with a role for CO₂ fixation by nodules in providing carbon skeletons for assimilation and transport of symbiotically fixed N₂.

Key Words: Source sink relations • Nitrogen fixation • Nitrogen assimilation • Carbon economy • Medicago sativa L. • Lotus corniculatus L.

² Research biologist, Monsanto Agricultural Products TIE, 800 N. Lindbergh Blvd., St. Louis, MO 63167 (formerly graduate research assistant, Dep. of Agronomy and Plant Genetics, Univ. of Minnesota), research plant physiologists, USDA-ARS, and associate scientist in the Dep. of Agronomy and Plant Genetics, Univ. of Minnesota, St. Paul, MN 55108. Reprint requests should be addressed to CPV.

Received for publication May 27, 1983.

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