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

Abstract

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.