The Carbon Balance of a Legume and the Functional Economy of its Root Nodules

Budgets for carbon and nitrogen in shoot, root, and nodulesof garden pea (Pisum sativum L.) are drawn up for a 9-d intervalin the life cycle, from data on nitrogen fixation, carbon accumulationin dry matter, respiratory output of plant organs, and organicsolute exchange between shoot and nodulated root. Of the carbon gained photosynthetically by the shoot from theatmosphere 26 per cent is incorporated directly into its drymatter, 32 per cent translocated to the nodules, and 42 percent to the supporting root. Of the nodules’ share, 5per cent is consumed in growth, 12 per cent in respiration,and 15 per cent returned to the shoot via the xylem, as aminocompounds generated in nitrogen fixation. Growth and respirationof the root utilize, respectively, 7 and 35 per cent. The respiratory efficiency of a nodulated root in terms of nitrogenfixation (5.9mg C per mg N₂-N fixed) is found to be very similarto that of an uninoculated root assimilating nitrate (6.2 mgC per mg NO₃-N reduced). The nodules require in growth, respiration,and export 4.1 mg C ( 10.3 mg carbohydrate) for each mg N whichthey fix. The nodules consume 3 ml O₂ for every 1 ml N₂ utilized in fixation. In exporting a milligram of fixed nitrogen the nodules requireat least 0.35 ml of water. Almost half of this requirement mightbe met by mass flow into the nodules via the phloem.