N2 Fixation and the Respiratory Costs of Nodules, Nitrogenase Activity, and Nodule Growth and Maintenance in Fiskeby Soyabean

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Abstract
The respiratory effluxes of nodules and of roots of Fiskeby V soyabean (Glycine max (L.) Merr.), grown in a controlled environment, were measured at intervals in air and 3% O2 from shortly after the onset of N2 fixation until plant senescence. The respiratory burdens linked with nitrogenase plus ammonia metabolism, and nodule growth and maintenance, were calculated from gas exchange data and related to the concurrent rates of N2 fixation. The specific respiration rates of nodules increased to a maximum of 21 mg CO2 g−1 h−1 at the time pods began development: the equivalent maximum for roots was c. 4.5 mg CO2 g−1 h−1. Maximum nodule and root respiration rates per plant were attained about 25 d later at the time N2 fixation peaked at 15 mg N d−1 plant−1. The relationship between nodule respiration and N2 fixation indicated an average respiratory cost of 13.2 mg CO2 mg−1 N until the last few days of plant development Separation of nodule respiration into the two components: nitrogenase (+ NH3 metabolism) respiration and nodule growth and maintenance respiration, indicated that the latter efflux accounted for c. 20% of nodule respiration while N2 fixation was increasing and new nodule tissue was being formed. When nodule growth ceased and N2 fixation declined, this component of respiration also declined. The respiratory cost of nitrogenase activity plus the associated metabolism of NH3 varied between 11 mg CO2 mg−1 N during vegetative and early reproductive growth, to 12.5 mg CO2 mg−1 N during the later stages of pod development.

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