Respiration and the energy requirement for nitrogen fixation in nodulated pea roots

Pisum sativum L. cv. Trapper plants were inoculated and grown in a controlled environment on N-free nutrient solution. After 4 weeks N was supplied to treatment plants as NH₄NO₃, KNO₃, or NH₄Cl and rates of C₂H₂ reduction, root + nodule respiration, and leaf photosynthesis were determined 1 week later. The increase in respiration per unit of C₂H₂ reduction was not affected by either the form of N added or the light conditions during growth, although the basal respiration rate with no C₂H₂ reduction increased with irradiance level. The mean regression coefficient from plots of respiration versus C₂H₂ reduction was 0.23 + 0.04 (P [unk] .01) mg of CO₂ (μmol of C₂H₂ reduced)⁻¹ which was very similar to the value for the coefficient of respiration associated with nitrogenase activity estimated by subtracting growth and maintenance respiration. Since the rate of N accumulation in N-free nutrient conditions was proportional to the rate of C₂H₂ reduction, it appears that the method gives a true estimate of the energy requirements for N fixation which for these conditions was equivalent to 17 grams of carbohydrate consumed per gram of N fixed.