Respiration and the Energy Requirement for Nitrogen Fixation in Nodulated Pea Roots

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Abstract
Pisum sativum L. cv. Trapper plants were inoculated and grown in a controlled environment on N-free nutrient solution. After 4 wk 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 wk 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 vs. C2H2 reduction was 0.23 + 0.04 (P .ltoreq. .01) mg of CO2 (.mu.mol 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/gram of N fixed.