Environmental and Genotypic Effects on the Respiration Associated with Symbiotic Nitrogen Fixation in Peas

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Abstract
Estimated values for the respiration associated with symbiotic N fixation in Pisum sativum L. were independent of irradiance, temperature, plant age and CO2 concentration, despite large variation in the total rates of C2H2 reduction and root + nodule respiration. Similar values were also found in Phaseolus vulgaris L., Vicia faba L. and Glycine max (L.) Merr. Among all combinations of 4 Pisum cultivars with 4 Rhizobium leguminosarum inoculants only the plant genotype significantly affected the fixation-linked respiration, although both plant and bacterial types significantly influenced the total rate of C2H2 reduction. On the basis of measured rates of H2 evolution and C2H2 reduction, or total N gain in the same system, the least respiration per unit of ammonia produced symbiotically was estimated as 4.8-6.9 mol CO2 (mol NH3)-1 in Laxton''s Progress and the greatest as 9.3-13.3 mol CO2 (mol NH3)-1 in an Indian cultivar, as compared to a theoretical minimum respiration requirement of 4.7 mol CO2 (mol NH3)-1 in peas.