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Simultaneous Measurement of Nitrogen Fixation Estimated by Acetylene-Ethylene Assay and Nitrate Absorption by Soybeans ¹

Department of Agronomy and Range Science, University of California, Davis, California 95616

An apparatus was designed for simultaneous measurement of rates of N₂ fixation estimated by C₂H₂-C₂H₄ assay (N₂[C₂H₂] fixation) and NO₃^– absorption by roots of intact, nodulated soybeans (Glycine max [L.] Merr.). The principal design features include: (a) a gas-tight mist chamber in which nodulated roots can be exposed simultaneously to C₂H₂ in the gas phase and to a liquid phase containing NO₃^– sprayed in a fine mist; and (b) provision for sampling the gas phase for C₂H₄ determination, and the liquid phase for NO₃^– determination.

We studied NO₃^– absorption by soybeans as affected by nodulation, NO₃^– concentration during assay, and previous N nutrition during growth in nutrient solution culture in controlled environment chambers. It was established that 0.5 mM NO₃^– nearly saturated the NO₃^– absorption system of both nodulated and unnodulated soybeans when the concentration dependence of NO₃^– absorption rate was measured just after flowering began. Nitrate absorption rates were measured after development of N stress in unnodulated plants, and during recovery from N stress in nodulated plants. The results suggested that the lower capacity for NO₃^– absorption of nodulated plants was a consequence of N stress during the period of nodule growth and development.

Nitrogen [C₂H₂] fixation rates were compared in intact plants assayed in the mist chamber and in excised roots assayed in both the mist chamber and in glass jars. Excised roots had a lower N₂[C₂H₂] fixation rate than intact plants. The decline observed during the first hour after shoot removal was more pronounced for glass jar-assayed excised roots than for mist chamber-assayed excised roots.

We discuss the advantages of our method for assessing the capability of a nodulated legume to acquire nitrogen through both N₂ fixation and absorption and assimilation of NO₃^–.

¹ Research supported in part by the Department of Agronomy and Range Science and in part by Grant NSF-RANN AER 77-07301 to D. W. R. Any opinions, findings, and conclusions or recommendations expressed in this publication are those of the authors and do not necessarily reflect the views of the National Science Foundation.

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