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

• Published in: Plant physiology (Bethesda) 1978-09, Vol.62 (3), p.443-448
• Main Authors: Wych, R D, Rains, D W
• Format: Article
• Language: English
• Subjects: Germination; Kinetics; Liquids; Nitrates; Nodules; Nutrient solutions; Plant roots; Plants; Soybeans; Vapor phases
• ISSN: 0032-0889; 1532-2548
• DOI: 10.1104/pp.62.3.443

An apparatus was designed for simultaneous measurement of rates of N2 fixation estimated by $\text{C}_{2}\text{H}_{2}-\text{C}_{2}\text{H}_{4}$ assay (N2 [$C_{2}H_{2}$] 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 [$C_{2}H_{2}$] 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 [$C_{2}H_{2}$] 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 -.