Abstract
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−.
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Selected References
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