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

Abstract

An apparatus was designed for simultaneous measurement of rates of N2 fixation estimated by C2H2-C2H4 assay (N21 C2H21 fixation) and N03-absorption by roots of intact, nodulated soybeans (Glycine max IL.I 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 lquid phase containing N03 sprayed in a fine mist; and (b) provision for sampling the gas phase for C2H4 determination, and the liquid phase for N03-determination.We studied N03-absorption by soybeans as affected by nodulation, N03-concentration during assay, and previous N nutrition during growth in nutrient solution culture in controlled environment chambers. It was established that 0.5 Mm N03-nearly saturated the N03-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 N03-absorption of nodulated plants was a consequence of N stress during the period of nodule growth and development.Nitrogen I C2H21 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 N21 C2H21 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 nist 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 N03-. A recent consequence has been a trend toward developing techniques and apparatus for in situ C2H2 reduction assays on intact nodulated legumes. Mederski and Streeter (11) devised a method for suspending soybean root systems in a gas-tight enclosure and spraying them with a nutrient mist. Acetylene reduction was then monitored over several days. Although that system was obviously well suited for ion absorption studies, no such investigation was reported. Zobel et al. (24) grew several legume species with well nodulated roots in an "aeroponic" system. Their success suggested that nodules both developed and functioned well in a nutrient mist. We were convinced that a mist or spray would serve equally well for short term studies of absorption kinetics for a single ion (NO3 ), and would also facilitate simultaneous measurements of the N2[C2H21-fixing activity of nodules.The objectives of the research reported here were to: (a) develop methodology that would facilitate simultaneous measurement on intact nodulated soybean plants of N03 absorption from a liquid phase, and reduction of C2H2 to C2H4 in a gas phase; (b) evaluate the method by comparison with previously developed techniques; and (c) determine w...

Limitations on Leaf Nitrate Reductase Activity during Flowering and Podfill in Soybean

“…has been investigated by a number of workers (10,28,29). Earlier work has established that NO3-reduction in the leaves is chiefly responsible for N accumulation during vegetative development, reaching a maximum at beginning to full flower (12, 28).…”

“…Earlier work has established that NO3-reduction in the leaves is chiefly responsible for N accumulation during vegetative development, reaching a maximum at beginning to full flower (12, 28). A sharp decline in leaf NR2 activity has been reported to follow within the 2 to 3 weeks after flowering (10,29 of N03-resulted in no increase in activity. Shaner and Boyer (24,25) reported that a variety of factors which decreased the movement of nitrate from roots to leaves resulted in a parallel decrease in NR activity in soybean.…”

Ecophysiological Effects of Nitrogen on Soybean [<i>Glycine max</i> (L.) Merr.]

“…In addition, soybeans can also absorb and assimilate inorganic nitrogen, in the form of nitrates, from the soil [18]. The combined process of nitrogen fixation from atmospheric nitrogen and nitrogen assimilation from the soil ensure that soybean meet its large nitrogen demand for the production of protein rich seeds [18].…”

Simultaneous Measurement of NH4+ Absorption and N2 Fixation by Glycine max L.

“…The interaction of these processes can be studied only by more novel approaches than those allowed by conventional techniques. To facilitate the simultaneous measurement of NH4' absorption and N2 (acetylene) fixation of intact soybeans, a mist apparatus developed by Wych and Rains (23) was used in this study.…”

“…Nodules were separated from the dried roots, counted, and weighed. Acetylene and ethylene were determined by gas chromatography (23 was 9 ,imol NH4', and, at flowering, was 10 ,umol NH4' for unnodulated and nodulated soybeans. Decreasing acidity produced no significant change in the acetylene reduction rate of flowering nodulated plants (Table III).…”