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

Abstract

Ammonium absorption rates by intact nodulated and unnodulated soybean plants (Grycie max IL.l Meff., Amsoy 71) were determined from the lquid phase of a mist assay chamber. From the gas phase, simultaneous measurements of acetylene reductio rates were made from nodulated plants. Ammonium absorption capacity was consistently greater in unnodulated plants.At the bnnig of flowering, plant roots were sprayed with an uptake solution ranging from 0.05 to 1.0 milUmolar NH4Cl, and root nodules concurrently were exposed to 0.12 atmosphere acetylene. The NH4' absorption system of both nodulated and unnodulated plants were nearly saturated at 0.3 to 0.5 millimolar NH4+.Increasing the pH from 4.0 to 6.8 stimulated NH4+ absorption rate in both flowering and preilowering nodulated and unnodulated plants. Acet The interaction between N2 fixation and inorganic N in leguminous plants has been the object ofmany studies (5, 15). Longterm experiments which quantify mineral composition and yield responses have led to insights on how N source and time of N application influence N2 fixation. Short-term studies are aimed at characterization of control mechanisms and the specific influence of various parameters. Since N03 is the predominant ionic form of N in the soil solution, most work has focused on the interaction of N03 and N2 fixation. Realization of the need to increase N use efficiency due to both economic and environmental concerns, coupled with the availability ofnitrification inhibitors, has aroused interest in the role of NH4' in this interaction.Initially, it appears that metabolic energy would be conserved if NH4' was supplied to the roots, since the processes of N2 fLxation and NO3 absorption and reduction require large amounts of reductant and ATP (19 incorporated into organic compounds without the expenditure of energy for reduction. Concentrations at which plants benefit from NH4' absorption are narrow, however, and excess levels can produce toxic reactions (4). This limits the use of NH4' as a source of nitrogen for most plants.Ammonium absorption kinetics have been observed in a number of higher plants (1,10,12,17,20,21) including soybean (9). A comparative study on the kinetics of NH4' absorption by nodulated and unnodulated soybeans would furnish information on the uptake mechanism and would describe the uptake pattern of plants dependent on N2 fixation as a source of N. Ammonium ion concentration (9, 10, 12), root and shoot temperature (3, 7, 10), solution pH (1,10,14,20), and other ions (10, 14) influence short-term NH4' absorption. Similar to NH4' absorption, the N2 fixing activity of root nodules is influenced by temperature (8,15,18), pH (15), and combined N (5, 15).Most conventional techniques measure N2 (acetylene) fixation3by incubating root systems in the gas phase of a solid rooting medium. In contrast, ion absorption is measured by ion depletion from substrate media or by accumulation in plant tissues. However, N2 fixation and ion absorption do not occur totally independently in the plant. The i...

Some regulatory aspects of [14C]methylamine influx intoPisum sativum L. cv. Feltham First seedlings

“…1975). The apparent affinity for ammonium when [14C]methylamine was used to 'trace' ammonium uptake was of the same order of magnitude as that reported for net uptake by soybean seedlings which had an apparent Km=0.15 mM for ammonium (Marcus-Wyner and Rains 1982).…”

“…The kinetics of net ammonium uptake have been reported for soybean (Marcus-Wyner and Rains 1982;Joseph et al 1975) and a number of cereals e.g, corn (Warncke and Barber 1975), maize (Becking 1956) and wheat (Cox and Reisenauer 1973). In all the forenamed cases, short-term ( < 15 min) tracer influx of NH + has not been measured, and the results reflect a stun of uptake, and other metabolic events such as reduction and translocation.…”

“…Nodulated soybeans have lowered rates of ammonium uptake (Marcus-Wyner and Rains 1982) and nitrate uptake (Wynch and Rains 1978) compared with nonnodulated plants. These results differ from those found from experiments with pea seedlings which showed that inoculation with Rhizobium stimulated nitrate uptake (Deane-Drummond and Chaffey 1985).…”

Utilization of Ammonium as a Nitrogen Source

“…per unit root mass has been reported (12)(13)(14) to be reduced during short-term exposure. The rates of uptake of NH4+ per g root dry weight calculated for this study (Fig.…”

“…The reduction in growth with NH4+ as the nitrogen source has been attributed to the combined effects of acidification of the rootzone associated with the excess influx of cations relative to influx of anions during absorption of NH4+ (8) and toxic accumulation of free NH4+ or ammonia in plant tissues (18,29 First, the rate of NH4+ uptake per unit root mass is decreased by increased acidity (12)(13)(14). Second, root growth is reduced as acidity is increased (23), which could further reduce uptake of nitrogen from an NH4+ source.…”