Responses of soybean to ammonium and nitrate supplied in combination to the whole root system or separately in a split-root system

Abstract

To address the questions of whether allocation of carbohydrates to roots is influenced by ionic form of nitrogen absorbed and whether allocation of carbohydrates to roots in turn influences proportionality between NH4+ and NO3- uptake from mixed sources, NH4+ and NO3- were supplied separately to halves of a split-root hydroponic system and were supplied in combination to a whole-root system. Dry matter accumulation in the split-root system was 18% less in the NH4(+)-fed axis than in the NO3(-)-fed axis. This, however, does not indicate that partitioning of carbohydrate between the two axes was different. Most of the reduction in dry matter accumulation in the NH4(+)-fed axis can be accounted for by the retransport of CH2O equivalents from the root back to the shoot with amino acids produced by NH4+ assimilation. Uptake of NH4+ or NO3- by the respective halves of the split-root system was proportional to the estimated allocation of carbohydrate to that half. When NH4+ and NO3- were supplied to separate halves of the split-root system, the cumulative NH4+ to NO3- uptake ratio was 0.81. When supplied in combination to the whole-root system, the cumulative NH4+ to NO3- uptake ratio was 1.67. Thus, while the shoot may affect total nitrogen uptake through the export of carbohydrates to roots, the shoot (common for halves of the split-root system) apparently does not exert a direct effect on proportionality of NH4+ and NO3- uptake by roots. For whole roots supplied with both NH4+ and NO3-, the restriction in uptake of NO3- may involve a stimulation of NO3- efflux rather than an inhibition of NO3- influx. While only the net uptake of NH4+ and NO3- was measured by ion chromatography, monitoring at approximately hourly intervals during the first 3 days of treatment revealed irregularly occurring intervals of both depletion (net influx) and enrichment (net efflux) in solutions. In the case of NH4+, numbers of net efflux events were similar (21 to 24 out of 65 sequential sampling intervals) whether NH4+ was supplied with NO3- to whole-root systems or separately to an axis of the split-root system. In the case of NO3-, however, the number of net efflux events increased from 8 when NO3- was supplied to a separate axis of the split-root system to between 19 and 24 when NO3- was supplied with NH4+ to whole-root systems.

Consequences of high loads of nitrogen for spruce (Picea abies) and beech (Fagus sylvatica) forests

“…This phenomenon cannot be explained by differences in kinetic properties and\or temperature dependencies between NO $ − and NH % + net uptake . From incubation experiments with constant NO $ − and varying NH % + concentrations applied simultaneously, inhibition of NO $ − net uptake in roots by NH % + has been reported in some studies (Glass, Thompson & Bordeleau, 1985 ;Lee & Drew, 1989 ;King et al, 1993 ;Chaillou et al, 1994 ;Kreuzwieser et al, 1997 ;, but few or no effects were observed in others (Smith & Thompson, 1971 ;Schrader et al, 1972 ;Oaks, Stulen & Boesel, 1979 ;Flaig & Mohr, 1992). Direct effects of NH % + on processes of NO $ − uptake have been assumed by some authors (Lee & Drew, 1989 ;King et al, 1993), whereas others have postulated that products of N assimilation such as amino compounds are responsible for the downregulation of NO $ − net uptake in roots simultaneously supplied with NH % + (Lee et al, 1992 ;Imsande & Touraine, 1994).…”

Nitrogen Metabolism in Citrus Based on Expressed Tag Analysis

“…Nitrate may be processed by booth root and leaf cells, and its accumulation in vacuoles re fl ects an important role as osmotic agent. In contrast, ammonium ions are toxic and have to be incorporated in organic compounds quickly (Chaillou et al 1994 ). Variation in metabolic processing of nitrogen sources is de fi ned by their chemical nature and diversity of plant and environmental conditions.…”

A convenient, soil‐free method for the production of root nodules in soybean to study the effects of exogenous additives

“…Conversely, it is difficult to perform similar assays with exogenous additives in crop legumes such as soybean, due to their large stature, long life cycle, the formation of a huge number of root nodules, and the requirement of soil for nodule formation. To circumvent these problems, split-root system was used in soybean to study the effects of various exogenous variable during rhizobia-legume symbioses (Chaillou, Rideout, Raper, & Morot-Gaudry, 1994;Gil-Quintana et al, 2013;Lin, Gresshoff, & Ferguson, 2012;Singleton & Bohlool, 1984). Most recently, a new split-root system was developed for continuous monitoring of soybean roots throughout the whole experiment after rhizobial infection (Hidalgo, Ruiz-Sainz, & Vinardell, 2018).…”