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.

Effects of different nitrogen forms on the growth and cytokinin content in xylem sap of tomato (Lycopersicon esculentum Mill.) seedlings

“…Many studies have shown that when roots are supplied a moderate N concentration completely by sole NH 4 + nutrition, the growth of roots, shoots, and even fruits is usually significantly inhibited (Schortemeyer et al 1997;Zou et al 2005;Claussen 2002;Wang et al 2003;Chaillou et al 1994;Feil 1994). However, if the roots were spatially half supplied with sole NH 4 + and half with sole NO 3 − , roots in the sole NH 4 + part suffered from depressed growth (Guo et al 2002(Guo et al , 2007Chaillou et al 1994), whereas the shoots were still able to accumulate a high dry matter (DM) content. This indicates that plants have the ability to control development at a whole-plant level (Gansel et al 2001) to adapt themselves to environments with uneven N-distribution (Zhang and Forde 1998).…”

“…This indicates that plants have the ability to control development at a whole-plant level (Gansel et al 2001) to adapt themselves to environments with uneven N-distribution (Zhang and Forde 1998). To our knowledge, although there are some comparisons of shoot and root growth responding to both homogeneous and localized supply of different N forms in tobacco and soybean plantlets (Walch-Liu et al 2000;Chaillou et al 1994), few reports have described differences in root morphology in response to different NO 3 − :NH 4 + ratios under either homogeneous or split-root systems.…”

“…The fact that, in nature, plants are always exposed to a heterogeneous NO 3 − and NH 4 + soil environment (Schortemeyer and Feil 1996) suggests that their roots could have the ability to acclimate to mixed and heterogeneous distributions of N forms. A split-root system (Schortemeyer et al 1993;Cramer and Lewis 1993;Chaillou et al 1994;Feil 1994;Schortemeyer and Feil 1996) was developed to study the response of plants to a simultaneous but spatially separated supply of NO 3 − and NH 4 + , in which half of the root system is exposed to NO 3 − only and the other half to NH 4 + only. This system is acknowledged as being superior to the homogeneous system to study the effects of different N forms on physiological and molecular responses (Chaillou et al 1994;Walch-Liu et al 2000Gansel et al 2001;Guo et al 2002Guo et al , 2007Schmidt et al 2003;Rahayu et al 2005;Boukcima et al 2006).…”

“…A split-root system (Schortemeyer et al 1993;Cramer and Lewis 1993;Chaillou et al 1994;Feil 1994;Schortemeyer and Feil 1996) was developed to study the response of plants to a simultaneous but spatially separated supply of NO 3 − and NH 4 + , in which half of the root system is exposed to NO 3 − only and the other half to NH 4 + only. This system is acknowledged as being superior to the homogeneous system to study the effects of different N forms on physiological and molecular responses (Chaillou et al 1994;Walch-Liu et al 2000Gansel et al 2001;Guo et al 2002Guo et al , 2007Schmidt et al 2003;Rahayu et al 2005;Boukcima et al 2006). Chaillou et al (1994) used the split-root system to study independent uptake of NO 3 − or NH 4 + , since it was difficult to assess the quantitative effects on assimilate partitioning when the two ions were supplied in combination.…”

“…This system is acknowledged as being superior to the homogeneous system to study the effects of different N forms on physiological and molecular responses (Chaillou et al 1994;Walch-Liu et al 2000Gansel et al 2001;Guo et al 2002Guo et al , 2007Schmidt et al 2003;Rahayu et al 2005;Boukcima et al 2006). Chaillou et al (1994) used the split-root system to study independent uptake of NO 3 − or NH 4 + , since it was difficult to assess the quantitative effects on assimilate partitioning when the two ions were supplied in combination.…”