Depletion of the heaviest stable N isotope is associated with NH4+/NH3 toxicity in NH4+-fed plants

Abstract

BackgroundIn plants, nitrate (NO3-) nutrition gives rise to a natural N isotopic signature (δ15N), which correlates with the δ15N of the N source. However, little is known about the relationship between the δ15N of the N source and the 14N/15N fractionation in plants under ammonium (NH4+) nutrition. When NH4+ is the major N source, the two forms, NH4+ and NH3, are present in the nutrient solution. There is a 1.025 thermodynamic isotope effect between NH3 (g) and NH4+ (aq) which drives to a different δ15N. Nine plant species with different NH4+-sensitivities were cultured hydroponically with NO3- or NH4+ as the sole N sources, and plant growth and δ15N were determined. Short-term NH4+/NH3 uptake experiments at pH 6.0 and 9.0 (which favours NH3 form) were carried out in order to support and substantiate our hypothesis. N source fractionation throughout the whole plant was interpreted on the basis of the relative transport of NH4+ and NH3.ResultsSeveral NO3--fed plants were consistently enriched in 15N, whereas plants under NH4+ nutrition were depleted of 15N. It was shown that more sensitive plants to NH4+ toxicity were the most depleted in 15N. In parallel, N-deficient pea and spinach plants fed with 15NH4+ showed an increased level of NH3 uptake at alkaline pH that was related to the 15N depletion of the plant. Tolerant to NH4+ pea plants or sensitive spinach plants showed similar trend on 15N depletion while slight differences in the time kinetics were observed during the initial stages. The use of RbNO3 as control discarded that the differences observed arise from pH detrimental effects.ConclusionsThis article proposes that the negative values of δ15N in NH4+-fed plants are originated from NH3 uptake by plants. Moreover, this depletion of the heavier N isotope is proportional to the NH4+/NH3 toxicity in plants species. Therefore, we hypothesise that the low affinity transport system for NH4+ may have two components: one that transports N in the molecular form and is associated with fractionation and another that transports N in the ionic form and is not associated with fractionation.

Foliar application of urea to “Sauvignon Blanc” and “Merlot” vines: doses and time of application

“…Subsamples of this material (1 mg of dry weight) were used to determine the d 15 N isotope content using a mass spectrometer (Delta Plus, Thermoquest, Finnigan) coupled to an NC 2500 elemental analyser (CE Instruments, Milan). The percentage of incorporated N in berries of N applied in leaves was calculated using the natural abundance calculated with d 15 N, as described previously by Ariz et al (2011). This value was then applied to the biomass produced and expressed as the amount of N added.…”

CO 2 enrichment modulates ammonium nutrition in tomato adjusting carbon and nitrogen metabolism to stomatal conductance

“…Such a biomass increment is in the range of the average increment (20-40 %) reported for C3 plants [20]. Therefore, taking into account the biomass ratio between nitrate and ammonium nutrition as an indicator of plant's sensitivity to NH 4 + [31,37], we considered 7.5 mM as a "nontoxic" ammonium concentration for both CO 2 growing conditions and 15 mM ammonium, as "potentially toxic" for this cultivar since at this N concentration, elevated CO 2 improved plant growth under ammonium conditions but did not ameliorate ammonium tolerance. Besides, at ambient CO 2 , N source had no effect on biomass partitioning (Table 1).…”

Patterns in δ15N in roots, stems, and leaves of sugar maple and American beech seedlings, saplings, and mature trees

“…Finally, the form of the N taken up may itself influence the extent of fractionation (Gessler et al 1998;Kreuzwieser et al 1997;Marschner et al 1991;Serna et al 1992). For example, a recent study by Ariz et al (2011) (Pardo et al 2002) and root d 15 N in the mineral soil (at 50 cm depth) was higher than that in the forest floor (Pardo et al 2006). Thus, we assumed that as rooting depth increased from seedlings to trees, the deeper roots would have access to inorganic N that would be more enriched in 15 (Pardo et al 2002), which does not appear to have altered nitrification rates (Houlton et al 2003).…”