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

Idoia Ariz,Pedro M Aparicio-Tejo,Carmen González-Murua,Cristina Cruz,Maria A Martins-Loução,María B González-Moro,Jose F Moran,Carmen García-Olaverri

doi:10.1186/1471-2229-11-83

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.

Highlights

In plants, nitrate (NO3-) nutrition gives rise to a natural N isotopic signature (δ15N), which correlates with the δ15N of the N source
Four trends emerged from the natural isotopic signature data (Figure 1): 1) NO3--fed plants tended to be enriched in the heavier N isotope, whereas NH4+-fed plants were depleted compared with their respective N sources; 2) for the same external N
Concentration, the degree of fractionation depended on the plant species; 3) the δ15N values of shoots and roots were not the same but followed similar patterns; and 4) in contrast to the NO3--fed plants, which had δ15N values that were insensitive to the N concentration, under NH4+ nutrition, fractionation tended to increase with the N concentration within plant species (Table 2)

Summary

Introduction

Nitrate (NO3-) nutrition gives rise to a natural N isotopic signature (δ15N), which correlates with the δ15N of the N source. Assessment under natural conditions is difficult because, under most circumstances, NO3- and NH4+ are simultaneously present in the soil and their concentrations change both spatially and temporally over a wide range (e.g., 20 μM to 20 mM) [1,2]. This situation becomes even more complex if the rhizosphere and its symbiotic interactions (N2-fixing organisms or mycorrhiza) are taken into account. Some studies assume that the δ15N of leaf tissue reflects that of the source in the soil (e.g., see [5]) This assumption implies that the isotope ratio of the N source is preserved during N absorption, assimilation and translocation. In a growth system where the quantity of substrate (N) is limited, and the organism exhausts the N source completely, the plant δ15N will be similar (or even identical) to the original N source [6,7]

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