Depletion of the heaviest stable N isotope is associated with NH₄⁺/NH₃toxicity in NH₄⁺-fed plants

<p>Abstract</p> <p>Background</p> <p>In 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⁺/NH₃uptake experiments at pH 6.0 and 9.0 (which favours NH₃form) 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.</p> <p>Results</p> <p>Several 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 NH₃uptake 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 RbNO₃as control discarded that the differences observed arise from pH detrimental effects.</p> <p>Conclusions</p> <p>This article proposes that the negative values of δ¹⁵N in NH₄⁺-fed plants are originated from NH₃uptake by plants. Moreover, this depletion of the heavier N isotope is proportional to the NH₄⁺/NH₃toxicity 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.</p>