Depletion of the heaviest stable N isotope is associated with NH.sub.4 .sup.+.sup./NH.sub.3 toxicity in NH.sub.4 .sup.+.sup.-fed plants

In plants, nitrate (NO.sub.3 .sup.-.sup.) nutrition gives rise to a natural N isotopic signature ([delta].sup.15.sup.N), which correlates with the [delta].sup.15.sup.N of the N source. However, little is known about the relationship between the [delta].sup.15.sup.N of the N source and the .sup.14.sup.N/.sup.15.sup.N fractionation in plants under ammonium (NH.sub.4 .sup.+.sup.) nutrition. When NH.sub.4 .sup.+ .sup.is the major N source, the two forms, NH.sub.4 .sup.+ .sup.and NH.sub.3 , are present in the nutrient solution. There is a 1.025 thermodynamic isotope effect between NH.sub.3 (g) and NH.sub.4 .sup.+ .sup.(aq) which drives to a different [delta].sup.15.sup.N. Nine plant species with different NH.sub.4 .sup.+.sup.-sensitivities were cultured hydroponically with NO.sub.3 .sup.- .sup.or NH.sub.4 .sup.+ .sup.as the sole N sources, and plant growth and [delta].sup.15.sup.N were determined. Short-term NH.sub.4 .sup.+.sup./NH.sub.3 uptake experiments at pH 6.0 and 9.0 (which favours NH.sub.3 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 NH.sub.4 .sup.+ .sup.and NH.sub.3 . Several NO.sub.3 .sup.-.sup.-fed plants were consistently enriched in .sup.15.sup.N, whereas plants under NH.sub.4 .sup.+ .sup.nutrition were depleted of .sup.15.sup.N. It was shown that more sensitive plants to NH.sub.4 .sup.+ .sup.toxicity were the most depleted in .sup.15.sup.N. In parallel, N-deficient pea and spinach plants fed with .sup.15.sup.NH.sub.4 .sup.+ .sup.showed an increased level of NH.sub.3 uptake at alkaline pH that was related to the .sup.15.sup.N depletion of the plant. Tolerant to NH.sub.4 .sup.+ .sup.pea plants or sensitive spinach plants showed similar trend on .sup.15.sup.N depletion while slight differences in the time kinetics were observed during the initial stages. The use of RbNO.sub.3 as control discarded that the differences observed arise from pH detrimental effects. This article proposes that the negative values of [delta].sup.15.sup.N in NH.sub.4 .sup.+.sup.-fed plants are originated from NH.sub.3 uptake by plants. Moreover, this depletion of the heavier N isotope is proportional to the NH.sub.4 .sup.+.sup./NH.sub.3 toxicity in plants species. Therefore, we hypothesise that the low affinity transport system for NH.sub.4 .sup.+ .sup.may have two components: one that transports N in the molecular form and is associated w