Resumen de Depletion of the heavier N isotope is associated with NH4+/NH3 toxicity in NH4 +-fed plants
Idoia Ariz Arnedo, Cristina Cruz Louçao, José Fernando Morán Juez, Iván Jáuregui Mosquera, Pedro María Aparicio Tejo
In plants, nitrate (NO3-) nutrition gives rise to a natural isotopic signature (δ15N) whichcorrelates with the δ15N level of the N source. However, little is known about 14N/15Nfractionation in plants under ammonium (NH4+) nutrition. We have studied several plant specieswith different NH4+-sensitivities under hydroponic culture with NO3- or NH4+ as the sole Nsource, and determined plant growth and δ15N.In parallel, N-deficient plants were fed with 15NH4+ at pH 6.0 or 9.0 (which favours NH3 form)for short periods and 15N uptake was recorded during the initial stages.NO3--fed plants were consistently enriched in 15N whereas all studied plants under NH4+nutrition became depleted in 15N. It was shown that those plants most sensitive to NH4+ toxicitywere the most depleted in 15N. Additionally, short-term NH4+/NH3 uptake experiments showedan increased NH3 uptake at alkaline pH that was related to the plant’s 15N depletion.Based on these results, this paper proposes that the low affinity uptake system for NH4+ has twocomponents, one of which transports N in the molecular form (NH3) and is associated withdiscrimination fractionation, whereas the other transports N in the ionic form and is notassociated with fractionation.
