Abstract
Plants are dependent on exogenous nitrogen (N) supply. Ammonium (NH₄(+)), together with nitrate (NO₃(-)), is one of the main nitrogenous compounds available in the soil. Paradoxically, although NH4 (+) assimilation requires less energy than that of NO₃(-), many plants display toxicity symptoms when grown with NH₄(+) as the sole N source. However, in addition to species-specific ammonium toxicity, intraspecific variability has also been shown. Thus, the aim of this work was to study the intraspecific ammonium tolerance in a large panel of Arabidopsis thaliana natural accessions. Plants were grown with either 1mM NO₃(-) or NH₄(+) as the N source, and several parameters related to ammonium tolerance and assimilation were determined. Overall, high variability was observed in A. thaliana shoot growth under both forms of N nutrition. From the parameters determined, tissue ammonium content was the one with the highest impact on shoot biomass, and interestingly this was also the case when N was supplied as NO₃(-). Enzymes of nitrogen assimilation did not have an impact on A. thaliana biomass variation, but the N source affected their activity. Glutamate dehydrogenase (GDH) aminating activity was, in general, higher in NH4 (+)-fed plants. In contrast, GDH deaminating activity was higher in NO₃(-)-fed plants, suggesting a differential role for this enzyme as a function of the N form supplied. Overall, NH4 (+) accumulation seems to be an important player in Arabidopsis natural variability in ammonium tolerance rather than the cell NH₄(+) assimilation capacity.
Highlights
Plants have a fundamental dependence on inorganic nitrogen (N), and intensive agriculture requires the use of N compounds to supplement the natural supply from the soil
To evaluate N use efficiency (NUE) with ammonium as the sole N source, Arabidopsis rosette biomass was compared after 3 weeks of growth under 1 mM NH4+ [0.5 mM (NH4)2SO4] or 1 mM NO3– [0.5 mM Ca(NO3)2], and the ratio between shoot biomass under NH4+ and NO3– conditions (SB NH4+/NO3–) was used to estimate ammonium tolerance as it has been previously used in other studies (Cruz et al, 2006; Ariz et al, 2011)
Significant natural variability was detected in NH4+ tolerance expressed as the SB NH4+/NO3– ratio, and, interestingly, NH4+ accumulation in NO3-fed plants was the parameter showing the highest relevance, which may indicate an evolutionary adaptation suggesting that plants that under NO3– nutrition are able to accumulate more ammonium could be genetically better adapted to an ammonium-based nutrition
Summary
Plants have a fundamental dependence on inorganic nitrogen (N), and intensive agriculture requires the use of N compounds to supplement the natural supply from the soil. In part because of the intense use of fertilizers, agriculture is a dominant force behind many environmental threats, including climate change and degradation of land and fresh water (Foley et al, 2011; Tilman et al, 2011). Incomplete capture and poor conversion of nitrogen fertilizer causes global warming through emissions of nitrous oxide. Due to these detrimental effects of adding high NO3− concentrations to ecosystems (Gruber and Galloway, 2008), the potential of NH4+ as an N source
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