Abstract
Plants are dependent on exogenous nitrogen (N) supply. Ammonium (NH&sub4;(+)), together with nitrate (NO&sub3;(-)), is one of the main nitrogenous compounds available in the soil. Paradoxically, although NH4 (+) assimilation requires less energy than that of NO&sub3;(-), many plants display toxicity symptoms when grown with NH&sub4;(+) 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 NO&sub3;(-) or NH&sub4;(+) 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 NO&sub3;(-). 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 NO&sub3;(-)-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 NH&sub4;(+) assimilation capacity.