Abstract
Ammonium is an important N source, but application of excess ammonium incurs
growth suppression. This ammonium toxicity is supposed to be due to deficiency of C skeleton, followed by increased ammonium assimilation. This ammonium toxicity is alleviated by supplying nitrate. The purpose of this study is to reveal whether the ammonium toxicity is due to deficiency of C skeleton. We focused whether nitrate can rescue growth suppression by supplying C skeleton. We measured C/N metabolites in shoots of Arabidopsis thaliana seedlings transferred to different inorganic N conditions. High ammonium nutrition caused remarkable growth suppression, which was rescued by nitrate. High ammonium nutrition increased amino acids and decreased organic acids, irrespective of nitrate. Starch and sugars accumulated in high ammonium nutrition, but these accumulation were decreased by nitrate. These results indicated that the ammonium toxicity is due to nitrate deficiency or carbohydrate accumulation, not deficiency of C skeleton.
