Ammonium and potassium effect on nitrate assimilation in cucumber seedlings

Abstract

The effect of ammonium present in the induction medium toghether with nitrate on the activity of nitrate reductase (NR), nitrite reductase (NiR), glutamic acid dehydrogenase (GDH) and absorption and accumulation of NO<sub>3</sub><sup>-</sup> in cucumber seedlings were investigated. Maximum NR and NiR activity in the cotyledons was observed when seedlings were supplied with KNO<sub>3</sub> as the sole source of nitrogen. When plants were supplied with NH<sub>4</sub>NO<sub>3</sub> the presence of NH<sub>4</sub><sup>+</sup> in the induction medium repressed by about 50 per cent the activity of both reductases in the cotyledons. Addition of K<sup>+</sup> to this medium abolished completely the inhibitory effect of NH<sub>4</sub><sup>+</sup>. The effect of K<sup>+</sup> cannot be replaced by that Na+ ions. On the other hand, ammonium has no effect on the level of NR activity in roots, while NiR was almost completely repressed. Under the experimental conditions ammonium, in the presence of nitrates, decreased the activity of GDH, but this diminution did not occur when the plants were supplied with K<sup>+</sup> simultaneously. It has found that NH<sub>4</sub><sup>+</sup> ions reduced NO<sub>3</sub><sup>-</sup> absorption but at the same time, the ratio of NO<sub>3</sub>- absorbed to that reduced was increased more than twice. The presumable mechanism of these phenomena is discussed.