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 NO₃^- in cucumber seedlings were investigated. Maximum NR and NiR activity in the cotyledons was observed when seedlings were supplied with KNO₃ as the sole source of nitrogen. When plants were supplied with NH₄NO₃ the presence of NH₄⁺ in the induction medium repressed by about 50 per cent the activity of both reductases in the cotyledons. Addition of K⁺ to this medium abolished completely the inhibitory effect of NH₄⁺. The effect of K⁺ 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⁺ simultaneously. It has found that NH₄⁺ ions reduced NO₃^- absorption but at the same time, the ratio of NO₃- absorbed to that reduced was increased more than twice. The presumable mechanism of these phenomena is discussed.