Uptake and assimilation of ammonium ions by cucumber seedlings from solutions with different pH and addition of heavy metals

Abstract

Influence of Cu²⁺, Cd²⁺, Pb²⁺ and Fe²⁺ in various pH conditions (5.0, 6.0, 7.0) on the uptake and assimilation of NH₄⁺ by cucumber seedlings was estimated. Every metal in different pH of uptake solution distinctly reduced ammonium absorption calculated from NH₄⁺ depletion. Copper and ferric ions, but not cadmium or lead, astonishingly decreased the uptake of ammonium from solution at pH 5.0. Cu²⁺ was also very active at pH 6.0. The accumulation of ammonium in roots of metals-treated seedlings at the same time was high. The high level of ammonium in root cells despite of its low uptake probably resulted from disturbance in NH₄⁺ assimilation. Both glutamine synthetase (GS) and NADH-glutamate dehydrogenase (NADH-GDH) - the major enzymes in ammonium assimilation, were inhibited after one hour of plant exposition to the metals. Similarly as in the case of ammonium uptake, the influence of pH was visible only in combination with copper and ferric ions. The strongest reduction of enzyme activities was observed for Cu²⁺ and Fe²⁺, in pH 5.0 and 6.0. The various metals absorption by roots from solutions with different pH was not dedected. The data show correlation between metal inhibition of GS and NADH-GDH activities and metal inhibition of ammonium absorption.