The effect of Cu2+ on uptake and assimilation of ammonium by cucumber seedlings

The ammonium uptake by cucumber seedlings was estimated from ammonium ions depletion in an uptake solution. The uptake of NH4+ was decreased by about 60% after one hour and by about 90% after two hours of 100 mu M Cu2+ treatment. On the contrary the accumulation of ammonium in roots of Cu2+-treated seedlings at the same time was higher than in the control. Cu2+ in the concentration inhibiting NH4+ absorption during one hour inhibited also glutamine synthetase (GS) (EC 6.3.1.2) and NADH-glutamate dehydrogenase (NADH-GDH) (EC 1.4.1.2) activities both localized in the roots of seedlings. After one hour and at least up to the 4th hour Cu2+ accumulated mainly in roots (95%). It was probably the reason of the GS activity in cotyledons of seedling treated with Cu2+ that it was at the same level as in the control. NADH-GDH activity in cotyledons after one hour of the Cu2+ treatment was lower than in the control but the influence of Cu2+ action on the activity of this enzyme in roots was by far stronger. 100 mu M Cu2+ did not affect the activities of both enzymes in in vitro experiments. Copper added into the incubation medium in 1000 mu M concentration decreased GS activity, but still did not change NADH-GDH activity. These results suggested the indirect Cu2+ action on the investigated enzymes in in vitro experiments. However, no substantial effect on enzyme activities extracted from control plants was observed after the addition of the extract from Cu2+-treated plants into the incubation medium. The data suggest that the influence of Cu2+ on uptake and assimilation of ammonium may be connected not only with changes of plasma membrane properties not only with changes of plasma membrane properties in the root cells of Cu2+ treated seedlings but also with Cu2+ action on two major enzymes involved in NH4+ assimilation: glutamate synthetase and NADH-glutamate dehydrogenase.