THE EFFECT OF CU2+ ON UPTAKE AND ASSIMILATION OF AMMONIUM BY CUCUMBERSEEDLINGS

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 m u M Cu2+ treatment. On the contrary the accumulation of ammonium in ro ots of Cu2+-treated seedlings at the same time was higher than in the control. Cu2+ in the concentration inhibiting NH4+ absorption during o ne hour inhibited also glutamine synthetase (GS) (EC 6.3.1.2) and NADH -glutamate dehydrogenase (NADH-GDH) (EC 1.4.1.2) activities both local ized 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 r eason of the GS activity in cotyledons of seedling treated with Cu2+ t hat it was at the same level as in the control. NADH-GDH activity in c otyledons after one hour of the Cu2+ treatment was lower than in the c ontrol 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 activi ties of both enzymes in in vitro experiments. Copper added into the in cubation medium in 1000 mu M concentration decreased GS activity, but still did not change NADH-GDH activity. These results suggested the in direct Cu2+ action on the investigated enzymes in in vitro experiments . However, no substantial effect on enzyme activities extracted from c ontrol 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 connec ted not only with changes of plasma membrane properties not only with changes of plasma membrane properties in the root cells of Cu2+ treate d seedlings but also with Cu2+ action on two major enzymes involved in NH4+ assimilation: glutamate synthetase and NADH-glutamate dehydrogen ase.