ABSTRACT
The effect of NH[4]Cl either as a sole nitrogen source or together with NO[3], on nitrate reductase activity [NR] of the water fern, Azolla caroliniana was studied. Ammonium ions induced slightly the NR activity until 1.0 mM, then the activity decreased. When NH[+4]was added to the induction medium [2/5 N[2-]free Hoagland solution plus 5.0 mM N0[3]] a marked decrease in NR activity was -recorded. Nitrate uptake, assimilation and NR activity of Azolla plants pretreated with 1.0 mM NH[4]for 0,1,2,3 or 4 days were also studied. The uptake of NO[-3]was roughly the same, while NO[3]assimilation was greatly suppressed and, accordingly, accumulation of NO[3]was markedly increased. Maximum NR activity was shown with the non-treated Azolla plants. Ammonium uptake and NTR activity of previously induced or non-induced plants were also investigated. Induced Azolla plant showed a gradual increase in NH[-4]uptake and a marked decrease in NR activity throughout the whole experimental period. However, the non-induced plants absorbed much greater amounts of ammonia than did the induced ones