The role of ammonium nitrogen in the distribution of two species of Elodea

The impact of ammonium nitrogen on the physiology of two species of Elodea, Elodea nuttallii St John and E. canadensis Michx. was evaluated in laboratory experiments, as was its role in their distribution in situ. The plants were grown in a system of glass aquaria on nutrient solutions containing 5 mg.l(-1) nitrogen at selected ratios of ammonium and nitrate nitrogen. The growth rate of plants, the Photosynthetic Activity (AP) and Nitrate Reductase Activity (NRA) of Elodea nuttallii St John and E. canadensis Michx. were investigated in two types of experiments. In the first type of experiment where the nitrate concentration was maintained constant at 2 mg.l(-1), high concentrations of ammonium nitrogen stimulated the photosynthesis of E. nuttallii and inhibit the AP of E. canadensis. Ammonium nitrogen caused a distinct inhibition of the enzyme activity (determined by in vivo assay) of Elodea canadensis whereas the nitrate reductase activity of E. nuttallii was greatly enhanced by concentrations of ammonium nitrogen of less than 0.5 mg.l(-1) (maximal rate of NRA was obtained with 0.5 mg.l(-1) of ammonium nitrogen). In the second type of experiment, when ammonium and nitrate were applied simultaneously, maximal NRA occurred when 3/4 of the nitrogen was supplied as ammonium for E. nuttallii whereas maximal activity was obtained in absence of ammonium for E. canadensis. The interspecific differences in response to ammonium nitrogen intoxication could explain their trophic status in situ.