The response to ammonium- and nitrate-nitrogen of seedlings of the calcicole orchid species Dactylorhiza incarnata (L.) Soó was tested in axenic in vitro culture of c. 3-momh-old protocorms. A pronounced toxicity of ammonium ions was observed. Seedlings raised from plants of a coastal population (f. dunensis (Druce) Soó) and those from plants of an inland population (f. incarnata) showed a different response to nitrogen form. Although ammonium toxicity was similar for seedlings of both sites, those of the coastal population showed a positive response to nitrate ions whereas those of the inland population did not. These differences could not be interpreted unequivocally as population differentiation. Ammonium toxicity was reduced in alkaline conditions. Yields increased with increasing pH and reached a maximum at pH 8.0. Calcium concentrations of 1 mM were optimal, but calcium levels did not affect the ammonium response. Seedlings did not respond to potassium concentrations in the range of 0.0–4.0 mM superimposed on a basal level of 0.4 mM. No interaction could be demonstrated between ammonium toxicity and potassium levels. Response to pH and ammonium provides a plausible mechanism for the sensitivity of the species to soil decalciftcation and decrease in seepage intensity of calcium rich groundwater in the field.
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