AXENIC IN-VITRO NITROGEN AND PHOSPHORUS RESPONSES OF SOME DUTCH MARSH ORCHIDS

Seedlings of Dactylorhiza majalis (Reichb.) Hunt & Summerh., D. praetermissa (Druce) Soo var.junialis (Vermen.) Sengh., D. incarnata (L.) Soo and D. maculata (L.) Soo ssp. maculata were grown axenically in vitro at various concentrations of ammonium nitrate and phosphate to assess the extent of growth depression owing to high nutrient availability. Only large nitrogen concentrations had a general negative effect on orchid axenic growth. Responses to both nutrients were highly species-specific. D. praetermissa responded positively to addition of mineral nitrogen in the media, whereas D. incarnata showed the most negative response. D. majalis differed in showing preference for the greatest phosphate concentrations, in contrast to D. praetermissa. Axenic nutrient responses might explain differences in distribution patterns between the tested species in the field.