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SUMMARYThe response to ammonium-and nitrate-nitrogen of seedlings of the calcicole orchid species Dactylorhiza incarnata (L.) Sod was tested in axenic in vitro culture of c. 3-month-old protocorms. A pronounced toxicity of ammonium ions was observed. Seedlings raised from plants of a coastal population (f. dunensis (Druce) Soo) 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 hoth 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, .\nnmonium 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 decaicification and decrease in seepage intensity of calcium rich groundwatrer in the field.

The effect of mycorrhizal infection on the response to mineral nitrogen was studied in Orchis morio L., Dactylorhiza praetermissa (Druce) So6 var. junialis (Verrnln.) Sengh., Dactylorhiza rnajalis (Reichb.) Hunt & Summerh., and Dactylorhiza incarnata (L.) So6, using two strains of Ceratorhiza sp. and two of Epulorhiza repens (Bernard) Moore as mycorrhizal symbionts. Both types of endophyte appeared to be able to stimulate orchid growth at relatively low nitrogen availability in the culture medium. The Ceratorhiza strains decreased nitrogen tolerance of associated orchid protocorms and had pronounced negative effects on the growth and survival of D. majalis and D. incarnata at high concentrations of ammonium nitrate. Such effects did not occur in symbioses involving the Epulorhiza strains. Within orchid species, differences were observed in the extent to which fungal strains stimulated growth. The Ceratorhiza isolates were incompatible with 0. morio while the Epulorhiza strains were generally less efficient for Dactylorhiza spp.Resume : En utilisant deux souches de Ceratorhiza sp. et deux de 1'Epulorhiza repens (Bernard) Moore, comme champignons rnycorhiziens symbiotiques, les auteurs ont CtudiC les effets de la colonisation rnycorhizienne sur la rCaction de I'Orchis mori L., du Dactylorhiza praetermissa (Druce) Soo var. junialis (Vermin.) Sengh., du Dactylorhiza majalis (Reichb.) Hunt & Summerh. et du Dactylorhiza incarnata (1.) Soo, h la nutrition azotCe. Les deux types d'endophytes sernblent capables de stimuler la croissance des orchidCes en prCsence de quantitC relativement faibles d'azote disponible dans le milieu de culture. Les souches de Ceratorhiza diminuent la tolCrance i l'azote des protocormes d'orchidCes associCs, et exercent des effets nCgatifs marquCs sur la croissance et la survie du D. majalis et du D. incarnata aux fortes concentrations de nitrate d'arnmonium. De tels effets ne se retrouvent pas dans les symbioses impliquant les souches d'Epulorhiza. Pour une espbce d'orchidCes donnCe, on observe des diffkrences dans le degrC de stimulation exercC par les souches fongiques. Les isolats du Ceratorhiza sont incompatibles avec 1'0. mori, alors que les souches d'Epulorhiza sont gCnCralement rnoins efficaces pour les Dactylorhiza spp.

SUMMARY The effects of nitrogen, potassium and phosphorus fertilization on performance of Dactylorhiza majalis (Rchb.f.) Hunt & Summerh. were studied in a Juncus acutiflorus‐dominated hayfield. Frequency, total shoot biomass and flowering incidence decreased as a result of both nitrogen and phosphorus application. Nutrient additions also affected yields of species showing positive and negative association with orchid performance. These changes were sufficient in explaining the nitrogen and phosphate response of orchid shoot biomass and extent of flowering. The most important competitors in this respect were Rumex acetosa and Holcus lanatus. The negative overall N‐ and P‐effect on orchid frequency seemed to be independent of changes in the surrounding vegetation however. The relative importance of nutrient limitations and additions is discussed in relation to the species decline in frequency and to its reappearance in grasslands under restoration management with decreasing nutrient availability.

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