SummaryThe effect of nitrogen (ammonium, urea and nitrate), applied during a conditioning or germination phase, on germination ofOrobanche crenataForsk, seeds was investigatedin vitro.Ammonium sulphate (8 mM) applied during conditioning in combination with the nitrification inhibitor nitrapyrin, reduced germination of seeds from 46 to 26. A lower concentration of 4 mM ammonium sulphate or supplying nitrogen as 8 mM urea or 16 mM nitrate did not inhibit germination. Applied after conditioning during the germination phase, 4 mM ammonium sulphate strongly inhibited germination, especially in combination with a nitrification inhibitor: germination was reduced to less than 5. Urea (8 mM) reduced germination to only a limited extent (from 58 to 40). When a urease inhibitor (carbon disulfide) was additionally applied, germination percentage was not affected. Nitrate had no effect. It is hypothesized that the inhibition by ammonium ofO. crenatagermination is connected with a reduced ability to detoxify ammonium, owing to the reduced activities of the enzymes GS and GOGAT. The inhibitory effect of urea may be indirect, occurring through conversion to ammonium. The effect of a short‐term exposure to ammonium was largely reversible and no increase in mortality could be detected. The presence of 6 mM phosphate, Hepes or Mops buffer was found to intervene with the inhibition by ammoniu
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