Production of clover broomrape seed germination stimulants by red clover root requires nitrate but is inhibited by phosphate and ammonium

The effect of nutrients (nitrate, ammonium, urea, phosphate and potassium) on the production and/or exudation of germination stimulants for clover broomrape (Orobanche minor Sm,), a root holoparasite, by its host red clover (Trifolium pratense L,) was examined using hydroponically grown material. Potassium (K2SO4) concentrations up to 100 mg l(-1) (based on P) did not affect the production of germination stimulants by red clover while, in contrast, phosphate (NaH2PO4) was highly inhibitory even at concentrations as low as 1 mg l(-1) (based on P). Nitrate (NaNO3) markedly promoted stimulant production in a dose-dependent manner from 2 to 50 mg l(-1) (based on N). Ammonium [(NH4)(2)SO4] had no effect at 2 mg l(-1) (based on N) but was inhibitory at higher concentrations. Ammonium is known to be a seed germination inhibitor of root parasites, indicating that ammonium has a dual inhibitory action. Urea had no effect at 2 mg l(-1) (based on N) but was promotive at higher concentrations. These results provide a basis for the inhibitory effects of nitrogen fertilizer on infection by root parasitic weeds, broomrapes and witchweeds, and explain why these parasites prevail in areas where there is lower phosphorus availability in soils.