Influence of nitrogen on germination and early development of broomrape (Orobanche spp.)

Broomrapes are parasitic angiosperms that live on the roots of many importa nt crop plants We have conducted a series of in vitro experiments to invest igate the direct effects of nitrogen fertilizers on broomrape seed germinat ion and early development. These experiments confirm that nitrogen in the a mmonium form is more inhibitory than nitrate, but we conclude that it is th e elongation of the seedling radicle that is primarily inhibited by ammoniu m, rather than the seed germination itself. Reductions in radicle length we re observed when ammonium solutions were applied during either precondition ing or germination periods. For germinating seeds, exposures to ammonium su lfate of 4 to 8 h (depending on the species) reduced radicle elongation by half, indicating a relatively rapid inhibition. Broomrape species varied in sensitivity to ammonium, with Orobanche aegyptiaca being the most tolerant , followed by O. crenata, O. ramosa, O. cernua and finally O. minor as comp ared in the presence of 5 mM ammonium. However, all broomrape species were much more sensitive to ammonium toxicity than were host crop species under similar treatment conditions.