Possible involvement of gibberellins and ethylene in Orobanche ramosa germination

Six inhibitors acting at different stages of gibberellin biosynthesis, applied during conditioning of Orobanche ramosa seeds, reduced subsequent germination in the presence of GR 24 (a strigol analogue). Ethylene seems to be involved in the induction of germination of conditioned seeds by GR 24 , as inhibitors of its synthesis or action, applied to conditioned seeds, also strongly reduced induction of germination by GR 24 . Exogenous ethylene did not induce germination of conditioned seeds, but 2-chloroethylphosphonic acid was able to do so. When inhibitors of gibberellin biosynthesis were applied to conditioned seeds in the presence of GR 24 , they inhibited germination. These same inhibitors also strongly inhibited germination of conditioned Striga hermonthica seeds in response to GR 24 ; this inhibitory effect was reversed by the addition of 2-chloroethylphosphonic acid. The effect of these inhibitors on S. hermonthica, in which ethylene is a necessary mediator of germination induction by GR 24 , strongly suggests that ethylene synthesis is also required for the induction of O. ramosa seed germination by GR 24 . These growth regulators, which inhibit the two steps of germination in O. ramosa, could be useful for the development of methods for early season control of this parasite.