INDUCTION OF GERMINATION BY A STRIGOL ANALOG REQUIRES ETHYLENE ACTION IN STRIGA-HERMONTHICA BUT NOT IN S-FORBESII

The synthetic strigol analogue GR-24 stimulated germination in two strains of Striga hermonthica (Del.) Benth., after moistened seeds were first <conditioned> by prior incubation at 30-degrees-C for at least 7 d. Ethylene (1.0-mu-L L-1) also promoted germination of conditioned seeds, but to a smaller extent. When 2,5-norbornadiene, a competitive inhibitor of ethylene action (approx. 2000-mu-L L-1), was given simultaneously with ethylene or GR-24 (1.0 mg L-1), germination was strongly inhibited. The activity of norbornadiene was diminished by applying it 5 h after GR-24, and lost completely by delaying the application for 17 h. All seeds produced ethylene but rates were increased substantially by GR-24 in conditioned seeds. The ability of GR-24 to stimulate ethylene production was not a consequence of promoting germination, since its ability to increase ethylene production was retained even when norbornadiene was supplied to block germination. S. forbesii behaved differently from S. hermonthica. Although seed of this species produced ethylene, especially in the presence of GR-24, it did not respond positively to exogenous ethylene. Furthermore, norbornadiene failed to reverse the promoting effect of GR-24 on germination. We conclude that in S. hermonthica, germination results from the joint action of GR-24 and the extra endogenous ethylene GR-24 induces. However, S. forbesii is unresponsiveness to ethylene, and does not require the gas for GR-24-induced germination.