Induction of germination by a strigol analogue requires ethylene action in Striga hermonthica but not in S. forbesii.
The synthetic strigol analogue GR-24 stimulated germination in 2 strains of S. hermonthica after moistened seeds were first conditioned by incubation at 30°C for ≥7 d. Ethylene at 1.0 µl/litre also promoted germination of conditioned seeds, but to a lesser extent. When approx. 2000 µl/litre 2,5-norbornadiene, a competitive inhibitor of ethylene activity, was provided simultaneously with ethylene or 1.0 mg/litre GR-24, germination was strongly inhibited. The activity of norbornadiene was diminished by applying it 5 h after GR-24, and was lost completely by delaying its 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 to S. hermonthica; although seeds of the former produced ethylene, especially in the presence of GR-24, they did not respond positively to exogenous ethylene. Furthermore, norbornadiene failed to reverse the promoting effect of GR-24 on germination in S. forbesii. It was concluded 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 unresponsive to ethylene, and does not require the gas for GC-24-induced germination.