Responsiveness of Orobanche ramosa L. Seeds to GR 24 as Related to Temperature, Oxygen Availability and Water Potential During Preconditioning and Subsequent Germination

Broomrape (Orobanche ramosa L.) is a root holoparasite responsible for important yield losses in numerous crops, particularly in the Mediterranean area. In this paper, the effects of temperature, oxygen concentration and water potential of the medium on broomrape seed germination were investigated. Seeds became able to germinate in the presence of a strigol analogue (GR 24) only after a preincubation period for at least 3 days at 20 °C. Their responsiveness to GR 24 increased with increasing duration of their preconditioning at 20 °C, and was optimal after 2–3 weeks. The preconditioning treatment was effective at temperatures ranging from 10 to 30 °C. At the optimal temperature (20 °C), it required at least 1% oxygen in the atmosphere and remained effective at a water potential of the medium of –2 MPa. A too prolonged preincubation of seeds at sub- or supraoptimal temperatures (5 and 30 °C) resulted in induction of a secondary dormancy. Seeds preconditioned for 14 days at 20 °C germinated in the presence of 1 mg L^–1 GR 24 at temperatures ranging from 10 to 25 °C, and the thermal optimum was the same (20 °C) than that of preconditioning. At 20 °C, seeds were able to germinate in the presence of GR 24 under atmospheres containing at least 3% oxygen and at a water potential of the medium as low as –3 MPa. The differences observed in the effects of environmental factors on preconditioning efficiency and germination of preconditioned seeds suggest that both processes involve different mechanisms. The results obtained might also help to better understand the regulation of O. ramosa spread in temperate areas.