Germination response of Orobanche seeds subjected to conditioning temperature, water potential and growth regulator treatments

Broomrapes (Orobanche spp.) are parasitic weeds that cause significant losses of crop yield. Experiments were conducted to investigate the seed response to the artificial germination stimulant GR(24) in three species of Orobanche subjected to preconditioning under various temperatures, water potentials and with plant growth regulators. The highest germination percentages were observed in Orobanche ramosa, Orobanche aegyptiaca and Orobanche minor seeds conditioned at 18 degrees C for 7 days followed by germination stimulation at 18 degrees C. With the increase of the conditioning period (7, 14, 21 and 28 days), the germination percentage of O. ramosa and O. aegyptiaca progressively decreased. When conditioned at -2 MPa, the germination percentage was lower than at 0 and -1 MPa, especially at 13 and 28 degrees C. Orobanche minor seeds could retain relatively high germination if conditioned at 18, 23 or 28 degrees C, even after significantly extended conditioning periods (up to 84 days). GA(3) (30-100 mg L-1), norflurazon and fluridone (10-100 mg L-1), and brassinolide (0.5-1.0 mg L-1) increased seed germination, while 0.01 mg L-1 uniconazole significantly reduced germination rates of all three Orobanche spp. The promotional effects of GA(3) and norflurazon and the inhibitory effect of uniconazole were evident, even when they were treated for 3 days. Germination of Orobanche seeds was much lower when the unconditioned seeds were directly exposed to GR(24) at 10(-6) M. This early GR(24)-induced inhibition was however alleviated or even eliminated by the inclusion of GA(3) or norflurazon (10-50 mg L-1) in the conditioning medium. On the contrary, the inclusion of uniconazole increased the inhibitory effect of GR(24), particularly in the case of O. ramosa.