PRIMARY SEED DORMANCY IN DIFFUSE AND SPOTTED KNAPWEED

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Abstract
Large numbers of viable, diffuse (Centaurea diffusa Lam.) and spotted knapweed (C. maculosa Lam.) seeds (achenes), collected in the interior of British Columbia, failed to germinate in darkness at 25 °C. This primary dormancy was released to varying degrees by gibberellic acid, exposure to red light, or excision of the distal end of the seed. The effect of red light was negated by subsequent exposure to far-red light. The demonstration of red/far-red reversibility implicates the phytochrome pigment system in the light-sensitive germination of knapweed seeds. Seeds collected from different sites, and from individual plants within sites, had different germination levels in darkness and following exposure to 2 min of red light. Three types of germination behavior were evident: nondormant seeds germinated in darkness; light-sensitive dormant seeds germinated in response to red light; and light-insensitive dormant seeds failed to germinate after 5 d of continuous red light. Seeds of all three germination types were found on individual plants.Key words: Centaurea diffusa, Centaurea maculosa, knapweed, seed dormancy, light-sensitive germination, germination polymorphism