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Seed germination characteristics of Centaurea diffusa and C. Maculosa Nolan, Daryl Guy


The problematic reinfestation of chemically-treated sites by diffuse and spotted knapweed {Centaurea diffusa and C. maculosa) is thought to occur from dormant seeds in the soil. This study confirmed that reserves of dormant seeds are present in the soil of infested sites, although greater numbers of seeds were recovered from senesced plants. Knapweed plants produce both non-dormant and dormant seeds (germination polymorphism), the relative proportions of which vary between individual plants within a site, as well as between bulk samples collected from different sites. Two types of dormant seeds were identified. Dormancy of some seeds was broken by exposure to red light ('light-sensitive seeds'). Light-sensitivity was evident at 10, 15, 20, 25, and 30°C. Germination in light-sensitive seeds was shown to be mediated by phytochrome. A lesser number of dormant seeds failed to respond to red light ('light-insensitive seeds'). Dry after-ripening released dormancy in both light-sensitive and light-insensitive seeds. However, no apparent loss of dormancy from after-ripening occurred when the relative humidity was too low or too high. At the highest relative humidity level tested (90.7%), dormancy was induced in some seeds while other seeds died. Dormancy was also induced when imbibed seeds were incubated in darkness at 25, 30, 35, and 40°C for 5 days. Dormancy induction was greatly enhanced by incubating submerged seeds in de-oxygenated water (anaerobiosis). However, some seeds died when incubated anaerobically for 5 days. Dormancy was broken in a small percentage of dormant seeds by incubation in a 10 mM solution of potassium nitrate or potassium nitrite; 100 mM potassium nitrite killed most seeds. Gibberellic acid was a much stronger germination stimulant. Some dormant seeds germinated at 25 °C if they were previously chilled at 3°C. To compare laboratory findings with field germination behaviour, seeds from two samples of each species were buried to a depth of about 3 cm in mesh packets during November, April and August near Salmon Arm, B.C. Seeds exhibiting higher levels of germination in darkness in vitro also germinated to higher levels in situ when burial occurred in November. However, burial in April and August led to lower germination levels in situ. Light sensitivity was still prominent following 17 months of burial. Most of the decline in viable seed numbers during burial were attributable to in situ germination. Theoretical discussions of the source of germination polymorphism in knapweed seeds, the importance of light to field germination and seedling mortality, and a potential strategy for controlling these weeds are presented.

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