Interpopulation variation in germination responses of Scotch thistle, Onopordum acanthium L., to various concentrations of GA₃, KNO₃, and NaHCO₃

The effects of GA₃, KNO₃, and NaHCO₃ on germination percentages and rates were determined for cypselas (seeds) of four local populations of Scotch thistle, Onopordum acanthium L., collected during a 2-year period. Three (1996, stored cypselas) and four (1997, fresh cypselas) concentrations (0.025, 0.05, and 0.1% for both years, plus 0.0125% for 1997) of the three compounds were used. Both freshly collected cypselas and those stored for 67 days at room temperature exhibited a range of germination responses from readily germinable to strongly dormant. GA₃, even at a very low concentration, caused most viable cypselas to germinate, but 5-6% of viable cypselas in two populations did not respond to any GA₃ treatment. In general, dry-stored cypselas germinated faster than fresh ones. Populations with more germinability (= proportion of cypselas capable of germinating) responded faster to GA₃ than those with less germinability. KNO₃ usually increased germination percentages but reduced the rate of germination. When treated with KNO₃, stored cypselas from the two populations with less germinability germinated more slowly than those of the other two. In general, germination percentage decreased with an increase in NaHCO₃ concentration. Overall, stored cypselas that were treated with NaHCO₃ germinated more slowly than those in the control. The large differences among four local populations in response to these chemicals suggest that the effects of germination stimulators and other chemicals should be observed on several populations per species.