Effects of dry heat on the germinability and viability of Scotch thistle (Onopordum acanthium) cypselas: Interpopulation and interposition variation

Cypselas (fruits) of four local populations of Scotch thistle, Onopordum acanthiun L., were stored for 3 months, heated gradually from room temperature to 50, 75, or 100°C and then gradually cooled to determine effects on germination. Populations responded similarly to this treatment. The lowest germination percentages and slowest rates followed the 100°C treatment, while the fastest germination rates followed 75°C. In a direct-heating experiment, cypselas (from central and peripheral positions in the capitulum) of one population were stored for 41/2 months and then exposed directly to 50, 75, or 100°C for 0.5, 1, or 2 min. This treatment at 75 and 100°C increased germination percentages, with higher percentages for the central cypselas. The 2 min treatment at 100°C yielded the highest percentage and fastest germination rate. Central cypselas were heavier but were similar to peripheral ones in imbibition and in the ratio between cypsela coat and embryo weights. In subsidiary experiments, wax-removed cypselas had smoother surfaces, heated cypselas had visible surface cracks, and germinated cypselas had thinner pericarps. Increases in the percentage and rate of germination in response to dry heat suggest that seedlings of O. acanthium may emerge sooner and in greater numbers following a fire event on disturbed or agricultural lands.