Variation in germination response among local populations of Scotch thistle, Onopordum acanthium L.

Cypselas of Scotch thistle, Onopordum acanthium L., collected from four populations in or near London, Ontario in September 1996, were incubated under seven diurnal light and temperature regimes (35/20, 25/10 and 10/5°C, both under alternating light and darkness and continuous dark conditions, plus a 20°C dark treatment). Although all populations originated on gravel soil of similar pH near the Thames River, there were large differences among populations in both germination rates and final percentages in several treatments. At 25/10°C, cypselas from all populations had 3.5 to 13 times as much germination in alternating light and dark as in continuous darkness. At 35/20°C three of the populations had higher germination percentages in continuous darkness than under alternating light and dark. Under both 10/5°C treatments, as well as at 20°C in darkness, less than 8% germination was recorded for any population. Under alternating light and dark, cypselas of all populations germinated fastest in the 25/10°C treatment. Variation in germination response among local populations gives a species a greater range of responses and a better chance of surviving changeable environmental conditions.