The influence of the rate of temperature change on the activation of dormant seeds of Rumex obtusifolius L.

1. One temperature shift from 20 to 30°C in darkness induces 30–40% germination in Rumex obtusifolius seeds. The same germination percentages are found with heat treatment varying between 1 and 6h duration, indicating that the total heat sum of the temperature shift is not important.
2. Germination is greatly enhanced by three consecutive heat shifts of 1h at 30°C separated by 1h periods at 20°C.
3. The seeds are activated to a small extent after a slow warming (+2°Ch^–1) from 20 to 30°C, followed by incubation for 1h at 30°C. Germination is much higher after rapid heating (+10°Ch^–1) to 30°C, followed by 1h incubation at this temperature. Repeated fast heating treatments on four consecutive days enhances germination. Moderately rapid heatings (+3·3°Ch^–1) give intermediate results.
4. The rate of cooling does not influence the germination percentage. Cooling alone cannot induce germination.
5. Heating alone from 15 to 25°C without cooling also activates germination. In this temperature range the seeds are more activated by rapid warming than by slow warming.
6. The ecological relevance of the response to different warming rate is discussed. The insensitivity of seeds to a slow warming might keep deeply buried seeds in a dormant stage.