Ecophysiology of Secondary Dormancy in Seeds of Ambrosia Artemisiifolia

As buried seeds of Ambrosia artemisiifolia came out of primary and secondary dormancy, they showed similar germination responses. In early spring at simulated habitat temperatures, seeds germinated well in light but not in darkness, but they germinated well in darkness at temperatures characteristic of the habitat in late spring and summer. By late spring, seeds had entered secondary dormancy and did not germinate at any temperature in light (14—h daily photoperiod) or darkness. Prolonged exposure to 5°C in darkness did not decrease germination in either light or darkness at a 12—h alternating temperature regime of 30°/15°C, but seeds subsequently kept at 15°/6°C in darkness for 1 mo required light for germination. Seeds then lost the ability to germinate in light as they were exposed sequentially to 20°/10°, 25°C/15°C, and 30°C/15°C in darkness. Thus, upon exposure to the sequence of natural temperature regimes in spring, induction of secondary dormancy of seeds in darkness occurs in two physiological stages. While buried seeds are at low marginal temperatures for germination they lose the ability to germinate in darkness, and then at higher temperatures they lose the ability to germinate in light.