Seed dormancy cycling: A driver of germination timing in a facultative winter annual

Timing of seed germination is critical for survival of annual plants in seasonal climates. We tested the hypothesis that seeds of the winter annual species Cardamine impatiens (Brassicaceae) "track" their thermal environment and synchronize germination with favorable growth conditions. We predicted that seeds buried in the field from maturity to autumn germinate best at autumn temperatures, while those buried from maturity to spring germinate best in early spring. We monitored seasonal changes in germinability by exhuming field-buried seeds monthly for 30 months and incubating them under laboratory conditions. The effects of temperature on the transition of dormancy status also were investigated. Seeds of C. impatiens were dormant at dispersal in May, and during summer dormancy transitioned to conditional dormancy and then to non-dormancy. By early autumn, seeds germinated in a wide range of temperature regimes in light. Nongerminated seeds re-entered conditional dormancy during winter, losing the ability to germinate at high, but not low, temperatures in light. The light requirement for germination was reduced during prolonged seed burial. Overall, our hypothesis is supported. Buried seeds of C. impatiens exhibited a seasonally synchronized conditional dormancy/non-dormancy cycle, enabling germination in both autumn and early spring; this information will facilitate management efforts of this weedy species. We conclude that dormancy cycling in C. impatiens is an adaptive functional trait that controls the timing of germination, thereby optimizing seedling emergence under favorable conditions while avoiding summer heat.