Seed dormancy in the wetland winter annual Ptilimnium nuttallii (Apiaceae)

Seeds (mericarps) of Ptilimnium nuttallii have linear, underdeveloped embryos that must grow from about 0.2 to 1.3 mm in length before the radicle emerges from the seed (fruit); thus, seeds have morphological dormancy (MD). in addition to MD, about 70% of the seeds also have physiological dormancy (PD) at maturity in mid-July; thus, seeds have morphophysiological dormancy (MPD). Whereas fresh seeds with only MD (26-29%) germinated in light during 2 weeks of incubation at 15/6, 20/10, and 25/15 degrees C, those with MPD required exposure to high temperatures (25/15, 30/15 degrees C), which broke PD, before they would germinate. The high temperature requirement for loss of PD means that seeds have nondeep simple MPD. Seeds buried in soil in July 1991 and exposed to natural seasonal temperature changes came out of PD during the summers of 1991-1996, and those failing to germinate in autumn of each year re-entered PD during winter. Thus, seeds have an annual dormancy/nondormancy cycle with respect to PD, and MD can be broken only while seeds are not physiologically dormant. Light was required for embryo growth in 55-98% of the seeds, but the light requirement varied with year. Consequently, embryo growth and germination are restricted to autumn, and seeds on the soil surface are more likely to germinate than those buried in soil. Results of this study indicate that P. nuttallii is a strict winter annual with a long-term soil seed bank. The annual dormancy/nondormancy cycle would prevent germination in spring when high water levels in the habitat might be inhibitory for successful seedling establishment.