Seeds (mericarps) of Ptilimnium nuttallii have linear, underdeveloped embry
os that must grow from about 0.2 to 1.3 mm in length before the radicle eme
rges from the seed (fruit); thus, seeds have morphological dormancy (MD). i
n 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 brok
e PD, before they would germinate. The high temperature requirement for los
s of PD means that seeds have nondeep simple MPD. Seeds buried in soil in J
uly 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 dorman
cy/nondormancy cycle with respect to PD, and MD can be broken only while se
eds are not physiologically dormant. Light was required for embryo growth i
n 55-98% of the seeds, but the light requirement varied with year. Conseque
ntly, embryo growth and germination are restricted to autumn, and seeds on
the soil surface are more likely to germinate than those buried in soil. Re
sults of this study indicate that P. nuttallii is a strict winter annual wi
th a long-term soil seed bank. The annual dormancy/nondormancy cycle would
prevent germination in spring when high water levels in the habitat might b
e inhibitory for successful seedling establishment.