Only 11 d afterripening (DAR) was used for seeds from the weedy strains and cultivars because we found this period of time was appropriate to distinguish difference in degree of dormancy between the two categories of genotypes (Foley et al., 2003).

It is most likely a combination of factors that led to the relatively high germination of achenes following warm pretreatment, but this period may have allowed or stimulated the process of afterripening. The use of heat as a pretreatment is similar to the exposure of achenes to the warm summer temperatures in their natural environment.

Germination response patterns to temperature during afterripening of achenes of four Texas winter annual Asteraceae.

arvense (LaLonde and Roitberg, 1994), Pitcher's thistLe seeds are dormant at the time of dispersal in the fall and dormancy is broken by a combination of afterripening and moist chilling.

Germination of intact seeds increased to >90% by afterripening for an additional 3 d past their normal drying period (data not shown).

However, for both species some afterripening may have occurred in the laboratory before the seeds were tested.

They often possess inhibitory mechanisms, including seed coat impermeability, afterripening requirements, or light and temperature sensitivities which must be overcome with specific stimuli or treatments before germination is triggered (Nikolaeva, 1977; Baskin and Baskin, 1991).

Some cultivars may have up to 95% of their seeds dormant at the time of harvest, and those seeds may need as long as 2 yr of afterripening to become germinable.

Although pasture establishment and management for atra paspalum have been studied (Kalmbacher et al., 1997a), little is known about seed dormancy and aging except for an observation that its seed underwent afterripening. In preliminary experiments conducted in 1992, 28-d germination at 65 d after harvest was only 7%, but it increased to 59% at 106 d.