Multiple mechanisms for seed dormancy regulation in shadscale (Atriplex confertifolia: Chenopodiaceae)

Shadscale (Atriplex confertifolia (Torr. & Frem.) Wats.) is a dominant shrub species of western North American deserts. Its one-seeded utricles are enclosed in bracteoles, and its seeds are highly dormant at dispersal. We examined the role of soluble inhibitors, bracteole effects, after-ripening, and chilling in regulating dormancy for six shadscale populations. Seeds in bracteole-enclosed fruits required chilling to become germinable, and the fraction responsive to chilling was small for recently harvested seeds. This fraction increased during dry storage for 24 weeks, but the rate of increase decreased asymptotically. After-ripening rate was an exponential function of storage temperature over the range 22-50degreesC. After-ripening models based on these nonlinear relationships for each of four seed populations accounted for 96% of the change in the chilling-responsive fraction during 3 years of laboratory storage. Rate of change in the chilling-responsive fraction during laboratory storage was log-linearly related to the chilling-responsive fraction for recently harvested seeds. Excision of utricles from recently harvested fruits increased the chilling-responsive fraction but did not generally eliminate the chilling requirement. Leaching or dewinging to remove soluble inhibitors increased germination of otherwise nondormant seeds but did not relieve primary dormancy. Knowledge of dormancy loss mechanisms will facilitate prediction of germination phenology and seed bank persistence for shadscale under fluctuating field conditions.