Seed Dormancy and Germination in Relation to Stand Establishment of Native Wildflowers and Grasses

Carol C. Baskin
University of Kentucky
Lexington, KY 40506
(859) 257-3996
ccbask0@pop.uky.edu

Freshly-matured seeds of some species germinate over a range of environmental conditions and thus are nondormant, while those of other species fail to germinate at any set of environmental conditions and thus are dormant. The classes of seed dormancy are physiological (PD), morphological (MD), morphophysiological (MPD), physical (PY), and combinational (PY+PD). A dichotomous key has been written to allow one to quickly and easily distinguish these five classes of seed dormancy [Baskin, C. C. and J. M. Baskin. How to get the most information on dormancy-breaking and germination requirements from the fewest seeds. IN: E. Guerrant, K. Havens, and M. Maunder (editors). Strategies for Survival. Island Press, Covelo, CA (in press)].

PD is caused by a physiological inhibiting mechanism in the embryo, and it is the most common class of seed dormancy on earth. PD may be broken during exposure to cool (0.05-10°C) moist conditions such as occur in winter (called cold stratification) or warm (>15°C) conditions such as occur in summer, depending on the species. In some species, dormancy-break during summer may occur under warm, dry conditions (after ripening), but in other species warm, moist conditions (called warm stratification) are required. Information on the phenological (ecological) life cycle of a species and seasonal temperature changes in the habitat will help one decide if seeds should be given warm and/or cold treatments to break dormancy.

PY is due to an impermeable seed or fruit coat, it is the second most common class of seed dormancy on earth. PY occurs in some members of 15 plant families, including the Anacardiaceae, Bixaceae, Cannaceae, Cistaceae, Cochlospermaceae, Convolvulaceae, Curcurbitaceae, Dipterocarpaceae, Fabaceae, Geraniaceae, Malvaceae (now includes the Bombacaceae, Sterculiaceae, and Tiliaceae), Nelumbonaceae, Rhamnaceae, Sapindaceae, and Sarcolaenaceae. Impermeable seeds (or fruits) in each of these families have a distinct "water plug" that must be dislodged or moved before imbibition can occur. In nature, seeds with PY may germinate after exposure to low winter temperatures (Melilotus spp.), drying (Geranium carolinianum), high daily temperature fluctuations (winter annual Trifolium spp.), or fire (Iliamna corei).