Non-Peer ReviewedThe timing of seed germination plays a critical role in the survival of plants in natural ecosystems. Population-based models for the prediction of seed germination as the function of temperature and water potential have been developed, which can also be used in predicting field emergence. We used winterfat (Eurotia lanata) to test variations in parameters of the thermal time and hydrothermal time model among seed mass classes and germination conditions. Germination rates (GR) of subpopulations were estimated from germination time courses over a water potential range from 0 to –1.33 MPa at 2, 5, 10, 15, 20, and 25 oC. Estimated base temperature (Tb) was lower in the large seed mass class (-4.5 oC) than the small seed mass class (-3.5 oC). The ζ b(50) was lowest at intermediate temperatures between 10 to 15 oC. A linear
increase of hydro time (ρH) with subpopulation was found at lower temperatures, especially at 2 oC. There were no significant differences in ζ b(50) between large and small seeds, but significant differences were observed in hydrothermal time requirement (ρHT(50)), which was lower at intermediate temperatures than at either lower or higher temperatures. The predictability of the thermal and hydrothermal time model was improved when parameters were allowed to change with seed size and germination conditions. Variations in Tb among seed mass classes favor large seeds, which accumulate more thermal time at a given temperature. This is particularly important for species such as winterfat, which germinates early in the season and early-emerged seedlings have better chance to establish and survive
