Using hydrothermal time concepts to model seed germination response to temperature, dormancy loss, and priming effects in Elymus elymoides

Hydrothermal time (HTT) describes progress toward seed germination under va rious combinations of incubation water potential (Psi) and temperature (T). To examine changes in HTT parameters during dormancy loss, seeds from two populations of the bunchgrass Elymus elymoides were incubated under seven t emperature regimes following dry storage at 10, 20 and 30 degreesC for inte rvals from 0 to 16 weeks. Fully after-ripened seeds were primed for 1 week at a range of Psis. Data on germination rate during priming were used to ob tain a HTT equation for each seed population, while data obtained following transfer to water were used to calculate HTT accumulation during priming. HTT equations accurately predicted germination time course curves if mean b ase water potential, Psi (b)(50), was allowed to vary with temperature. Psi (b)(50) values increased linearly with temperature, explaining why germina tion rate does not increase with temperature in this species. Psi (b)(50) s howed a linear decrease as a function of thermal time in storage. Slopes fo r the T x Psi (b)(50) relationship did not change during after-ripening. Th is thermal after-ripening time model was characterized by a single base tem perature and a constant slope across temperatures for each collection. Beca use the difference between initial and final Psi (b)(50)s was uniform acros s temperatures, the thermal after-ripening requirement was also a constant. When seeds were primed for 1 week at -4 to -20 MPa, accumulation of HTT wa s a uniform 20% of the total HTT requirement. When primed at 0 to -4 MPa, H TT accumulation decreased linearly with decreasing priming potential, and a hydrothermal priming time model using a constant minimum priming potential adequately described priming effects. Use of these simple HTT relationship s will facilitate modelling of germination phenology in the field.