A simulation model to predict seed dormancy loss in the field for Bromus tectorum L.

Bromus tectorum L, (cheatgrass) is an invasive winter annual whose seeds lose dormancy through dry after-ripening. In this paper a thermal after-ripening time model for simulating seed dormancy loss of B, tecforum in the field is presented. The model employs the hydrothermal time parameter mean base water potential (psi(b)(50)) as an index of dormancy status. Other parameters of the hydrothermal time equation (the hydrothermal time constant theta(HT), the standard deviation of base water potentials sigma(psi b), and the base temperature T(b)) are held constant, while psi(b)(50) is allowed to vary and accounts for changes in germination time-course curves due to stage of after-ripening or incubation temperature. To obtain hydrothemal time parameters for each of four collections, seeds were stored dry at 20 degrees C for different intervals, then incubated in water (0 MPa) or polyethylene glycol (PEG) solutions (-0.5, -1.0, -1.5 MPa) at 15 and 25 degrees C, Germination data for the thermal after-ripening time model were obtained from seeds stored dry in the laboratory at 10, 15, 20, 30, 40, and 50 degrees C for 0 to 42 weeks, then incubated at two alternating temperatures in water. Change in psi(b)(50) was characterized for each collection and incubation temperature as a linear function of thermal time in storage. Measurements of seed zone temperature at a field site were combined with equations describing changes in psi(b)(50) during after-ripening to make predictions of seed dormancy loss in the field. Model predictions were compared with values derived from incubation of seeds retrieved weekly from the field site. Predictions of changes in psi(b)(50) were generally close to observed values, suggesting the model is useful for simulating seed dormancy loss during after-ripening in the field.