A HYDROTHERMAL TIME MODEL OF SEED AFTER-RIPENING IN BROMUS-TECTORUM L

Bromus tectorum L. is an invasive winter annual grass with seeds that lose dormancy through the process of dry after-ripening. This paper pr oposes a model for after-ripening of B. tectorum seeds based on the co ncept of hydrothermal time. Seed germination time course curves are mo delled using five parameters: a hydrothermal time constant, the fracti on of viable seeds in the population, base temperature, mean base wate r potential and the standard deviation of base water potentials in the population. It is considered that only mean base water potential vari es as a function of storage duration and incubation temperature follow ing after-ripening. All other parameters are held constant throughout after-ripening and at all incubation temperatures. Data for model deve lopment are from seed germination studies carried out at four water po tentials (0, -0.5, -1.0 and -1.5 MPa) at each of two constant incubati on temperatures (15 and 25 degrees C) following different storage inte rvals including recently harvested, partially after-ripened (stored fo r 4, 9 or 16 weeks at 20 degrees C) and fully after-ripened (stored fo r 14 weeks at 40 degrees C). The model was fitted using a repeated pro bit regression method, and for the two seed populations studied gave R (2) values of 0.898 and 0.829. Germination time course curves predicte d by the model generally had a good fit when compared with observed cu rves at the incubation temperature/water potential treatment combinati ons for different after-ripening intervals. Changes in germination tim e course curves during after-ripening of B. tectorum can largely be ex plained by decreases in the mean base water potential. The simplicity and good fit of the model give it considerable potential for extension to simulation of after-ripening under field conditions.