Seed germination has been modeled extensively using hydrothermal time (HTT) models. Generally, a normal distribution for base water potential w b(g) and a linear upward shift in w b(g) with increasing temperature were used within these models to relate variation in w b(g) to the variation in time to germination of a given fraction of seeds. However, little research has tested these assumptions. Therefore, the main purpose of this study was to test the assumption of normality in the distribution of w b(g) by germinating zucchini (Cucurbita pepo L.) seed with nondormant seed across a range of constant w at sub-and supra-optimal temperatures. Zucchini seeds were incubated at six constant temperature regimes (15, 20, 25, 30, 35, and 37°C) at each of the following osmotic potentials (0, -0.15, -0.30, -0.45, and -0.60 MPa). Results indicate that when incorporated into an HTT model, the Weibull distribution (R 2 = 0.85; RMSE = 11.89) more accurately described w b(g) and germination course over sub-and supra-optimal temperatures than HTT based on a normal distribution (R 2 = 0.77; RMSE = 14.80). According to these models, base temperature and base water potential for the 50th percentile (w b(50) ) were estimated at 9.38°C for both models and -1.27 and -1.31 MPa by the normal and Weibull distribution, respectively. Given the flexibility of the Weibull distribution this model not only provides a useful method for predicting germination but also a means for determining the distribution of w b(g) .

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