Application of a Hydrothermal Time Seed Germination Model Using the Weibull Distribution to Describe Base Water Potential in Zucchini (Cucurbita pepo L.)

Seed germination has been modeled extensively using hydrothermal time (HTT) models. Generally, a normal distribution for base water potential psi (b(g)) and a linear upward shift in psi (b(g)) with increasing temperature were used within these models to relate variation in psi (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 psi (b(g)) by germinating zucchini (Cucurbita pepo L.) seed with non-dormant seed across a range of constant psi at sub- and supra-optimal temperatures. Zucchini seeds were incubated at six constant temperature regimes (15, 20, 25, 30, 35, and 37 A degrees 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 psi (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 (psi (b(50)) ) were estimated at 9.38 A degrees 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 psi (b(g)) .