comparison of hydrothermal time models to seed germination modeling of phalaris minor on the basis of normal, weibull and gumbel distributions

the hydrothermal time models have been widely applied to describe the germination responses of seeds to temperature and water potential. in this study, three hydrothermal time models developed on the basis of different statistical distributions (normal, weibull and gumbel) were compared to describe the phalaris minor seed germination. results showed that gumbel (aicc=-762.8) and weibull (aicc=-762.6) hydrothermal time models more accurately predicted p. minor germination than normal (aicc=-737.6) hydrothermal time model. however, there was no significant difference among values of predicted hydrothermal time constant, base temperature and median base water potential (ψb(50)) for germination of p. minor using all three models and were estimated to be 1000 mpa c h, 4.2 c and -1 mpa, respectively. based on the weibull hydrothermal time model, water potential threshold for the onset of germination (ψb(0)) was equal to -1.5 mpa. also, the distribution of base water potential for germination was right skewed (λ=1.57). weibull hydrothermal time model can be widely used to predict seed germination by considering the flexibility of distribution, the possibility of determining the base water potential distribution skewness and estimation of water potential threshold for the onset of germination.