Determination of Cardinal Temperatures of Milk Thistle (Silybum marianum L.) Germination

Plants response to temperature is an important issue. In order to determine cardinal temperatures and its related germination responses to temperature three varieties seeds of milk thistle (Shomal, Mollasani and Majarestan) were exposed to six constant temperatures (10, 15, 20, 25, 30 and 35 °C) in a four replicated factorial arranged, completely randomized design. Results indicated that the temperature and variety had a significant effect on the maximum germination percentage (MGR), germination uniformity (GU), germination rate (reciprocal time to 50% germination), and time to 5, 10, 50, 90 and 95% germination, but the interacted effect of temperature and variety was not significant. Assessment of three non-linear regression models, including segmented, dent-like and beta models revealed that the beta model was an appropriate model for all varieties. Base and ceiling temperatures for all three varieties were estimated at near to 0 and 40 °C, respectively. Optimum temperature for Shomal, Mollasani and Majarestan varieties was estimated as (28.79, 29.97 and 29.58 °C), respectively. Biological time for 50% germination for Shomal, Mollasani and Majarestan varieties also was estimated at 43.61, 42.61 and 45.85 hours, correspondingly. It was concluded that these model can be used to quantify response of milk thistle germination to temperature and to obtain cardinal temperature of germination. These parameters are required to predict milk thistle germination and emergence.