Understanding of thermal requirements of medicinal plants is necessary for their appropriate germination, growth and development. In this experiment germination cardinal temperatures of flixweed (Descurainia sophia) was calculated using relationship of germination rate with temperature. Nine constant temperatures (0, 5, 10, 15, 20, 25, 30, 35 and 40 °C) were considered and three regression models: intersected-lines, quadratic polynomial and five-parameters beta models were tested. The highest germination percentage (84%) and germination rate (5.83 seeds day−1) were obtained at 15 and 20 °C, respectively. Among three used models, the intersected-lines model was the best for prediction of germination responses of flixweed (R2 = 0.85). The base, optimum and ceiling temperatures obtained from intersected-lines model, were 1.21, 18.29 and 32.44 °C, respectively. In addition, temperature range (Tm−Tb) for the plant germination was 32.2 °C. Overall, the results of the three tested models confirmed the generally known fact that flixweed requires lower temperatures for its proper germination.

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