Estimation of cardinal temperatures for seed germination of evening primrose (Oenothera biennis L.) using non-linear regression models

Evening primrose (Oenothera biennis L.) is medicinal plant of moderate region and its seed containing about 20 to 30 percent of oil. Germination of seed and establishment of seedling are determining factors of crop production ripening time. Models apply for predicting of cumulative germination response and can estimate the cardinal temperatures too. Numerous thermal functions can describe germination response to temperature, amongst them segmented, beta and dent-like functions are most popular. Although many investigations have conducted to determine cardinal temperatures of germination and required thermal time of emergence, there is no sufficient information about evening primrose in Iran from this regard. This investigation has conducted in order to evaluate non-linear regression models to describe the evening primrose seed germination rate response to temperature and estimate its seed germination cardinal temperatures.
Experiment conducted as Completely Randomized Design (CRD) with four replications at ARYA TINA GENE® Co. laboratory. Evening primrose seeds were placed at eight constant temperatures regimes involves 5, 10, 15, 20, 25, 30, 35 and 40 °C. One hundred evening primrose seeds were placed at 9 cm. petri dishes containing two Whatman No. 1 filter papers and drained with 5 ml. distilled water. Germinated seeds were counted at 12 hr. intervals. To obtain maximum germination rate, cumulative germination progress curve were plotted versus time and the time required to 50 percentile germination estimated using 3 parameters sigmoidal function. Cardinal temperature of germination described using quadratic, four parameters beta, segmented and dent-like functions. The best fitted model selected according to Root Mean Square Error (RMSE), Mean Absolute Error (MAE), adjusted Regression coefficient (adj. R2) values and Akaike Information Criteria (AIC and AICc) indices.
Results showed that temperature effects on germination percentile and time to reach 50 % germination (germination rate) were significant at 5% error level. Comparison of RMSE, MAE, adjusted R2 and AICc criteria improved that evening primrose seed germination response to temperature has the best fitting to dent-like function. According to dent-like model, the base, lower and upper range and ceiling temperatures were 4.7, 24.3 to 34.7 and 41.2 °C, respectively. There were no different estimation of base and ceiling temperatures between models. However, segmented model have overestimated optimum temperature among models.
Evening primrose seed germination followed by dent-like function clear that seeds are able to germinate at a wide range of temperatures. In cultural circumstances, cultivation of evening primrose is possible at autumn or spring which is completely confirmed with behavior of the plant in natural stands.