Estimation of Cardinal Temperatures of Carthamus oxycantha Germination Using Different Regression Models

The genus Carthamus includes 25 species and subspecies in Iran, the two species C. oxycantha and C. lanatus have the most diversity, distribution and adaptation to the climatic conditions of Iran. C. oxycantha is a broadleaf weed and belongs to sunflower (Asteraceae) family. As seed germination is the beginning of the life cycle of plants, seedling emergence is critical for the establishment of plant populations. Germination and early seedling growth of many plants are the most sensitive stages to environmental stresses. Environmental factors, such as temperature, soil solution osmotic potential, solution pH, light quality, management practices and seed location in the soil seed bank, affect weed seed germination and emergence. Temperature is the most important environmental factors that control plant development, growth and yield. All biological processes respond to temperature, and all responses can be summarized in terms of three cardinal temperatures, namely the base or minimum (Tmin), the optimum (Topt), and the maximum (Tm) temperatures. Modeling of seed germination is considered an effective approach to determining cardinal temperatures for most plant species. Determination of cardinal temperatures could be a useful guidance to introduce new species in a new area or in selection of the sowing time. A clear understanding of cardinal temperatures could also be the first step for domestication of new species. There are various mathematical models describing seed germination responses to temperature, among which three have been used more often: intersected lines (ISL), quadratic polynomial (QPN) and five parameters beta (FPB). In order to investigate percentage and germination rate of C. oxycantha seeds, a laboratory experiment was conducted in complete randomized design (CRD) with four replications and under 7 constant temperatures 5, 10, 15, 20, 25, 30 and 35 ºC. Seeds were sterilized with 0.5% sodium hypochlorite solution for 1 min. Followed by washing with distilled water. Then, they were transferred to 9 cm diameter sterilized petri dish containing single layer of filter paper (Wathman #1). The germinated seeds were counted daily and continued until a cumulative germination reached a fixed amount (up to 14 days) or when 100% germination was achieved. Seeds were considered as germinated if the radicle was visible. To estimate the effects of temperature on germination rate of C. oxycantha seeds, three regression models included: Five-parameters Beta (FPB), Intersected-lines (ISL) and Quadratic Polynomial (QPN), were used. The germination data were tested for normality before analysis of variance. Data were analyzed using SAS 9.1 and Microsoft Excel 2007, and figures were designed by Sigmaplot 12.5. The results of the experiment showed that the temperature had a significant effect on the percentage and rate of germination. The lowest germination percentage was at 30°C (23%), while the lowest germination rate (0.62) occurred at 5°C. The highest germination percentage and germination rate occurred in 15-20 ºC. Generally, by increasing temperature from 5 °C to 20 °C, the percentage and germination rate increased and decreased after 20 °C, so that at 35 ºC, no seeds of wild safflower germinated. Based on the regression models the cardinal temperatures (Tbase, Topt and Tmax) were (4.4-5), (19.6-19.91) and (28.4-33.66) °C, respectively. ISL was the best model to estimate cardinal temperature of C. oxycantha based on the root-mean-square error, determination coefficient and residual values. According this model, the base, optimum and maximum temperatures were estimated as 4.41°C, 19.6°C and 33.3 °C. Khalaj et al (2015) modeled the germination rate of three medicinal plants, including wild oat (Avena fatua L.), wild mustard (Sinapis arvensis L.) and Descurania Sophia (L.). They showed that the segmented model was the best. But parmoon et al (2015) showed that the beta model was found to be the best model for predicting the germination rate and cardinal temperature of milk thistle (Silybum marianum L.). Soltani et al (2006) showed that the response of chickpea (Cicer arietinum L.) emergence to temperature is best described by a dent-like function. The result of this experiment showed the best model to estimate cardinal temperature of C. oxycantha was ISL. According this model Tb, To and Tm were estimated as1.4.41°C, 19.6°C and 33.3°C, respectively. It should be noted that although according to the results of this experiment, the optimum germination temperature in the wild safflower was about 20 °C and high temperatures were effective in reducing germination percentage and consequently inducing dormancy in the safflower seedlings, with climate change, plants such as wild safflower adapted to the new conditions, and the cardinal temperatures of this plant may also be changed in accordance with the new conditions.