The models based on thermal time concept have been widely applied to quantify the germination responses of seeds to temperature. The majority of these models assume a Normal distribution for both sub-optimal thermal time θT(G) and maximum temperature Tc(G) to describe the variation in time to germination. In this study, the response of germination to temperature in six spring canola (Brassica napus L.) cultivars was described using the thermal time model.
Germination tests were carried out at constant temperatures of 8, 12, 16, 20, 24, 28, 32, 33, 34, 35 and 36 °C. The thermal time model accurately described germination patterns of different cultivars in response to temperature over sub- and supra-optimal. The thermal thresholds for seed germination, base temperature (Tb), suboptimal thermal time needed to achieve 50% germination (θT(50)), maximum germination temperature for induction of 50% thermoinhibition in seeds (Tc(50)) and supra-optimal thermal time to complete germination (θTc) differed significantly among the canola cultivars studied. The values of Tb, θT(50), Tc(50) and θTc ranged from 4.86 to 7.10 °C, 358.89–407.19 °C h, 33.90–34.42 °C and 27.66–38.26 °C h, respectively. Within each cultivar optimum temperature (To(G)) showed little variation amongst different germination percentiles. The magnitude of To(50) ranged from 31.86 to 32.25 °C depending on the cultivar. The thermal thresholds for seed germination identified here explained the differences in seed germination found among cultivars. All model parameters may be readily used in crop simulation models.