Estimation of thermal thresholds for seedling emergence of spring canola in the field

Accurate prediction of seedling emergence in the field is crucial for the performance of growth models. In order to determine thresholds of seedling emergence response to temperature, two replicated field experiments were carried out at Ramin Agriculture and Natural Resource University of Khuzestan during 2015-2016 and 2016-2017. In these experiments, seedling emergence of two spring canola cultivars (Hyola 401 and Sarigol) was evaluated in a randomized complete block design with four replications in fifteen planting dates (as environment). Thermal-time model was developed based on the Weibull probability distribution function, and thermal thresholds for seedling emergence response of two spring canola cultivars in field conditions was modeled based on this function. Based on model outputs, the base temperature for seedling emergence (Tb) was estimated to be 5.83 °C in the hybrid Hyola 401 and 4.16 °C in the cultivar Sarigol. The thermal-time required to initiate seedling emergence at sub-optimal temperature range (θT(0)) and the thermal-time needed to complete seedling emergence at supra-optimal temperature regimes without significant differences between two cultivars was estimated to be 55.5 and 5.65 °C d, respectively. Maximum temperature for the 50% probability of the thermoinhibition of seedling emergence (Tm(50)) in hybrid Hyola 401 and cultivar Sarigol was estimated to be 33.02 and 33.30 °C, respectively. The optimum temperature for 50% seedling emergence in the field (To(50)) for hybrid Hyola 401 and cultivar Sarigol was determined to be 30.99 and 31.22 °C, respectively.