Quantification of Critical Temperature Thresholds and Thermal Time Required for Seedling Emergence of Spring Rapeseed Mandana Mirbakhsh 1, Mehdi Hayati 2, Mohammad Roozkhosh 3, Fakher Kardoni 4* 1- Department Agronomy, Purdue University, West Lafayette, IN 47907, USA 2- Department of Agronomy and Plant Breeding, College of Agriculture, Islamic Azad University, Ilam Branch, Ilam, Iran. 3- Department of Agrotechnology, College of Agriculture, Ferdowsi University of Mashhad, Iranand Department of Crop Protection, Faculty of Agriculture, University of Jiroft, Iran 4- Department of Agronomy, College of Agriculture, Birjand University, Birjand, Iran. *Corresponding Author: Email: kardoni1986@gmail.com Received: 8 April 2021 Accepted: 10 June 2021 Abstract Quantitative knowledge about temperature T effect on seedling emergence in spring rapeseed (Brassica napus L.) is rare. Therefore, the main purpose of the present study was to determine the critical T thresholds and thermal time required for seedling emergence of spring rapeseed. To do this, two field trials, each with 15 different planting dates, were performed to evaluate the seedling emergence responses of five spring rapeseed cultivars to a wide range of T environments. A Normal-based thermal time model was used to describe the relationship between time-to-emergence and T. In this model, a Normal distribution of both sub-optimal thermal time θT(e) and maximum temperature Tm(e) was assumed while base temperature Tb and supra-optimal thermal time remained constant for all emergence fractions. The model correctly explained the emergence dynamics of various cultivars in response to T over sub- and supra-optimal ranges. The thermal thresholds for seedling emergence, Tb, sub-optimal thermal time to reach 50% emergence (θT(50)), maximum emergence T for induction of 50% thermoinhibition in seeds (Tm(50)), and depended on the rapeseed cultivar studied. The values of Tb, θT(50), Tm(50) and ranged from 3.90 to 5.94 °C, 71.44 to 76.67 °C d, 32.90 to 33.48 °C and 4.76 to 6.67 °C d, respectively. Within each cultivar, optimum T (To(e)) exhibited a slight variation among various emergence fractions. The value of To(50) varied between 30.96 and 31.22 °C, depending on the cultivar. The outputs obtained from this work may be readily used in crop simulation models. Manuscript profile