Seyed Amir Moosavi - Department of Plant Production and Genetics, Faculty of Agriculture, Agricultural Sciences and Natural Resources University of Khuzestan, Iran
Seyed Ataollah Siadat - Department of Plant Production and Genetics, Faculty of Agriculture, Agricultural Sciences and Natural Resources University of Khuzestan, Iran
Ahmad Koochekzadeh - Department of Plant Production and Genetics, Faculty of Agriculture, Agricultural Sciences and Natural Resources University of Khuzestan, Iran
Ghasem Parmoon - Department of Agronomy, Faculty of Agriculture, University of Mohaghegh Ardabili, Ardabil, Iran
Samaneh Kiani - Department of Plant Production and Genetics, Faculty of Agriculture, Agricultural Sciences and Natural Resources University of Khuzestan, Iran

Many studies have focused on the cardinal temperatures and thermal time requirements of seed germination, but how seed size and color affect germination rate and thermal thresholds is poorly understood. In this study, nonlinear regression models were used to examine the relationships among seed size, seed color, and germination rate, and assess the extent to which these seed traits influence cardinal temperature and thermal time requirements for seed germination. The beta-modified model was found to be the best model for predicting the required time to reach ۵۰% germination. Based on the model output, the base, optimum, and maximum temperatures were ۴.۴۹-۸.۵۹, ۱۹.۷۶-۲۱.۸۸ and ۳۴.۱۲-۴۱.۶۸ ○C, respectively. Larger seeds have a higher base and ceiling temperatures compared to smaller seeds. The thermal time of ۵۰% germination was ۱۸۹۰, ۹۵۴, ۱۵۵۱, and ۱۱۸۸ degree-hours for small and large greenish-gay and reddish-brown seeds, respectively. The lower germination rate in greenish-gray seeds compared with reddish-brown colored seeds could be due to the lower seed vigor or viability. Not all castor seeds are produced at the same time during the growing season. Therefore, the last produced grains lose ideal grain filling conditions, which cause them to be smaller, less dense, and have low vigor.

Base temperatures, Germination rate, Regression, Seed morphology