Cardinal temperatures and thermal time are valuable information for scheduling year-round production of seed-propagated species. In this study, thermal germination behavior of three species of the family Fabaceae, Onobrychis subnitens Bornm., Onobrychis scrobiculata Boiss. and Vicia variabilis Grossh., from Kurdistan Province, Iran were quantified. Seeds of these species were germinated at temperatures of 5– 35°C. Three nonlinear regression models (segmented, beta, and dent-like) were used to estimate cardinal temperatures. After examining O.subnitens, findings showed that the segmented model is the best model for predicting cardinal temperatures. The base, optimum, and maximum temperatures were 1.23, 17.22 and 41.10°C, respectively. The thermal time required for 50% germination was 21.29 degree-days. For O.scrobiculata, segmented and dent-like models were equal. The base, optimum and maximum temperatures of seed germination were 5.5, 18.8–19.76 and 38.14°C, respectively. The thermal time required for 50% germination was 25.87 degree-days. For V.variabilis, there was no difference between the segmented and dentlike models and each one could be chosen. Accordingly, the base, optimum, and maximum temperatures of seed germination were -1.2, 20 and 35.13°C, respectively. The thermal time required for 50% germination was 60.9 degree-days. Based on the germination percentage, germination rate and thermal time requirement, O.subnitens is preferred for use in the arid and semi-arid rangeland improvement project.
