Estimation of the cardinal temperatures for germination of four Satureja species growing in Iran
Introduction: Seed germination is a complex physiological process regulated by genetic and environmental factors including temperature, water, oxygen, light and pH. Among them, temperature is one of the most important factors controlling the maximum rate and percentage of diaspore germination. Objective: The aim of the study was to determine the cardinal temperatures (Tb, To, Tc) of four Satureja species growing in Iran. Methods: Seeds of Satureja mutica Fish. et C. A. Mey., S. macrantha C. A. Mey., S. sahandica Bornm and S. bachtiarica Bunge were germinated at nine constant temperatures (from 0 to 40°C) with 5°C intervals. A factorial experiment based on completely randomized design with four replications was applied to determine the cardinal temperatures estimated by three regression models including intersected-lines (ISL), quadratic polynomial (QPN) and five-parameters beta (FPB). Results: The highest germination percentage (GP) occurred at 20°C for S. mutica (86%), S. macrantha (55%), S. sahandica (81%) and S. bachtiarica (89%), but there was no significant difference between 20 and 25oC, except S. sahandica. Germination stopped at 0°C and 40°C. The highest germination rate (GR), the lowest mean germination time (MGT) and time to 50% germination (D50) were obtained at 20–25°C for all species. The GRm for S. bachtiarica was significantly (p≤0.05) higher than for three other species in all temperatures. None of the species did reach to 50% germination at temperatures higher than 30°C. Conclusion: Obtained results revealed the superiority of S. bachtiarica over the other species, v.s. S. macrantha was inferior. FPB and ISL models were most reliable for predicting cardinal temperatures, because of higher R2 value and the lower root mean square error (RMSE). S. macrantha and S. mutica showed the lowest and the highest cardinal temperatures, respectively, in all three models.