How salinity stress influences the thermal time requirements of seed germination in Silybum marianum and Calendula officinalis

Salinity is a major problem of many agricultural lands that is usually associated with drought stress in arid and semi-arid regions. In this study we examine the role of salinity stress on temperature requirements of two herbaceous species and how it could be modeled to quantify alterations. We applied four non-linear regression models (segmented, beta, beta modified, and dent-like) to describe the germination rate–temperature relationships of Silybum marinum L. and Calendua officinalis L. over six constant temperatures exposed to different levels of salinity stress. Our results revealed that salinity could affect the cardinal temperatures in both plants and, as a result, it is not possible to suggest one model for all levels of salinity stress. The best model to fit data to predict cardinal temperatures of Silybum marianum and Calendula officinalis at the no-salinity condition were dent-like (AICc = 4.03) and beta (AICc = − 2.30), respectively. Knowing the thermal time constant ( f o ) value helps us predict the minimum number of hours required for completion of germination at the optimal temperature. All models in this study were estimated higher f o due to higher salinity stress in both Silybum marianum and Calendula officinalis seeds. The highest estimated f o for Silybum marianum (91.5 ± 59.6) and Calendula officinalis (178.9 ± 26.5) was obtained from the results of germination rate prediction using a dent-like model at 200 mM salinity.