Quantifying of germination response in dill (Anethum graveolens L.) to temperature and drought stress by hydrothermal time model

Temperature and water are the most important environmental factors controlling seed germination in ‎plants. In order to investigate the effect of temperature and drought stress on seed germination and ‎quantifying of germination; a factorial experiment was conducted with eight temperature levels including ‎‎5, 10, 15, 20, 25, 30, 35 and 40 degrees Celsius and the seven levels of drought stress including 0, 0.5-‎‎0.0, -0.2, -0.3, -0.4, -0.5 and -0.6 MPa, respectively. The results showed that the germination was ‎decreased by decreasing osmotic potential at all temperature levels. However, the intensity of this ‎decrease was less in the range of 20 to 30°C. Using two different methods, the base and maximum ‎temperature of dill germination was estimated in the range of 2.3 and 2.9 °C and 43.0 and 47.3 °C, ‎respectively. The optimum temperature for dill germination was 26 °C. The estimated value of base ‎water potential for germination was -0.53 Mpa with the hydrothermal time model. The results also ‎showed that water base potential for dill germination was increased by -0.5 Mpa as temperature ‎increased. Finally, it was found that if the base water potential taking into account variable in response to ‎changing temperature, then the hydrothermal time model can be good enough to quntify dill germination ‎response to temperature and water.‎