Modelling the effects of water stress and temperature on germination rate of Orobanche aegyptiaca seeds

Orobanche aegyptiaca seeds were germinated at a range of water potentials and temperatures and the progress of germination within the seed population was modelled. Base water potentials (at which the rate of progress towards germination is zero) varied between individual seeds according to a normal distribution with a mean of -1.96 MPa and standard deviation of 0.33 MPa at 20C. Contrary to the underlying assumption of the hydrothermal time model in the literature, the median base water potential varied significantly with temperature, being c. -2 MPa at 14-23C and increasing at both higher and lower temperatures. Thermal times to germination also varied according to a normal distribution between individual seeds with a mean of 49°Cd and standard deviation of 18°Cd in water. The median thermal time to germination varied with water potential. Again, however, an assumption of the hydrothermal time model was found to be invalid since the base temperature for rate of germination also varied significantly with water potential. The relationship of both base temperature and thermal time to water potential were linear such that germination progress curves in 33 different hydrothermal environments (8-26°C and 0 to -1.2 MPa) could be described according to a new modified thermal time model which accounted for 78% of the variation in the data.Keywords: Water stress, temperature, Orobanche aegyptiaca, germination rate, hydrothermal time models.