Mathematical models based on thermal time (degree day) and hydrotime (Ψw day) can be used as an input to more general models of seed germination and seedling emergence in the field, and may be an important tool for understanding weed biology and control. Germination of Drymaria cordata seeds in response to reduced water potentials in the medium was evaluated by using the hydrotime model. Both the germinability and germination rate decreased linearly with the reduction of the water potential (Ψw), attaining values near a zero at -0.8 MPa. The rate of decrease was higher at optimum temperature as compared to sub- and supra-optimum temperatures. The mean base water potential (Ψwb) was similar between sub- (19 ºC) and supra-optimum (32 ºC) temperatures, and was higher (less negative) at optimum temperature (25 ºC), showing that D. cordata seeds are less sensitive to reduced water potential at 19 ºC than at 25 ºC. Hydrotime was higher for seeds germinated at sub-optimum than at optimum temperature, showing that the germination rate for a given water potential is higher at optimum temperature. The hydrotime required for seeds to complete germination was higher at supra-optimum than at optimum temperature, and lower at supra-optimum than at sub-optimum temperature. In general, hydrotime was relatively constant among subpopulations. The hydrotime model can well describe the effects of water potential on the time-course of seed germination of D. cordata, particularly at sub- and supra-optimum temperatures.
hydrotime; modelling; temperature; weeds
