A model of the effects of water stress on seed advancement and germination

A model of seed germination is proposed which uses a variable with the unit s of an osmotic potential (virtual osmotic potential) to integrate the effe ct of a constant or a varying water potential. This differs from existing m odels that describe the effects of fixed water potentials on germination, o r the effects of fixed priming water potentials on the subsequent germinati on at a fixed water potential. When a seed is sown, the virtual osmotic pot ential is assumed to fall at a rate that depends on the ambient water poten tial, and on the difference between its current and a minimum value. Radicl e growth is assumed to initiate when the difference between the ambient wat er potential and the virtual osmotic potential exceeds a threshold. The ger mination of carrot and onion seeds at various fixed potentials below 0 MPa was well described by the virtual osmotic potential model. The model was al so used to simulate the results of experiments in which seeds were given a single step change in water potential.