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Cotton is an important crop commercially produced in many semi-arid regions of the world where unfavorable conditions are common during the crop establishment. Under these conditions priming, incomplete hydration of seeds, is one of the treatments used to accelerate field emergence, With a water potential reduced below critical value, there will be no radicle emergence, however, advancement toward germination can still occur. A hydrotime model was used to predict this water potential for zero germination, as well as other water relations useful to understand the physiological process. The objectives of this research were to test the model validity for cotton, to use the model parameters to determine critical water potential for zero germination and to evaluate cotton response to priming. The best adjustment to the model was obtained at a hydrotime (theta(H)) = 1.395 MPa d, a mean base potential (psi(b(50))) = -0.823 MPa and standard deviation (sigma(psib)) = 0,423 MPa. The critical potential for zero germination was -2.2 MPa. The priming for 96 hours in polyethilenglycol (PEG) 6000 at this water potential decreased the percentage of seed germination in water as well as in PEG -0.8 MPa whereas the speed of germination was reduced only in PEG -0.8 MPa.