Relationship between accumulated hydrothermal time during seed priming and subsequent seed germination rates

Seed germination rates are sensitive to both temperature (T) and water potential (psi) The times to germination of seeds imbibed at suboptimal T and/or reduced psi are inversely proportional to the amounts by which T exceeds a base temperature (T-b) and psi exceeds a base water potential (psi(b)) Germination rates across a range of suboptimal T and psi can be normalized on the basis of the hydrothermal time accumulated in excess of these thresholds However, seeds can also progress metabohcally toward germination even at T or psi too low to allow radicle emergence to occur Seeds preimbibed at low psi and dried back, or primed, germinate more rapidly upon subsequent reimbibition We show here that the increase in germination rates of tomato {Lycopersicon esculentum Mill) seeds resulting from seed priming is linearly related to the hydrothermal time accumulated during the priming treatment The threshold temperature (T-min = 7 05 degrees C) and water potential (psi(min) = - 2 50 MPa) for metabolic advancement were considerably lower than the corresponding thresholds for radicle emergence of the same seed lot (T-b = 11 degrees C, psi(b) = - 0 7 1 MPa), allowing the accumulation of hydrothermal priming time that is subsequently expressed as more rapid germination when T or psi increase The hydrothermal time model can now be applied to quantify and analyse germination rates of seeds across the entire range of suboptimal T and psi at which metabolic progress toward radicle emergence is possible.