Hydrothermal time analysis of tomato seed germination responses to primingtreatments

Controlled hydration of seeds followed by drying (seed priming) is used to break dormancy, speed germination, and improve uniformity of radicle emerge nce, To date, empirical trials are used to predict optimal priming conditio ns for a given seed lot. Since priming is based upon seed water relations, it was hypothesized that the sensitivity of germination to reduced water po tential before priming might be mechanistically related to, and therefore p redictive of, priming responsiveness, Analyses of germination of 13 tomato (Lycopersicon esculentum Mill.) seed lots at two temperatures (15 degrees C and 20 degrees C) and three water potentials (0, -0.28 and -0.43 MPa) show ed that seed lot germination responses could be quantitatively characterize d by parameters derived from thermal time. hydrotime, and hydrothermal time models (R-2 = 0.73-0.99). Six of the seed lots were primed at two temperat ures (15 degrees C and 20 degrees C) and three water potentials (-1.0, -1.5 and -2.0 MPa) for various durations, dried, and their subsequent germinati on rates analysed according to hydropriming time and hydrothermal priming t ime models. The responses of germination rates to priming were characterize d by hydropriming time (theta(HP)) and hydrothermal priming time (theta(HTP )) constants and the minimum water potential (psi(min)) and temperature (T- min) for achieving a priming effect. The values of psi(min) and T-min varie d relatively little among tomato seed lots, and the generalized values of p si(min) = -2.39 MPa and T-min=9.10 degrees C accounted for 74% (15 degrees C), 57% (20 degrees C), and 62% (across both temperatures) of the increase in germination rates following priming. Nonetheless, while the hydrothermal time models described germination patterns both before and after priming, there was relatively little predictive relationship between them.