Hydrothermal time analysis of tomato seed germination responses to priming treatments

Controlled hydration of seeds followed by drying (seed priming) is used to break dormancy, speed germination, and improve uniformity of radicle emergence. To date, empirical trials are used to predict optimal priming conditions for a given seed lot. Since priming is based upon seed water relations, it was hypothesized that the sensitivity of germination to reduced water potential before priming might be mechanistically related to, and therefore predictive of, priming responsiveness. Analyses of germination of 13 tomato (Lycopersicon esculentum Mill.) seed lots at two temperatures (15 °C and 20 °C) and three water potentials (0, −0.28 and −0.43 MPa) showed that seed lot germination responses could be quantitatively characterized by parameters derived from thermal time, hydrotime, and hydrothermal time models (R2=0.73–0.99). Six of the seed lots were primed at two temperatures (15 °C and 20 °C) and three water potentials (−1.0, −1.5 and −2.0 MPa) for various durations, dried, and their subsequent germination rates analysed according to hydropriming time and hydrothermal priming time models. The responses of germination rates to priming were characterized by hydropriming time (θHP) and hydrothermal priming time (θHTP) constants and the minimum water potential (ψmin) and temperature (Tmin) for achieving a priming effect. The values of ψmin and Tmin varied relatively little among tomato seed lots, and the generalized values of ψmin= −2.39 MPa and Tmin=9.10 °C accounted for 74% (15 °C), 57% (20 °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.