The Hydrotime Concept in Seed Germination and Dormancy

Abstract

The hydrotime concept has been developed to describe the relationship between water potential (Ijf) and seed germination rates. In analogy with thermal time, hydrotime is the accumulated Ijf units (MPa) above a base or threshold value (ljfb) multiplied by the time of imbibition at that Ijf. The germination rate (inverse of time to radicle emergence) increases linearly as the seed Ijf increases above Ijfb' As Ijfb values vary among individual seeds, the amount by which the Ijf exceeds Ijfb also varies among seeds, resulting in the typical right-skewed sigmoid germination time courses. Factors which influence seed germination kinetics alter the mean Ijfb ofthe seed lot, with higher (more positive) values resulting in an inhibition of germination or imposition of dormancy and lower (more negative) values being associated with rapid germination and high vigour. The induction or alleviation of dormancy involves physiological shifts in the Ijfb distribution, allowing a seed population to track both long-term and short-term environmental conditions to enhance seedling survival.