The germination of intact, moist-chilled white spruce (Picea glauca [Moench.] Voss.) seeds and liquid–nitrogen–decoated (LN2), unchilled seeds is significantly more than of intact, unchilled seeds and intact, unchilled seeds exposed to LN2. The testa is largely responsible for imposition of dormancy in these seeds, although the megagametophyte and/or nucellus also play a role. One or both of these tissues undergoes significant weakening during moist chilling. Excised embryos from dormant and nondormant white spruce seeds elongate when placed on solid Murashige and Skoog minimal organics medium supplemented with a carbon source (sucrose, glucose or galactose), but elongate very little on medium without sugar. They are not killed by rapid imbibition on unsupplemented media. Thus, embryos of white spruce do not exhibit innate dormancy, but are dependent on a carbon source for elongation, and have dormancy imposed on them by their surrounding structures.

The relationship between endosperm cap weakening and endo-β-mannanase activity during priming and the time to germination after priming was studied in tomato (Lycopersicon esculentum) seeds. During priming of seeds in −0.4 MPa PEG, the mechanical restraint of the endosperm cap decreased while the endo-β-mannanase activity in the endosperm cap increased. There was no decrease in required puncture force and no increase in endo-β-mannanase activity in seeds during priming in −1.0 MPa PEG. Two classes of seeds could be distinguished during priming in −0.7 MPa PEG: one with decreased required puncture force and one without. A strong correlation was found between the lowering of the mechanical restraint and endo-β-mannanase activity. It was concluded that individual seeds have to cross a threshold water potential in order to develop enzyme activity and lower their mechanical restraint. A decrease in required puncture force and increase in endo-β-mannanase activity correlated with ice crystal-induced porosity in the endosperm cap cell walls in scanning micrographs. It was presumed that ice crystal-induced porosity reflects cell-wall hydrolysis. Germination time after priming correlated positively with required puncture force during priming, depending on the osmotic potential. Seeds in −1.0 MPa PEG improved their time to germination, without a decrease in the required puncture force. Therefore it was concluded that lowering of the endosperm restraint during priming positively affects the time to germination of primed seeds but is not a prerequisite for rapid germination.

In the physiological sense, germination begins with seed water uptake and ends with the initiation of elongation by the embryonic axis, usually the radicle. The driving forces and constraints on expansion by the embryo are examined, particularly for seeds in which the embryo is surrounded by endosperm and testa tissues that restrict growth. Models have been developed to predict germination based on thermal time, hydrotime and combined hydrothermal time. These population-based models indicate that the timing of germination is closely tied to physiologically determined temperature and water potential thresholds for radicle emergence which vary among individual seeds in a population. The restraint imposed by tissues surrounding the radicle is a major determinant of the threshold water potential. Enzymatic weakening of these tissues is a key event regulating the timing of radicle emergence. Considerable evidence suggests that endo-β-mannanase is involved in this process in a number of species, although it is doubtful that it is the sole determinant of when radicle emergence occurs. Molecular and biochemical studies are revealing the complexity of events occurring in endosperm and embryo cells associated with the completion of germination. Unique permeability properties and the presence of enzymes associated with pathogen resistance suggest additional functional roles for the tissues enclosing the embryo. The insights gained from physiology and modelling are being extended by the application of molecular techniques to identify and determine the function of genes expressed in association with germination. Single-seed assay methods, in vivo reporters, specific modification of gene expression and mutagenesis will be critical technologies for advancing our understanding of germination