seeds, indicating that they are ready to sprout. Exposure to freezing temperatures at this time may be in-
jurious to the germinating seedlings.
Stratification, if required, varies with each species. The closer the required temperature and duration
are to optimum, the better will be the outcome (Tables 2 and 3). Thus, knowing the proper temperatures
and stratification duration can result in more effective and efficient seedling production. Refrigeration (ar-
tificial stratification) is more predictable than outdoor sowing (natural stratification) and refrigeration
usually provides better and more consistent results.
- Warm
Some seeds require moist, warm stratification. Seeds with double dormancy, such as those of Taxusspp.
(yew) and Viburnumspp. (viburnum), require both warm and cold stratification, and others with even
more complex dormancy such as Halesia carolina(Carolina silverbell) require cold-warm-cold expo-
sures in this sequence (Tables 2 and 3).
During exposure to warm temperatures, usually between 20 and 30°C but possibly more or less de-
pending on the species, immature or rudimentary embryos develop. If sufficient heat is gained after out-
door summer or fall seeding to satisfy the warm temperature requirement, germination will occur the first
spring after seeding. Otherwise, germination will occur the second spring after seeding. Dormancy of
some seeds is also broken by storage in hot, dry conditions [11].
- Embryo Culture
The technique of in vitro embryo culture or embryo rescue is used by plant breeders and seed laborato-
ries to obtain seedlings from otherwise nongerminable seeds that are not sufficiently mature when the fruit
is ripe or from seeds with very complex dormancies [2]. The procedure involves aseptically excising the
embryo from the seed and culturing it in a suitable, sterilized nutrient medium. When an immature or rudi-
mentary embryo is cultured in this way, it may bypass the need for warm and /or cold stratification.
C. Growth Regulators
Seed dormancy and germination are believed to be controlled by the balance and interaction of growth-
promoting and growth-inhibiting substances. These regulatory hormones accumulate in seeds during
embryo development, although not necessarily in the embryo itself [8]. Gibberellic acid, in particular,
appears to be essential for seed germination. It mobilizes food sources and stimulates growth of em-
bryonic tissue. Differences in endogenous gibberellin concentrations of some cold-requiring seeds have
been related to the amount of chilling exposure of the seeds [45]. Abscisic acid appears to be a specific
antagonist of gibberellin action to promote germination. It is present in the seed coat, endosperm, or
embryo.
Exogenously applied growth regulators sometimes influence seed germination. Some dormant seeds,
particularly of wild plants that require light or cold for germination, may be induced to germinate by ap-
plying gibberellins. Seeds of these plants typically germinate in pockets of leaf mold where fungal activ-
ity releases gibberellins [21]. Ethylene also breaks dormancy and initiates germination, but its effect is
not as well documented as that of other growth regulators.
According to Bell et al. [11], water-soluble chemical factors from charred wood or smoke may stim-
ulate germination of some seeds. Interestingly, ethylene is a component of wood smoke. Ethylene re-
leased from seeds may also stimulate their germination [12]. Also, applied cytokinins overcome dor-
mancy in many species. Conversely, abscisic acid often inhibits germination when applied to nondormant
seeds. In some instances, the abscisic acid–induced germination inhibition can be reversed by cytokinin.
Gibberellins do not usually reverse abscisic acid–induced germination inhibition. However, the combined
application of gibberellin and cytokinin can induce germination of dormant seeds in a wider range of
species than either chemical administered separately.
D. Priming
Postharvest treatments that improve germination and seedling vigor are termed seed enhancements [46].
Seed priming (osmoconditioning) is a seed enhancement technique that has proved effective for improv-
ing germination, seedling emergence, and yield of many early-planted, small-seeded vegetable and flower
GERMINATION AND EMERGENCE 111