Chapter 19

Plant Propagation from Seed

Methods of Breaking Seed Dormancy

Dormancy is an adaptation that ensures seeds will germinate only when environmental conditions are favorable for survival. The conditions necessary to allow seeds to break dormancy and germinate can be highly variable among species, within a species, or among seed sources of the same species. External seed dormancy may be physical, physical-physiological, chemical, or mechanical. Internal dormancy may be morphological, physiological, or both. A variety of seed treatments have been developed in response to the diversity of seed types grown in greenhouses. For dormant seeds, dormancy must be overcome using one or more of the methods outlined in the following sections of this chapter. The greenhouse operations must determine whether seeds will need to be cleaned, scarified, soaked, stimulated, stratified, and treated in other ways before sowing on a species-by-species basis. The following sections describe the seed treatment options available.

Seed Scarification

Seeds with external dormancy require scarification. Scarification is any method of disrupting an impermeable seed coat so that water and oxygen can enter the seeds. Seeds can be scarified many ways. How well the method works depends on the species and the thickness of the seed coats. Whichever method is chosen, it is very important not to damage the endosperm, cotyledons, or embryo during the treatment. In acid scarification, seeds are put in a glass container and covered with concentrated sulfuric acid at about twice the volume of the seeds. The seeds are stirred gently and allowed to soak from 10 minutes to several hours, depending on the hardness of the seed coat.

Seed Stratification

Seeds of some species of plants contain abscisic acid, a chemical that inhibits germination. For these species, germination occurs after certain changes take place within the seed in response to environmental conditions. In nature, this period of adjustment, called after-ripening, prevents fall-produced seeds from germinating until the following spring when conditions (especially temperature) are more conducive to the survival of newly emerged seedlings.

Leaching

Some seeds contain chemical inhibitors within the seed or seed coverings. These inhibitors serve as rain gauges that prevent germination until there is adequate precipitation for germination. This type of dormancy is most common in plants from arid regions. To artificially overcome chemical dormancy, soak the seeds in running water or in water that is changed frequently to leach the chemicals from the seeds. Once completed, plant the seed immediately, because the moisture has hydrated the embryos that are ready to germinate. If they dry again, they may lose viability or vigor.

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