Dormancy is an important survival mechanism found in seeds aiming to delay the germination process until the time and ambient conditions are right. This may cause seeds of numerous species to remain ungerminated in the soil for many years. Seed dormancy is a genetically inherited trait whose intensity is modified by the environment during seed development. Plants with a long history of domestication generally show less seed dormancy than wild or more recently domesticated species. While domesticated species exhibiting dormancy might become a problem in a seed production setting, it is sometimes desirable in winter cereals as it prevents preharvest sprouting and helps maintain seed quality.
Types of Dormancy
Dormancy is often confused with quiescence, which is merely a resting state in the absence of suitable germination conditions. On the other hand, true dormancy is referred to as a state in which seeds are prevented from germinating even under environmental conditions normally favorable for germination. We distinguish two main types of dormancy: primary and secondary dormancy, with each one using several physical and biological dormancy mechanisms.
Primary dormancy is the most common type of dormancy and takes two forms: exogenous and endogenous dormancy.
- Exogenous dormancy is a state in which the essential germination components (i.e., water, light, temperature) are not available to the seed, which fails to germinate as a result. This form of dormancy is generally related to the physical properties of the testa (seed coat). Under natural conditions, exogenous dormancy can be broken by the freezing-thawing of the soil, ingestion by animals, microorganism activity, forest fires, or natural soil acidity, all affecting the integrity of the testa.
- Endogenous dormancy is mainly caused by the inherent properties of the seed (i.e., moisture status, day length, temperature during seed development, age of the mother plant), which inhibit (or promote) germination. It is the most prevalent form of dormancy found is seeds. Endogenous dormancy can be alleviated through biological changes such as rudimentary embryo maturation, response to growth regulators, changes in temperature, exposure to light, or endogenous rhythms.
In some cases, seeds that are initially non-dormant may become dormant when exposed to environmental conditions conducive to germination but where one essential element is lacking or in excess (i.e., warm/cold temperature, light/darkness, wet/dry soil). This secondary dormancy mechanism is thought to be due to an unfavorable balance of growth-promoting versus growth-inhibiting substances within the seed.
Breaking Dormancy Artificially
Some of the vegetable crops that we consume today still exhibit some dormancy such as lettuce, pepper, and spinach, which might not germinate well during the first months after being harvested. In most cases, a period of dry storage is all that is required for enabling dormant seeds to germinate. Seeds that are under endogenous dormancy due to osmotic inhibition (compounds possessing osmotic pressures inhibiting seed germination) can be germinated after removing the seeds from the influence of the inhibitor (i.e., tomatoes and cucumbers). In a seed production setting, dormancy is generally broken by time in storage or over the winter. In most cases, it is therefore rarely an issue for the typical home seed saver.
Some seeds might require a cold stratification treatment, aiming to simulate natural conditions that the seeds must experience before germination can occur. When the stratification process is controlled, the pretreatment amounts to nothing more than subjecting the seeds to storage in a cool (but not freezing) and moist environment for a period found to be sufficient for the species in question. This period of time may vary from one to three months.
The seeds are sown in small pots filled with moist soil and then enclosed inside a plastic bag before placing inside a common refrigerator. Note that soaking the seeds in cold water for 6–12 hours immediately before placing them in cold stratification can cut down on the amount of time needed for stratification, as the seed needs to absorb some moisture to enable the chemical changes that take place. Alternatively, the seeds may be placed in a sealed plastic bag with three times as much moistened vermiculite. The vermiculite must be slightly dampened, as excessive moisture can cause the seeds to mold in the bag. Sand or paper towels can be used as an alternative. After undergoing the recommended period of stratification, the seeds are removed and sown in a nursery bed for germination.