Why is seed dormancy important?

Seed dormancy is a crucial process that affects when and how seeds germinate. It refers to the state in which seeds are prevented from germinating despite favorable environmental conditions. Understanding seed dormancy is important for managing plant growth and ensuring successful cultivation.

Plant hormones play a significant role in regulating seed dormancy and germination. Embryo dormancy is characterized by a high ratio of abscisic acid (ABA) to gibberellins (GA), combined with high ABA sensitivity and low GA sensitivity. To overcome this dormancy and initiate germination, the hormone balance must shift towards a lower ABA/GA ratio, with decreased ABA sensitivity and increased GA sensitivity.

ABA is responsible for maintaining embryo dormancy, while GA promotes embryo germination. Seed coat dormancy arises from the mechanical restrictions imposed by the seed coat and a low growth potential of the embryo. GA helps to break this dormancy by enhancing embryo growth potential or weakening the seed coat, allowing the seedling to emerge. The seed coat, which may consist of living or dead cells, can be influenced by hormones differently depending on its composition. ABA affects the growth characteristics of the seed coat, including its thickness, and interacts with GA to influence embryo growth.

Additionally, endosperm dormancy, which involves living tissue within the seed, is mediated by hormones as well. The endosperm can act as a barrier to germination and is influenced by the ABA/GA ratio and cellular sensitivity to these hormones. GA can weaken the endosperm and increase embryo growth potential, influencing both ABA-independent and ABA-inhibiting processes within the endosperm.

Seed dormancy is often classified into several categories: exogenous, endogenous, combinational, and secondary. A more recent classification system identifies five types of dormancy: morphological, physiological, morphophysiological, physical, and combinational. Understanding these categories helps in managing seed germination and optimizing plant growth.

Exogenous dormancy 

• Physical Dormancy: This occurs when seeds have hard, impermeable coats that prevent water from entering. To break this dormancy, a specialized structure called the 'water gap' must be disrupted in response to environmental cues such as temperature changes, allowing water to penetrate the seed and initiate germination. Plant families exhibiting physical dormancy include Anacardiaceae, Cannaceae, Convolvulaceae, Fabaceae, and Malvaceae.

• Chemical Dormancy: This type arises when seeds lack physiological dormancy but are inhibited by a chemical compound. This chemical can be removed or deactivated through leaching by rain or snowmelt. While leaching is often cited as a key factor in breaking dormancy for desert plants, concrete evidence supporting this mechanism is limited.

Endogenous dormancy 

• Morphological Dormancy: Germination is hindered by the embryo's developmental state. In some species, embryos are undifferentiated or underdeveloped at dispersal, requiring differentiation and growth before germination can occur. In other cases, embryos are differentiated but need to grow further to a specific size before germination. Plant families with morphological dormancy include Apiaceae, Cycadaceae, Liliaceae, Magnoliaceae, and Ranunculaceae.

• Morphophysiological Dormancy: Seeds with this type of dormancy have both underdeveloped embryos and physiological barriers. These seeds require both dormancy-breaking treatments and a period for embryo development. Plant families exhibiting morphophysiological dormancy include Apiaceae, Aquifoliaceae, Liliaceae, Magnoliaceae, Papaveraceae, and Ranunculaceae. Some species with this type of dormancy, such as Asarum and Trillium, experience multiple forms of dormancy affecting different parts of the embryo. Terms like double dormancy and 2-year seeds describe species that require extended periods, such as two winters and one summer, to complete germination.

• Physiological Dormancy: This occurs when the embryo cannot generate enough energy to break through covering structures due to physiological limitations. Dormancy is typically broken under cool, wet, warm, wet, or warm, dry conditions. Abscisic acid often inhibits growth, and its production can be influenced by light.

• Drying: Some seeds, especially those from arid regions or seasonal climates, require a drying period before they can germinate. If seeds remain moist post-dispersal, germination may be delayed for months or even years. Drying often alleviates physiological dormancy in many herbaceous plants from temperate zones. Seeds that require narrow temperature ranges for germination can broaden their range after drying.

• Combinational Dormancy: This type involves seeds with both impermeable coats and physiological dormancy. In such seeds, physical dormancy can be broken either before or after physiological dormancy.

• Secondary Dormancy: This occurs when seeds that are initially non-dormant are exposed to unfavorable conditions, often high temperatures, after dispersal. The mechanisms of secondary dormancy are not fully understood but may involve reduced receptor sensitivity in the seed's plasma membrane.

Additional dormancy strategies:

• Photo-dormancy: Some seeds require light or darkness to germinate. Seeds with thin coats may have light penetrate and trigger germination. Light or the lack thereof can influence germination, particularly in seeds buried at different depths.

• Thermo-dormancy: Sensitivity to temperature affects some seeds, such as cocklebur and amaranth, which germinate only at high temperatures (around 30°C). Other seeds need cool soils to germinate, while some, like celery, are inhibited by high temperatures. Thermo-dormancy requirements often diminish as seeds age or dry.

Not all seeds experience dormancy. For example, some mangroves exhibit viviparity, germinating while still attached to the parent plant. Cultivated garden plants may also lack seed dormancy due to selective breeding. Annuals and ephemeral plants use seeds to survive unfavorable conditions, with some annuals completing their life cycle from seed to seed in as little as six weeks.