Physiological Dormancy

The composition and dynamics of plant communities are determined by the survival, dissemination, and permanence of seeds in soil, and these processes are at the heart of our current understanding of plant biology. Most plant species produce seeds that are dormant at maturity in order to regulate the timing of germination. Physiological dormancy is the most common type of dormancy, and it is found in nearly all plant species. It acts as a reversible switch that allows plants to fine-tune the germination response of seeds in response to specific environmental cues. Physiological dormancy is defined as the inability of ingested seeds to germinate until specific environmental cues are perceived. While buried in soil seed banks, these seeds may lose and regain dormancy in a predictable yearly cycle for several decades.

Some species produce ‘hard’ seeds, which are incapable of germinating because their seed coats are impermeable to water. ‘Hard seededness’, also known as ‘physical dormancy,’ is found in only a few plant families, the largest of which being the Leguminosae.

Photodormancy

For some seeds, photodormancy or light sensitivity has an impact on their ability to germinate. To germinate, these photoblastic seeds require either darkness or light for a period of time. In species with thin seed coats, light may be able to penetrate into the dormant embryo and cause it to become active. The presence or absence of light may cause the germination process to begin, with some seeds buried too deeply in the soil and others not buried at all inhibiting germination.

Thermodormancy

Thermodormancy refers to a seed’s sensitivity to extremes of temperature. Some seeds, such as cocklebur and amaranth, germinate only when exposed to extremely high temperatures (30oC or 86F). Several plants that produce seeds that germinate in the early to mid summer have thermodormancy, which means that they will only germinate when the soil temperature is warm enough. Other seeds, such as celery, require cool soils to germinate, whereas others, such as sunflower seeds, are inhibited when soil temperatures are too high. When a seed ages or dries, the requirement for thermodormancy is no longer present.

Seeds are classified as having deep physiological dormancy under the following conditions: applications of GA3 do not result in increased germination; excised embryos produce abnormal seedlings; and seeds require more than three months of cold stratification before germinating before being classified as such.

Example of physical dormancy

Physical dormancy has been observed in the seeds of plants belonging to 16 different angiosperm families, including the following:

• Anacardiaceae
• Asteraceae
• Bixaceae
• Cistaceae 
• Cannaceae (monocot)
• Cochlospermaceae
• Convolvulaceae
• Cucurbitaceae
• Dipterocarpaceae
• Geraniaceae
• Fabaceae
• Malvaceae
• Nelumbonaceae
• Rhamnaceae
• Sarcolaenaceae
• Sapindaceae

However, only a few species of Cycadales, including Cycas revoluta and Zamia floridana (which have thick sclerotesta and slow embryo development), have been observed to be in physical dormancy, whereas no other groups of extant gymnosperms have done so.

Seed dormancy

When seeds do not germinate during unsuitable ecological conditions, such as those that would result in a low probability of seedling survival, seed dormancy is a form of evolutionary adaptation. A combination of environmental conditions that are ordinarily conducive to the germination of non-dormant seeds does not result in the germination of dormant seeds within a given amount of time in dormant seeds.

One of the most important functions of seed dormancy is delayed germination, which allows for dispersal of seeds and prevents the germination of all seeds at the same time. In this way, some seeds and seedlings are protected from damage or death caused by brief periods of inclement weather or transient herbivores. It also allows some seeds to germinate when competition for light and water from other plants is less intense, allowing them to thrive in the long run. Seed quiescence is a type of delayed seed germination that differs from genuine seed dormancy in that it happens when a seed fails to germinate because the external environmental conditions are too dry, too warm, or too cold for germination to take place.

Conclusion:

While embryo growth and seed germination are prevented until chemical changes take place, physiological dormancy is maintained.Physiological dormancy stops embryo development and seed germination until chemical changes take place.It is possible to break physiological dormancy when inhibiting chemicals are broken down or are no longer produced by the seed. 

When seeds do not germinate during unsuitable ecological conditions, such as those that would result in a low probability of seedling survival, seed dormancy is a form of evolutionary adaptation.One of the most important functions of seed dormancy is delayed germination, which allows for dispersal of seeds and prevents the germination of all seeds at the same time.