Endogenous Dormancy

When it comes to the survival of a species, the seed is an important stage in the higher plant’s life cycle. This plant dispersion unit, which survives the interval between seed maturation and the establishment of the next generation as a seedling once it has germinated, is referred to as the reproductive unit of the plant. For this reason, the seed, which is primarily in a dry state, is well adapted to withstand prolonged periods of unfavourable environmental conditions. The seed goes into a latent state to maximise germination over time. Dormancy also prevents the germination of seeds before harvest. Numerous research has been carried out to gain a better understanding of how germination is influenced by various environmental conditions and chemical treatments.

Embryonic dormancy (also known as endogenous dormancy) occurs when an embryo is still in its infancy or is not fully formed at the time of ripening or maturity. There are various varieties of dormancy, including morphological, physiological, double dormancy, and secondary dormancy, among others.

Dormancy occurs in some seeds when the embryo is not fully grown at the time of seed dispersal due to morphological dormancy (rudimentary and linear embryos). If such seeds are sown immediately after harvesting, they will not germinate. The seeds of plants with rudimentary embryos are little more than a pro-embryo lodged in a large endosperm as the fruit matures, indicating that the embryos are still developing. The embryo grows in size after the seeds have ingested water, but before the seeds begin to germinate. The formation of a rudimentary embryo is frequent in several plant families, including the Ranunculaceae (Ranunculus), and the Papaveraceae (poppy). Some temperate zone plants, such as holly and snowberry, have also been found to have rudimentary embryos.

Physiological dormancy

1. Non-deep physiological dormancy: After ripening, it takes time for seeds in dry storage to lose their dormancy. b) Deep physiological dormancy: This sort of dormancy is usually only temporary and fades when the product is placed in dry storage. Temperature-sensitive fruits such as apple, pear and cherry, as well as farmed cereals, vegetables and flower crops, go into a state of physiological dormancy that lasts one to six months and fades after being stored in a dry environment.
2. Photo dormancy: Photo-dormant seeds are seeds that do not germinate unless they are exposed to light or are kept in a dark environment. It is caused by a photochemically reactive pigment known as phytochrome, which is found in high concentrations in some plants. After being imbibed seeds are exposed to red light (660-760 nm), and the phytochrome converts to a red form (Pfr), effectively replacing the germination process for the imbibed process. When exposed to far-red light (760–800), Pfr is converted to Pf, which prevents the germination process from taking place.
3. Thermo dormancy: Some seeds require a specified temperature range to germinate, and if the temperature range is not met, the seeds remain dormant. The seeds in this category are referred to as thermo dormant. Seeds of lettuce, celery, and pansy, for example, will not germinate if the temperature is below 25 degrees Celsius.

Physiological dormancy is of 3 types:

1. Intermediate physiological dormancy: Some species’ seeds require a specified duration of one to three months of chilling while in an imbibed and aerated state, a process known as wet chilling, to achieve physiological dormancy. For example, to overcome seed dormancy in temperate fruit seeds, wet cooling is required in the majority of cases. This necessity resulted in the standardisation of stratification, a horticultural method that is now world-renowned. In this procedure, the seeds are placed between layers of damp sand in boxes and exposed to chilling temperatures ranging from 2 to 70 degrees Celsius for a duration ranging from 3-6 months to break dormancy.
2. Deep physiological dormancy: Seeds that require a long (>8 weeks) moist cooling stratification process to break hibernation. For example, Lilium, Hepatica antiloba, and Trillium have epicotyl dormancy.
3. Epicotyl dormancy: is a term used to describe seeds that have distinct dormancy conditions for the radicle hypocotyl and epicotyl, such as those seen in Lilium, Hepatica antiloba, and Trillium.

Double dormancy

Some species’ seeds are inactive due to stiff seed coats and latent embryos, which cause dormancy in their seeds. For example, some tree legume seed coats are impenetrable while at the same time their embryos are latent, as is the case with some tree legumes.

In nature, it takes two years for such seeds to emerge from their dormancy. The microorganisms attack the seed in the first spring, weakening and softening it, and then the embryo’s dormancy is interrupted by a cooling temperature in the winter the next year, breaking the cycle once more.

Double dormancy is a term used to describe the combination of two or more forms of dormancy. Dormancy can be classified as morpho-physiological, which is defined as a combination of a poorly formed embryo with physiological dormancy, or exo-endodormancy, which is defined as a mix of exogenous and endogenous dormancy characteristics, such as a hard seed coat (physical plus intermediate physiological dormancy).

Secondary dormancy

Secondary dormancy occurs as a result of poor germination circumstances. It is a further adaptation to limit the germination of an ingested seed if the surrounding environment is not appropriate for seed germination. Temperatures that are too high or too low, prolonged darkness, and water stress are examples of these types of circumstances. There are two kinds of it:

1. Thermal dormancy (thermo dormancy) is defined as dormancy caused by high temperatures.
2. Conditional dormancy: The ability of a plant to germinate varies depending on the time of year.

Advantages:

1. Allowing germination only when climatic conditions are favourable for seedling survival, as in the case of fruit trees in temperate climates.
2. Contributes to the establishment of a “seed bank.”
3. Dormancy can also be used to time the germination of seeds to a specific time of the year.

Dormancy conditions that are specifically designed to aid seed disposal are available. For example, through the digestive tract of a bird or other animal, the seed covering can be modified.

Conclusion

As a result, seed dormancy is critical for plant ecology and agriculture because it permits seeds to survive periods that are unfavourable for seedling establishment. The induction of dormancy, as well as the transition from the dormant to germination stage, are both known to be mediated by several mechanisms. Seed dormancy was caused primarily by the presence of endogenous inhibitors and an embryo that had not completed its development.