Seeds do not germinate as soon as they are formed and dispersed, they wait until suitable conditions occur. Thus dormancy or dormancy occurs and the seed can remain viable for months or even years. Seed dormancy can also be defined as the state or condition in which the germination of seeds is inhibited even in environmental conditions suitable for germination, including temperature, water, light, gas, seed coat and other mechanical constraints. The main reason for this is that they need a period of rest in order to germinate. The mechanism of dormancy may differ between species, depending on the diversity of climates and habitats.
There are certain causes of seed dormancy. Below are some of these reasons.
-Hard seed coat
– Germination inhibitors
-immatureness of the seed embryo
– Impermeability of seed coat to water
– Oxygen impermeability of the seed coat
-Mechanically durable seed coat
– Presence of highly concentrated solutes
Stages of dormancy
The development of dormancy, or sleepiness, typically occurs in stages.
The first stage of dormancy is called pre-dormancy. This early stage is reversible because, for example, the seed will continue to grow if it is returned to suitable growing conditions in a greenhouse. As pre-dormancy develops, the environmental conditions shrink, allowing growth to resume.
Following the pre-dormant period, the seed enters true dormancy. In true dormancy, growth will not resume even if the plant is returned to optimum growing conditions.
The final stage of dormancy is post-dormancy or post-sleep. This stage is typical for post-winter and early spring. During the post dormancy stage, the seed may grow, but growth may still be suppressed by adverse environmental conditions (eg low temperatures).
Innate, Compulsive, and Triggered Dormancy
Seed dormancy can be studied by dividing it into the following types:
It is the situation of seeds that cannot germinate even if suitable conditions are provided for seedling cultivation. This failure to germinate may be due to the embryo being immature at the time of dispersal in some species.
It is the situation of seeds that cannot germinate due to environmental restrictions, that is, adverse conditions. Adverse conditions include insufficient moisture, oxygen, light and a suitable temperature. This dormancy is forced into seed by a limitation of the germination medium. This type of sleep state disappears after the missing conditions are met.
Although the initial (primary) dormancy has been removed, seeds may re-dormancy due to unsuitable environmental conditions. Some of the conditions that initiate induced or induced seed dormancy are anoxia, darkness, excessively strong light, temperatures higher (lower) than the maximum (minimum) temperatures for germination to complete, and water stress. For example, it occurs when the seed absorbs water, but is subjected to extremely unfavorable conditions for germination. Triggered dormancy is also called secondary dormancy. Triggering conditions do not universally induce secondary dormancy for all species, with some species affected and others unaffected. Often, secondary dormancy reduces seed susceptibility to chemicals known to promote germination of dormant seeds.
Types of Dormancy Depending on the Source
Dormancy may result from conditions outside the embryo or from within the embryo. Depending on its source, seed dormancy is divided into two types:
This type of dormancy is exerted by factors other than the embryo. The tissues surrounding the embryo can affect germination by preventing water uptake, providing mechanical resistance to embryo expansion and radicle emergence, limiting oxygen to the embryo, preventing the inhibitor from leaking from the embryo, and providing the inhibitor to the embryo. The dormancy caused by an impermeable seed coat is called physical dormancy. It originates from the impermeable palisade cell layers in the seed or fruit shell that control water movement. Such layers prevent the seed from getting water or gas. As a result, seeds cannot germinate until dormancy is broken. Physical dormancy was observed in the seeds of plants belonging to 16 families of Angiosperms or angiosperms, but physical dormancy was not recorded in any gymnosperm.
Seeds that are sensitive to light need a dark or light period to germinate. This is called photodormancy. In species with a thin seed coat, light can penetrate the dormant embryo. The presence or absence of light can trigger the germination process, inhibiting germination in some seeds that are buried too deeply or in others that are not buried in the soil. Similarly, some seeds are sensitive to heat or cold (thermo dormancy), for example, larch and amaranth germinate only at high temperatures (30C).
Endogenous dormancy occurs due to chemical changes in the embryo of the seed. One reason a plant may fail to germinate due to endogenous dormancy is because the embryo is not yet fully developed or specific seasonal cues have not emerged. Germination can also be suppressed due to endogenous chemical inhibitors.
Methods of Breaking Dormancy
Seed dormancy is when a seed cannot germinate even under ideal growing conditions. Seeds germinate when they are most likely to develop, as dormancy can be broken with the most ideal growing conditions (which are different and specific for each species). Different methods of breaking dormancy are mentioned below:
Natural Breaking of Dormancy
The nature of dormancy ceases when the embryo receives the appropriate environment such as adaptive humidity and temperature. The seed coat, which is found in many species, becomes permeable due to the disintegration of the digestive system of birds and animals that feed on these seeds, due to the softening effect of natural factors such as microorganisms, temperature and abrasion. Completion of the excessive maturation period, leakage of inhibitors in the seed coat, inactivation of inhibitors by cold, heat and light source, leakage of excess and high concentration solutes from seeds, production of growth hormones that can neutralize the effect of inhibitors, are among other natural breaking methods.
Artificial Breaking of Dormancy
Some of the artificial methods used to break seed dormancy are listed below:
-In seed coats, waxes, surface inhibitors, etc., which are on the surface, preventing permeability. It can be treated with hot water to terminate it.
-The seed coat is broken up by filing, chipping or shaking in machines. These processes soften or crack a hard seed coat, making it permeable to water or gases. This process is called scarification. The process may be chemical or physical in nature.
-Exposed to heat, cold or light, depending on the type of seed dormancy. Incubation at 40-50°C for several hours to several days may be effective in overcoming dormancy in some species. For example, rice seeds are treated with hot water at 40 C for at least 4 hours. A suitable low temperature incubation (chilling process) of seeds on a moist substrate before transferring them to a temperature suitable for germination can also break dormancy. This processing process is called stratification.
-Hydraulic pressure can be applied for 5 to 20 minutes to weaken the hard seed coats.
-Seed hulls can be treated with concentrated sulfuric acid to remove all traces of mineral acid.
Role of Plant Hormones in Dormanside
Some researchers believe that during the short days in autumn, Abscisic Acid (ABA) reaches high levels and induces dormancy. Accordingly, dormancy is initiated by an increase in ABA content, which occurs in about half of normal seed development. The ABA concentration then decreases slightly in the later stages of development until seed maturity. ABA plays an important role in inducing dormancy in seeds, but it is not alone in this process. Auxin is one of the classical phytohormones that is effective during tropism (orientation) growth and tissue differentiation. Recent studies show that auxin has positive effects on seed dormancy. This suggests that auxin is the second phytohormone besides ABA to induce seed dormancy. Seed dormancy can be broken using gibberellins. As the soil begins to warm, growth promoters such as gibberellin and cytokinins accumulate and signal the radicle within the seed to continue growing.
The Importance of Dormancy in Seeds
Species with dormant seeds have developed dormancy, which is beneficial for survival. Plants use seed dormancy so that the seed can withstand unfavorable conditions and not all germinate at the same time, dying by unfavorable weather conditions. Dormancy provides the storage of seeds for later use by animals and humans. It helps the seeds to disperse in an unfavorable environment. Dormancy induced by inhibitors in seed pods is highly beneficial for desert plants. The dormancy state allows seeds to continue in a state of suspended or interrupted viability. Thus, the seeds are not damaged in cold or high summer temperatures or even in drought conditions. Dormancy also helps seeds survive in the soil for several years and provides a constant supply of new plants even when all mature plants in the area have died due to natural disasters. Seeds are dormant for a long time and this acts as a survival mechanism. The seed can adapt to adverse conditions and resist improper germination. The disadvantages of dormancy are the long times required to overcome dormancy and the longevity of weed seeds.
Writer: Musharref Ozdas