Some general knowledge about seeds – Seed dormancy.

SOME GENERAL KNOWLEDGE ABOUT SEEDS – SEED DORMANCY. 

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The concept of seed dormancy encompasses all forms of inhibition of current germination capacity, or a lack of readiness or even inclination to germinate, regardless of the cause.

Types of seed dormancy.

Two types of seed dormancy should be distinguished: relative, or conditional, and absolute, also known as deep or organic.

Relative dormancy occurs in seeds that have already reached physiological maturity but do not germinate due to unsuitable external conditions (inappropriate temperature, water or air shortage, etc.). However, if the right conditions are met, these seeds will germinate. Absolute dormancy, on the other hand, occurs in seeds that have not yet reached physiological maturity and therefore cannot germinate without certain preliminary treatments, even with optimal factors. It lasts as long as the post-harvest ripening period continues. This dormancy period ends after a shorter or longer period of seed storage under appropriate conditions, or as a result of deliberate human intervention. The seeds then reach physiological maturity and, under favorable conditions, are capable of germinating.

Absolute dormancy is a typical property of wild plant seeds, facilitating their survival and dispersal in various climatic conditions.

Thanks to absolute dormancy, wild plant seeds can sometimes remain in the soil for many years before reaching conditions conducive to germination.

The duration of absolute dormancy depends on the genus, species, and cultivar. It can last from several to several dozen days, or even years. The seeds of most cultivated plant species have a significantly shorter dormancy period than wild plants.

Seeds stored in unsuitable conditions can revert to a period of absolute dormancy, referred to as secondary dormancy.

Two types of absolute dormancy can be distinguished: absolute dormancy, or primary dormancy, characteristic of seeds undergoing post-ripening, and secondary dormancy, also known as induced dormancy, characteristic of seeds that are already mature.

Secondary dormancy can last for a very long time, even several years. Breaking it is very difficult and often requires complex procedures.

Causes of absolute seed dormancy

The primary cause is embryo dormancy, caused by its physiological state preventing seed germination. Another cause is the accumulation of growth inhibitors in the seed at the end of ripening, which inhibit germination. Other causes may include the impermeability of the seed coat to water and the special thermal or light requirements of the seed.

Inhibitors are found primarily in the seed coat, some in the endosperm, and in very small amounts in the embryo. Their mechanism of action involves inactivating growth substances, individual enzymes, or entire enzyme systems. This inhibits metabolism, which in turn leads to inhibited germination. Inhibitors are therefore one of the most important factors influencing seed dormancy.

Seeds can enter secondary dormancy under the influence of stimuli such as climatic, chemical, and physical factors. Primarily, secondary dormancy is triggered by conditions opposite to those that break or shorten primary absolute dormancy. For example, in seeds that normally undergo post-ripening in an air-dry state, moist conditions, allowing them to swell, can induce secondary dormancy. Seeds that require prolonged stratification at a cool temperature can, when slightly dried and moved to an elevated temperature, enter secondary dormancy.

External factors influencing the duration of absolute dormancy.

The duration of dormancy depends equally on the habitat in which the seeds are produced and on the storage conditions of the collected seeds. Weather conditions during seed ripening and harvest play a particularly important role here.

The most important habitat factors in this case are temperature, light, humidity, and oxygen access.

Temperature influences the rate of seed ripening and the degree of seed maturity at harvest, and therefore, in turn, the depth of absolute dormancy. Seeds ripening at high temperatures respond differently to temperature during germination. The longer high temperatures persist during ripening, the lower and narrower the range of temperatures within which seeds can subsequently germinate. During post-ripening, seeds undergo a gradual change in their response to temperature. Over time, seeds are capable of germinating in an increasingly wider temperature range. There is also a consequential effect of seed ripening temperature on the length of the stratification period. Seeds ripening at higher temperatures generally require longer stratification before sowing.

The effect of light depends not only on light intensity but also on day length.

Relatively high humidity in the habitat, combined with cold, prolongs post-ripening. Conversely, seeds produced in dry and hot conditions have a shortened dormancy period.

The oxygen and carbon dioxide content in the soil or in storage can alter the depth of dormancy. High oxygen concentrations reduce it, while significant CO2 concentrations deepen dormancy.

Storing seeds under appropriate conditions significantly influences the course of post-harvest ripening. Seeds of most crop species undergo this process most effectively when air-dried. This group includes, among others, seeds of cereals and grasses.

Seed longevity.

The measure of seed longevity, in the sense of industrial crops, etc., is the period during which the seeds of a given species retain their ability to germinate within the limits of applicable sowing standards. However, from a natural perspective, this period lasts longer, until individual seeds are still capable of germinating.

A symptom of seed aging is a decline in germination energy and growth power, followed by germination capacity. An additional characteristic of old seeds is the formation of a large number of abnormal, distorted, and unhealthy-looking sprouts during germination.

Seed lifespan, which varies among species, is a hereditary property, primarily related to the structure of the seed coat. Legume seeds have the longest lifespan. However, seed longevity also depends largely on the conditions of seed formation and ripening, as well as storage conditions.

Seeds of the same species and variety produced in different climatic regions have different longevity, as do seeds produced in the same region but in years with different weather patterns.

The degree of seed maturity is crucial for their longevity, as is their subsequent storage.

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Best regards, Hemp.