GERMINATION: STRUCTURE OF A SEED, PROCESS, TYPES - BIOLOGY - 2025

The germination is the process through which the embryo content in the seeds of spermatophyte plants develops to result in a new plant, and is characterized by the protrusion of the root outward testa or seedcoat.

In the plant kingdom, spermatophytes are the group of plants known as "higher plants", which have as a defining characteristic the production of seeds as a result of their sexual reproduction, from which it derives its name, since "sperma" in Greek means seed.

Germination of a dicotyledonous plant (Source: MAKY.OREL via Wikimedia Commons)

The group of spermatophytes is composed of flowering plants or angiosperms and non-flowering plants or gymnosperms, which produce seeds enclosed within a structure called "ovary" or bare seeds, respectively.

The germination of a seed, whatever its type, can be understood as the set of consecutive steps that make a quiescent or dormant seed, with a low water content, show an increase in its general metabolic activity and begin the formation of a seedling from the embryo inside.

The exact moment where germination ends and growth begins is very difficult to define, since germination has been specifically defined as the rupture of the seminal cover that, by itself, is already the result of growth (cell division and elongation).

There are multiple factors that affect the germination process, many of them endogenous (viability, degree of development of the embryo, etc.) and exogenous (availability of water, temperature and atmospheric composition, for example).

Seed structure

Angiosperm plants have seeds with a relatively simple structure, since they consist of an embryo (product of the fertilization of the ovum by the pollen grain) that is surrounded by a covering known as the “embryo sac”, which also derives from the fertilization process.

The seed coat is known as the testa and is the product of the development of the internal integuments of the ovule. The embryo feeds on a substance in which it is immersed, the endosperm, which can also become a rudimentary tissue in those plants with cotyledons.

Cotyledons are primary leaves that can fulfill nutritional functions for the embryo and can take care of the photosynthesis of the seedling that is formed when the seed germinates.

The amount of reserve substance is highly variable among the seeds, especially regarding the composition of proteins, fats and carbohydrates they have. However, the main storage substance in the seeds, to a greater or lesser extent, is usually starch.

The embryo is the fundamental structure of a seed. It can be seen as a "miniature plant" and consists of a radicle, a plumule or epicotyl (above where the cotyledons are), one or more cotyledons, and a hypocotyl (below the cotyledons).

From the radicle the root is subsequently formed, which is the underground portion of a plant; the epicotyl will later be the main axis of the stem, in the aerial portion; while the hypocotyl is the portion of the embryo that unites the radicle with the plumule or epicotyl, that is, that unites the stem with the root in the adult plant.

It is important to note that there is a great diversity of seeds in nature, especially with regard to size, shape, color and general structure, not counting their intrinsic physiological characteristics.

Process (stages)

All mature seeds are in a condition known as quiescence, whereby these propagation structures can withstand prolonged periods in which the favorable conditions necessary for germination are not in place.

The quiescence of a seed is reversed in the presence of water, of a suitable atmospheric composition and temperature (depending on the type of seed, of course).

Germination, once quiescence has passed, includes processes that are common in the physiology of plants:

- the breathing

- water absorption

- the conversion of "food" into soluble substances

- the synthesis of enzymes and hormones

- nitrogen and phosphorus metabolism

- the translocation of carbohydrates, hormones, water and minerals towards the meristems and

- the formation of tissues.

However, plant physiologists have defined three specific stages which are: imbibition, cell elongation and increase in the number of cells (cell division), the latter depending on different genetic and molecular events.

Imbibition

The water content in a mature seed is considerably low, which favors the metabolic lethargy of the tissues within. Thus, the first step in the germination of a seed is the absorption of water, which is known as imbibition.

The imbibition restores the turgor of the embryo cells, which were previously plasmolyzed due to the small size of their almost empty vacuoles.

During the first hours of this stage, no chemical changes are observed in the seeds, as well as any type of activity associated with the elongation or elongation of the cell walls, etc.

Shortly after, the hydration of the tissues (under favorable conditions of atmosphere and temperature), allows the activation of organelles and cellular enzymes, especially mitochondria. This activation also promotes the synthesis of hormones and proteins, necessary for subsequent events.

Elongation and increase in the number of cells (division)

After a few hours of imbibition (depending on the degree of desiccation of the seeds), the elongation of the cells belonging to the radicle can be appreciated, which allow this structure to extend and emerge from the surface that covers it.

The first cell divisions occur in the root meristem, just at the time when the radicle "breaks" the tissue that covers it. At this time, some cytological changes are observed, such as the more prominent appearance of the nucleus of each cell.

Stages in the germination of an A. thaliana seed (Source: Alena Kravchenko via Wikimedia Commons)

The seed coat or testa is traversed or broken by the primary root, which is represented by the radicle, after which the hypocotyledonous axis continues the process of elongation. The cotyledons remain inside the testa during this process, regardless of the type of germination.

While this process is taking place, the nutrition of the embryonic cells depends on the activity of the enzymes responsible for the degradation of carbohydrates and reserve fats in the endosperm and / or cotyledons, activity completely dependent on the previous imbibition process.

Types of germination

The types of germination have been defined according to the fate of the cotyledons when the seedling is formed from the embryo. The two best known types are epigeal germination and hypogeal germination.

Diagram of the germination process of a pea seed (Source: Germination.svg: * Germination.png: Kat1992derivative work: Begoonderivative work: Begoon via Wikimedia commons)

Epigeal germination

It occurs in many woody plants, including gymnosperms, and is characterized by the cotyledons emerging from the soil as "pushed" by the elongated epicotyl.

Hypogeal germination

It occurs when the cotyledons remain in the underground portion, meanwhile the epicotyl grows erect and photosynthetic leaves develop from it. It is common for many plant species, being the maple, chestnut and rubber tree examples.

References

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2. Copeland, LO, & McDonald, MF (2012). Principles of seed science and technology. Springer Science & Business Media.
3. Nabors, MW (2004). Introduction to botany (No. 580 N117i). Pearson.
4. Srivastava, LM (2002). Seed germination, mobilization of food reserves, and seed dormancy. Plant Growth and Development: Hormones and Environment. Academic Press: Cambridge, MA, 447-471.
5. Taiz, L., Zeiger, E., Møller, IM, & Murphy, A. (2015). Plant physiology and development.
6. Toole, EH, Hendricks, SB, Borthwick, HA, & Toole, VK (1956). Physiology of seed germination. Annual review of plant physiology, 7 (1), 299-324.
7. Tuan, PA, Sun, M., Nguyen, TN, Park, S., & Ayele, BT (2019). Molecular mechanisms of seed germination. In Sprouted Grains (pp. 1-24). AACC International Press.