Seed Germination

What is Seed Germination?

In biology, a seed is a plant in its embryonic stage that is encapsulated in a seed coat. In addition to a plant embryo, the seeds of most plant species also have food reserves.

When the soil moisture and temperature conditions are favourable for growth, seeds “wake up.” The collection of active metabolic processes that lead to the emergence of a new seedling is called seed germination.

Stages of Seed Germination

Seed germination takes place in the following three stages:

1. Inhibition: the seed imbibes (takes up) a lot of water, causing the seed coat to expand and soften.

2. Interim or lag phase: the seed’s internal physiology is activated, so its cells respire and the seed begins to produce proteins and use up its food reserves.

3. Radicle and root emergence: the cells begin to lengthen and divide, allowing the root and radicle to emerge from the seed.

A seed is considered to have completed germination as soon as the radicle breaks open the coleorhiza (root sheath) and emerge. A seed becomes a “seedling” as soon as the first true leaf emerges.

Types of Seed Germination

There are two main types of seed germination--epigeal and hypogeal germination--which indicate whether the cotyledonary node is above or below ground during germination.

The rate of cell division in the plant's hypocotyl area during germination and early seedling growth influences the position of the cotyledon. The epicotyl is the embryonic shoot region above the cotyledon attachment point, while the hypocotyl is the embryonic region below the cotyledon attachment point and continues down to the root.

We will briefly discuss the two types of seed germination and then present a set of diagrams using specific examples to help you visualize the processes.

Epigeal Germination

In epigeal germination, cell division in the hypocotyl is faster and more vigorous than cell division in the epicotyl. The hypocotyl's actively growing meristem induces cell growth and extension, pushing portions of the hypocotyl, the cotyledonary node, and epicotyl, above the ground.

See the image below for a visual representation of epigeal germination using the pinto bean seed as an example.

Hypogeal Germination

In hypogeal germination, cell division in the apical meristem at the end of the epicotyl is faster and more vigorous than in the hypocotyl. The cotyledons and all the hypocotyl stay below the soil surface as a result of this cell division and elongation. The cotyledonary node, then, is found below ground.

See the image below for a visual representation of epigeal germination using the pea seed as an example.

If you are having trouble distinguishing between epicotyl and hypocotyl as well as epigeal and hypogeal, it might help to remember that the prefix "epi" means above, while "hypo" means below. As such:

• Epicotyl = embryonic region above the cotyledon attachment point.

• Hypocotyl = embryonic region below the cotyledon attachment point.

• Epigeal = cotyledonary node is above ground.

• Hypogeal = cotyledonary node is below ground.

The Favorable Environment for Seed Germination

Seeds require specific temperature, moisture, and light to germinate. Without an optimal environment, germination may take longer and at an inconsistent pace. In this section, we will discuss each of these requirements.

Seed Germination Temperature Requirements

Seeds have ideal temperature ranges for germination:

• The minimum temperature is the lowest temperature at which seeds can successfully germinate.

• The maximum temperature is the highest temperature at which seeds can successfully germinate.

• The ideal temperature is the temperature at which seeds germinate rapidly and consistently.

Temperatures lower than the minimum or higher than the maximum temperature can cause seed damage or dormancy, which we will discuss further later on. To measure soil temperature, insert a soil thermometer around 3 to 4 inches deep from the soil surface and note the temperature reading.

One way to obtain optimal germination temperature in growing media is using germination mats. Such mats can be used to adjust the temperature depending on the needs of the seeds. Peppers, for example, germinate in 8 days at 86°F but take more than 13 days at 58°F.

Seed Germination Moisture Requirements

Seeds also require the right amount of moisture to initiate the metabolic processes that lead to germination. For example, field soil usually requires moisture at 50-75% of field capacity. Additionally, seeds need proper soil aeration which can be achieved if the seedbed has a fine texture and enough seed-to-soil contact.

Without proper soil aeration–for example, it has too much moisture or is too compact—the carbon dioxide released by the seeds will not dissolve and move away from the seeds, causing them to suffocate.

Seed Germination Light Requirements

Seeds have different light requirements: some need light to germinate, while others germinate best in the dark. There are even plants whose seed germination can be restricted by light. As such, it is important to look into the specific light requirements of the seeds.

It is also important to note that seeds and seedlings have different light requirements. After germination, most seedlings need light; without enough light, seedlings can become spindly or leggy.

Seed dormancy

Since we talked about how seeds germinate, it may also be helpful to touch on why seeds might not germinate, a state called seed dormancy. Seed dormancy is a condition that prevents seeds from germinating even under ideal conditions.

What are the types of seed dormancy and how can they be broken?

Seed dormancy can be physical or chemical:

• Physical dormancy is where seeds have a hard or thick seed coat.

  • Soaking or scratching the surface can break this type of seed dormancy and allow the seed to germinate.

• Chemical dormancy is where seeds have internal or metabolic conditions that hinder germination. Certain plant hormones like abscisic acid can prevent germination.

  • This type of dormancy can be broken by leaching the seed, using cold or moist stratification (putting seeds under both cold and moist environments such as the refrigerator), or fire scarification (using fire to weaken or alter the seed coat).

There may be various reasons why seed dormancy exists in nature. For instance, delayed germination can protect seedlings against potentially bad weather or even animals that can harm or consume them.

Planting germinated seeds

Seeds can be planted directly in the container or space where they will grow, a process called direct seeding. They can also be germinated in one container or space and then moved to a different container or space where they will continue to grow and develop, a process called transplanting. Transplanting is done after the seed has germinated and its first true leaves have developed.

Planting germinated seeds can be beneficial in several ways. One is that it can help prevent wasting space in a propagation area that could otherwise be filled with ungerminated seeds. Another is that it is a way to manage seeds with declining or unknown viability.

For instance, if seeds germinate and turn out healthy, then they can be transplanted; if they do not germinate, then maybe they are in dormancy and need to undergo other measures, or maybe they are no longer viable and can simply be discarded. Another benefit of planting germinated seeds is that they can be prepared ahead of time and transplanted outdoors when the conditions are most favourable for growth.

Some plants that can benefit from transplanting include broccoli, cabbage, and tomato.