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26th Oct 2022

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As a grower, unless you buy plot-ready plants, you will be purchasing seeds and carrying out your own propagation. Have you ever wondered just what happens between sowing seed and plant emergence? GrowMad's Claire explains the process...

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How Do Seeds Germinate?

| Introduction

Take a seed, plant it, water it and watch it grow - simple? But have you ever stopped to think about how amazing that is?

Unless you are buying your vegetable plants via mail order or from your local garden centre, germinating your own seeds is a process you will have to be involved in.

Seed germination is one of nature's miracles so I will try and shed some light on this particular wonder.

There's much to discover so let's unearth the mystery...

| Anatomy of a seed

Seed Image 4

A seed generally consists of three basic parts, an embryo, an endosperm, and a seed coat.

The embryo contains the parts that will grow into a full-size plant once germination has occurred, consisting of precursor tissues for the leaves and stem (cotyledons), and root (radicle).

The endosperm is a tissue that surrounds and nourishes the embryo. It mainly contains carbohydrates and proteins but in some species contains oils as well. This is why brassicas such as Oil seed rape can be harvested for their oil.

The Testa, or seed coat, is the tough outer part that protects the structures inside.

| Germination

Germination is the process of seeds developing into new plants.

Seeds stay dormant until they are exposed to the right temperature, moisture, air, and light necessary to germinate according to their species.

In essence, germination follows four stages, which can all be initiated or stopped depending on growing conditions.

Professor Deno states that-

1. Every species of plant has one or more mechanisms for delaying germination until the seed is dispersed.

2. Temperatures of 40 or 70 degrees are the most common for germination.

3. Members of the same family, genus, and even closely related species may have different mechanisms for delaying germination.

4. Some species germinate under conditions of oscillating temperatures.

5. Light is an important variable and is a requirement for germination in some species.

6. Seeds in fruits often have chemical inhibitors in the flesh of the fruit that block germination, so must be removed by washing before germination can take place.

7. Some species have two-step germinations.

8. Some species produce quantities of normal-size seed coats which are empty.

This may explain why sometimes your Brussels or your beetroot just don’t germinate- it’s a case of everything being in the right place at the right time!

Let's take a look at the individual germination stages:

Stage 1. Imbibition - absorption of water:

Seeds contain the genetic material that is required for the embryo to be transformed into a seedling. This is in the form of messenger RNA (mRNA), which essentially carries the instructions that a cell can use to make proteins. However, the embryo cannot begin to grow without air and water, so the outer coat needs to be breached for these resources to get in.

As most seeds are very dry, there is a great propensity to take in water, which occurs rapidly. Be aware though, that dead seeds can also take in water, so if you are soaking seeds, they may swell, but will never germinate.

During imbibition, chemical changes occur to the structure of the seed coat. The water rehydrates the structural and storage molecules in the cell wall. In brassicas, in particular, a mucilage (a thick gluey substance) made of pectins and cellulose is produced, which may make the seed appear slimy. It is theorised that this mucilage may help it stick to surfaces and animals to help with seed dispersal. As this layer is very hydrophilic (water-loving) it can help aid germination in dry conditions as reduces water loss.

Stage 2. Respiration:

Initially, the dormant seed is held in a state of suspended animation, where life processes are halted.
However, as the seed coat swells and cracks, water and oxygen are allowed in. As water is taken into the seed, enzymes become reactivated and metabolic activity starts. The stored mRNA starts to direct enzymes and hormones to be produced.
Respiration begins, which is the process by which simple sugars such as stored glucose within the endosperm, are reacted with oxygen to produce carbon dioxide and release energy.

Seed Image 9

The respiration process

This energy can then be utilised for growth processes. The absorbed water also increases the turgor pressure within individual cells allowing them to enlarge and thus grow.

Stage 3. Mobilization of Reserves and Role of Growth Regulators:

Stored starch, protein, or fats need to be digested for the embryo to continue to grow. Depending upon the type of seed, these food reserves may be stored in the endosperm (in monocotyledons, such as wheat, rice, and onion) or in the cotyledons (in dicotyledons such as peas and beans).
The embryo produces a hormone called Gibberellin (GA) which is a growth-regulating hormone. This activates a special outer layer of cells (aleurone layer) of the endosperm, causing it to then produce and secrete enzymes such as amylase and proteases. These enzymes break down, respectively, starches and proteins into simpler food solutions comprising sugars and amino acids.

Stage 4. Development of Embryo into a Seedling:

The simple food solutions produced above are diluted by water and passed towards the growing epicotyl, hypocotyl, and radicle. The cells of the embryo in these growing regions become very active. They grow in size and start dividing.
The radicle emerges from the seed coat- to become the root. This is one of the first outward signs that the seed is viable. Then the shoot, the hypocotyl, emerges and begins to grow towards a light source.

Seed Image2

Development of embryo into a seedling

| Effect of light levels on seed germination and growth

Plants vary greatly in response to light with respect to seed germination. The seeds which respond to light for their germination are called photoblastic and are grouped into three different categories.

a) Negative photoblastic seeds, e.g. onions, do not germinate if exposed to sunlight, so require planting reasonably deeply away from light.

b) Positive photoblastic seeds require the presence of sunlight, e.g. lettuce- so these would do well with the seeds germinating on a wettened paper towel on your windowsill or sown onto the surface of your soil.

c) Non-photoblastic seeds will germinate regardless of light levels.

If you have planted your seeds in the soil, germination occurs in the dark. Once the radicle has emerged, root growth slows whilst shoot elongation accelerates. As the seed has a limited supply of nutrients available, the seedling needs to start making its own food. This occurs via photosynthesis, a process that requires light, so the shoot grows rapidly towards the sunlight.

You can discover more about Photosynthesis here.

| Seed dormancy

A seed can remain dormant for years before suddenly bursting into life! Why might a seed remain dormant?

Poppy field2

Poppy seeds can lay dormant for centuries

Dormancy is crucial to the survival of plant species because it ensures that seed germination will occur only when environmental conditions are optimal for growth.

This process is also important for vegetable growing to ensure good crop rotation and prevent disease. A plant has various dispersal methods to ensure that any offspring can grow away from the mother plant, otherwise, you would have all the young plants smothering the original one. Therefore, a plant needs a way of halting germination until the conditions are right. For some, this can be a chemical delay mechanism, such as a period to allow it to dry out (e.g. peas, beans) for others it’s the fact that the seed is protected by a hard physical coating that must be penetrated.

A seed needs to break either its physical or chemical dormancy to germinate.

However, sometimes, the seed may not germinate even under favourable conditions. Below are listed some reasons and possible remedies.

Hard Seed Coat

Seeds of many species of legumes possess a hard seed coat and may remain dormant.

Hard seed coats prevent the entry of water into the seed and obstruct the exchange of gases, in particular, oxygen. A hard seed coat can also prevent a radicle from emerging.

A hard seed coat can, however, be weakened by a process called scarification. Scarification is a process whereby the gardener will manually abrade the seed. This can be achieved by nicking with a sharp knife or rubbing with sandpaper, ensuring that the embryo is not damaged in the process. Soaking in warm water can also help by softening some of the coat and allowing the structural proteins to become slightly unstable, thus more susceptible to imbibing water.

Incorrect light levels

As explained above, ensure you know the light conditions required by the particular seed you are planning on growing.

Temperature requirements

Some seeds need a period of chilling before they germinate. In the ground, this can occur naturally, but if you are planting new seeds, they may need a period of refrigeration first.

Brassicas will not germinate if the temperature is too high, so avoid sowing in a hot greenhouse. This anomaly is known as summer dormancy.

Oxygen requirements

Land plants, as opposed to aquatic species, cannot germinate underwater (apart from rice) as they require more oxygen than is available dissolved in water. These seeds need to obtain oxygen from the air in the surrounding soil. Therefore to ensure germination, most seeds need to be sown in loose soil near the surface. Hoeing can also aerate the soil and help encourage germination- but please do this before seed sowing!

| Conclusion and Takeaway

So there you have it! Not quite as straightforward as you thought...

It is now obvious that the seed-sowing and germination stars have to be aligned for seed-sowing success.

So, if you can germinate seeds and grow veg despite all this, you have reached seed-sowing Eden.

Take your time over sowing and don't be despondent if it sometimes results in failure. Seeds are fickle things, simply go again and resow.

As an aside, if you are interested in seeds in general, you may be intrigued to discover more with our comprehensive range of seed suppliers and their various array of seed offerings, in the GrowMad Seed Directory.

Happy sowing!