The following is from a link that no longer works:

http://koning.ecsu.ctstateu.edu/seedg/seed.html

Seed Germination

A seed certainly looks dead. It does not seem to move, to grow, nor do anything. In fact, even with biochemical tests for the metabolic processes we associate with

life (respiration, etc.) the rate of these processes is so slow that it would be difficult to determine whether there really was anything alive in a seed.

Indeed if a seed is not allowed to germinate (sprout) within some certain length of time, the embryo inside will die. Each species of seed has a certain length of

viability. Some maple species have seeds that need to sprout within two weeks of being dispersed, or they die. Some seeds of Lotus plants are known to be up to

2000 years old and still can be germinated.

Assuming the seed is still viable, the embryo inside the seed coat needs something to get its metabolism actived to start the embryo growing. The process of getting a

seed to germinate can be simple or complicated, and this our present subject.

Seeds Lacking True Dormancy

Common vegetable garden seeds generally lack any kind of dormancy. The seeds are ready to sprout. All they need is some moisture to get their biochemistry

activated, and temperature warm enough to allow the chemistry of life to proceed. Seeds taken from the wild, however, are frequently endowed with deeper forms

of dormancy.

Seeds With Truly Dormant Embryos

There are several mechanisms that permit seeds to be truly dormant.

Thick Seed Coat

Many kinds of seeds have very thick seed coats. These obviously keep water out of the seed, so the embryo cannot get the water needed to activate its metabolism

and start growing. The lotus seeds are an example of this. An outstanding example from the northern temperate zone is the Kentucky coffee tree (Gymnocladus

dioica). The seed coat is perhaps two millimeters thick! You can throw them as hard as you can against a concrete sidewalk and they just bounce! How could such a

seed actually sprout?

The Kentucky coffee tree holds its seed pods in the the top of the tree all winter. The inside of the pod is fleshy (lots of water). The pods are very dark in color. If

you put the fickle winter and sunshine and darkness into this picture, I think you can come up with the answer. Here is a hint: you might want to recall what happens

if you fill the ice cube trays in your freezer too full with water, or you might recall what happens to a container filled with soda that is then frozen.

Other species might use some pounding along a river or drop seeds into seacoast surf to abrade the thick seed coat. Some of the sea beans do this. Other seeds

might need an vertebrate or other animal to attack the seed coat (but give up trying to eat the seed) and thereby weaken the coat. The process of nicking the thick

seed coat to initiate germination is called scarification.

A final, and very common, example of a way to scarify a seed coat is observed in strawberry and raspberry. The thick seed coat is designed to be swallowed by the

frugivore. The animal digests the fruit pulp, but the seed coat passes through the digestive system still protecting the viable embryo inside, but weakened enough to

allow sprouting! The seed is deposited with a little organic fertilizer in the environment and can now sprout!

Thin Seed Coat

A thin seed coat is so thin that it is no barrier to water. Some other kind of dormancy mechanism is needed. Knowing that light can penetrate thin layers of plant

tissue (leaves for example) should give you the idea that light might be a signal. That plants can absorb light and respond biochemically is a fact you know from

your study of photosynthesis. All we need is a pigment molecule that can absorb light and cause a change in the behavior of the embryo.

The pigment is phytochrome. Like chlorophyll, it is made of a chromophore with tetrapyrole structure and is associated with proteins. This pigment is different from

chlorophyll, however, in one critical way. It exists in two inter-convertible forms.

One form of phytochrome, named Pfr, is the form of the phytochome found in plant cells that are exposed to red (660 nm) or common white light. This form of

phytochrome is biologically very active and plays a role in all systems when a plant needs to know if the lights are "on" or "off." In lettuce (Lactuca sativa) seeds,

Pfr causes the seeds to begin to germinate as we will soon see. Thus lettuce seeds germinate only when placed in white or red light. Buried in deep soil, they will not

germinate. Given that lettuce has a small seed, I think you can figure out why evolution arrived at this solution.

The other form of phytochrome, named Pr, is formed when phytochrome is exposed to far-red (730 nm) light. This form is biologically inactive or inhibits

responses. Thus if lettuce seeds are placed in far-red light they do not germinate.

Large seeds have lots of storage material. If their seed coat is very thin, their evolution may have arrived at a completely different response. Think about pea seeds.

They are large and have very thin seed coats. How would they respond to light?

Insufficient Development

If a seed's embryo is not completely developed, some additional maturation may be needed before the seed can sprout. This happens in seeds with little-to-no

storage material invested in the seed. Examples include orchid seeds. They are the size of dust and have almost nothing but a very immature embryo on-board. Such

a seed needs an association with fungi in the soil or other environments to feed the developing embryo until the embryo is mature enough to actually penetrate the

seed coat. These seeds are also likely to have a very brief viability. The fungal association must be established rapidly or the embryo dies.

Inhibitors Present

Many plant species invest chemicals in the developing seeds, and these chemicals inhibit the development of the embryos. They keep the embryos dormant.

Obviously the seed must have some way to eliminate these chemicals before they can sprout.

Abscisic Acid

Many temperate zone species that use inhibitors use abscisic acid. This chemical induces dormancy in the embryo. The chemical is produced in abundance in the

late summer and early fall. The seeds in the fruits become dormant so, even if they are dispersed in autumn, they cannot sprout. During the winter enzymes in the

seeds degrade the abscisic acid. By spring the abscisic acid is gone and the seed can sprout.

We can collect seeds of these species and get them to sprout early. The seeds are put in moist soil and refrigerated for about four weeks (a process often called

stratification). This is sufficient time to degrade the abscisic acid. Then the planted seeds are placed in a warm greenhouse. The seeds assume winter is over,

spring has come, and they begin to sprout. This process is called vernalization. If you think of "vernal" as meaning "spring" then you understand how we got this

name!

Phenolic Compounds

Plants that live in deserts have a different problem. There is no cold, moist, winter to allow vernalization of abscisic acid. These plants instead use more potent toxins,

phenolic compounds, to keep their seeds dormant until the proper season for germination. Phenolic compounds are freely water-soluble, the plant is living in a

desert. Deserts typically have very long dry seasons and a short wet season accompanied by flash floods and so on. How do you think the phenolic compounds are

lost? How would the mechanism ensure that seeds do not sprout in the dry season, but only after the seed could be sure it is in the wet season? The word leaching

might give you a hint?

This is a diagram of the seed of barley (Hordeum jubatum)

As any "typical" seed it has three fundamental parts:

a seed coat

a storage area (in this case the endosperm) and

a dormant embryo

The seed coat is really a fruit coat. In all grains, the seed coat (former ovule integument) is fused to the ovary wall (the true fruit wall). So in fact, the grain is technically

a fruit (caryopsis) even though we often call it a seed.

Barley is used primarily in beer making. Brewers use barley as a source of sugar to make the alcohol for beer. Brewers knew that sprouting the barley seeds improves

the sugar yields tremendously, but they wanted even more sugar yield. The brewers came to plant physiologists to find out if anything could be done.

Study of the seed showed that the seed/fruit coat is water-resistant and thereby reduces the rate of water uptake by the seed, and water uptake is essential for seed

germination and the improved sugar yields.

The sugar content of a dry barley seed is actually quite low, but the endosperm holds a huge reserve of starch. Starch is a polymer of sugars. In a sense it is a long

chain of sugar molecules linked together. This is probably the source of the sugar.

The barley embryo has three parts:

the cotyledon (or seed leaf). Since there is only one, barley is a monocot.

the epicotyl (becomes the shoot)

the radicle (becomes the root)

Seed germination is said to have occurred when growth of the radicle bursts the seed coat and protrudes as a young root. The energy for seed germination probably

comes from respiration of the sugar in the endosperm. However, the embryo and the starch are separated from each other. There must be some chemical

communications that physiologists could manipulate to enhance sugar yield for the brewers.

Move to the next stage.

The endosperm of the seed has two parts. The bulk of the volume is a starch storage area. The covering layer is called the aleurone layer. The aleurone is made of cells

that store protein in abundance.

You probably know about this in the case of rice. Rice grains are treated before we buy them for cooking. The fruit/seed coat is milled off and discarded as it has little

food value. The embryo breaks out and is called the "germ." In wheat, the embryos are sold as wheat germ. The milling process leaves the rice endosperm surrounded

by aleurone. This is sold as brown rice and has food value in starch and protein. It has an interesting texture after cooking. If the endosperm is polished before it is

sold, the aleurone can be removed. Polishing results in white rice which has food value in starch only. Thus white rice, while considered much "finer" than brown rice,

is actually a lower-quality food.

Now we will proceed with what the physiologists figured out...

The first step in barley seed germination is imbibition. In this process, water penetrates the seed coat and begins to soften the hard, dry tissues inside. The water

uptake causes the grain to swell up. The seed/fruit coat usually splits open allowing water to enter even faster. The water begins to activate the biochemistry of the

dormant embryo.

The water coming into the seed and embryo dissolves a chemical made inside the embryo. This chemical is called Gibberellic Acid (GA). It is a plant hormone, not

too different from steroids.

The dissolved GA is transported with the water through the rest of the seed tissues until it arrives at the aleurone layer.

The Gibberelic Acid which is transported by the water arrives at the aleurone layer.

The GA crosses into the cytoplasm of the aleurone cells and turns on certain genes in the nuclear DNA. DNA is, of course, the hereditary molecule and contains the

instructions for making every protein needed for the survival of a barley plant. The precise mechanism of how GA turns on the DNA is unknown at present. It is clear

however that the mode of action is to turn on just certain genes in the DNA.

The genes that are turned on are transcribed. The information archived in the DNA is precious, so the aleurone cells make a disposable RNA copy of the gene that is

turned on. This disposable copy of the information, a kind of blueprint, is often called messenger RNA. The process of making this RNA copy is called transcription.

The RNA that was made in the transcription process is transported into the cytoplasm of the aleurone cells.

In the cytoplasm, the messenger RNA joins up with a ribosome to begin the process of making a protein. This process is often called protein synthesis or translation.

In this process, the ribosome examines the information held in the sequence of bases in the RNA.

Transfer RNAs charged with particular amino acids are moved into the position specified by the instructions in the messenger RNA, and the amino acids are joined in a

proper sequence by the ribosome. The sequence of amino acids determines the properties of the protein being assembled.

In this case, the critical protein made with the information held in the RNA is amylase. This protein turns out to be an enzyme of great importance.

The process of information held in the genes of DNA being transcribed into RNA and then translated into protein constitutes the central dogma of genetics. You will

want to remember later, that it is the signal of Gibberellic acid that initiates this whole chain of events.

A question one might ask right away is: from where do the amino acid building blocks for the amylase come?

The answer is: from some other biochemistry in the aleurone cells. This biochemistry causes the storage proteins in the aleurone cells to be digested by hydrolytic

enzymes. The hydrolysis is accelerated by enzymes known as proteases. These enzymes increase the rate at which the storage protein is cut into individual amino

acids.

The amino acids released by the hydrolysis are then free to be reassembled by the ribosomes into the structure of amylase.

The same thing happens in people. You are not what you eat! You do not slowly become a steer by eating beef. The protein in your hamburger is digested into

amino acids. Those amino acids are then reassembled into human proteins. Since the instructions for reassembling the amino acids come from your human DNA,

the proteins produced are human, not bovine!

In a similar way, barley aleurone storage proteins are digested and amylase is made from the released amino acids. So, we have our amylase produced, so what?

This is when the payoff begins!

The amylase is secreted (transported out) from the aleurone cells into the endosperm.

Amylase is not just any old protein. It happens to be an enzyme. It catalyzes (accelerates) a particular chemical reaction. Specifically, the amylase speeds the

hydrolysis of starch into its component sugar units.

Hurrah! The physiologists now know how the sugar is made in a seed. They can also tell the brewers how to increase the sugar yields for beer making. What do they

tell them? I am sure your powers of recall will help you. What signal started this whole cascade of events?

Note that Gibberellic Acid was the first plant hormone purified in large quantities to be successfully used to increase the yield of any economically-important

biochemical in plants!

Move to the next stage.

Okay, so we helped out our brewer friends, but we need to finish up this project with barley seed germination.

Remember that we wanted to get the seed germinated (sprouted). This does not occur until after the released sugar is transported from the endosperm to the embryo.

The cotyledon is a major sugar transfer area in grains. Active sugar uptake is its specialized function. The sugar is transported into the embryo and used as a fuel

and building block for growth of the embryo itself.

The resulting embryo growth will include the emergence of the radicle from the seed/fruit coat. Germination is thus complete! Almost the end of the story...

Go to the lettuce variation on seed germination.

Our barley seed story is great, but it does not explain some other forms of seed germination.

The next great finding in the study of seed germination is what happens in lettuce seeds (Lactuca sativa). Lettuce seeds look somewhat different from barley seeds,

because they are dicots. They also have separate seed coat and fruit walls. But the things you buy as lettuce "seeds" are really lettuce fruits (called cypselas). As

different as they are, the seed germination process in lettuce needs just one step more than barley to get its seeds germinating.

Lettuce cypselas have very thin fruit walls and seed coats, so light can penetrate these coverings. As is typical of small seeds, light is needed to stimulate seed

germination. One might wonder how this would fit into what we just found out with barley.

To keep things simple, I am not drawing a lettuce seed here, but am showing you what happens in lettuce seeds by attaching the additional step to the diagram of a

barley seed we have been studying (remember lettuce is a dicot and lacks an aleurone, etc.):

Notice the new step in the diagram. The DNA must first be photoactivated before GA can have its effect. This is accomplished by the formation of a chemical signal

known as Pfr.

Pfr is one of two possible chemical forms of a molecule known as phytochrome. Phytochrome is a pigment that absorbs light energy. When it does absorb light energy

its internal chemical structure is altered instantly. Thus Pfr can be converted to the alternate Pr form of phytochrome by simply hitting the seeds with a flash of far-red

light (730 nm). Similarly, the Pr form of phytochrome can be converted back into Pfr with a flash of red light (660 nm).

Dry lettuce seeds have slightly more than half of their phytochrome in the Pfr form. So if they are moistened and kept in the dark, some of the seeds will actually

germinate.

If we shine red (or it turns out even white) light on imbibing seeds, then virtually all of the phytochrome is converted to the Pfr form and this photoactivates the genes in

the DNA. Almost all of the seeds will germinate in this light!

On the other hand, if we shine far-red light on imbibing seeds, then virtually all of the phytochrome in the seeds is converted to the Pr form and none of the seeds will

germinate.

Thus phytochrome and light tell lettuce seeds when they are exposed to sunlight and can begin to germinate. Small seeds, such as those in lettuce, do not have reserves

enough to start germinating without being able to quickly get to light. It is no surprise that small-seeded species have evolved this mechanism for photoactivation of

seed germination. It prevents a deeply-buried seed from trying to germinate!

As a challenge to your own thinking, you might wonder what happens in the case of a species with large seeds (peas = Pisum sativum) but with thin seed coats. How

might the strategy be similar? How might it differ significantly?