Seeds and Seed Germination

Seed Structure

Keep in mind that the ovule in the ovary is what becomes the seed. The integument of the ovule becomes the seed coat. Inside the integument of the ovule was the embryo sac. The antipodals and synergids senesce and disintegrate. The central cell united with one sperm cell to make endosperm...a nutritive tissue that accumulates starch, protein, and fats to provide for the growth of the embryo. The egg cell of the embryo sac united with the other sperm cell to make a zygote. The zygote grows and becomes a true embryo inside the integument.

We will recall that the seed developing in the carpel produces hormones that make the carpel develop into a fruit. The mother plant also produces abscisic acid to make the embryo inside the seed become dormant

When you have a Dormant Embryo, a Storage Tissue, and a Seed Coat, then you have a SEED. In some seeds, the endosperm is retained as the storage tissue. In other seeds the endosperm is more or less used up to put storage chemicals into the embryo itself (commonly in the cotyledons). Below are diagrams and a photo of some seeds.

This first example above, is a longitudinal section through a Capsella seed. You may observe several cell layers of seed coat (integument) on the outside. In the interior is an embryo. There is a root apex, a radicle that transitions into a hypocotyl terminating at the attachment node for two cotyledons between which we can see the shoot apex. The embryo may be bent, but it has all the parts of a typical embryo. Having two cotyledons means that Capsella is a dicot plant. Between the embryo and the seed coat is a layer of endosperm remaining; some is found between the radicle and the two bent cotyledons; more is found at the lower end of the seed.

The structure of dicot and monocot seeds is illustrated below. Obviously the dicot will have two cotyledons, and the monocot will have one cotyledon. The particular dicot chosen for this diagram is not bent, and has moved virtually all the endosperm into the cotyledons. That would be a typical member of the bean family. The monocot chosen has much remaining endosperm, and is typical of members of the grass family.

Seed Germination

The seeds are carried away from the mother plant by animals called dispersers. In some cases the animal carries away an entire fruit, in other cases the animal is carrying seeds alone. The animal usually feeds on the fruit (ovary wall), but "tosses" the seeds (or passes them through its digestive system). This assures that seeds end up far away from the "mother" plant, where they can develop without competition from "mom."

Having been taken away from the parent, it is time for the seeds to sprout or germinate. Seed germination is defined as the emergence of the radicle through the seed coat. Common garden seeds germinate if given just water and reasonably warm temperatures.

Wild species usually have some kind of deeper dormancy to avoid sprouting in late summer or fall when the seeds are commonly dispersed. This assures that tender seedlings are not frozen at a young age by the approaching winter. Instead the seedlings do not appear until warm weather arrives in springtime.

If a species has evolved a very thick seed coat, it may require scarification of that seed coat before water can enter the seed and initiate germination. Perhaps the coat has to be gnawed at by animals, or frozen and thawed repeatedly to crack the coat, or rund down the rocks in a stream bed or up beach in pounding surf to wear the coat down. Perhaps the seed is swallowed whole by the disperser. The thick seed coat protects the dormant embryo as it passes through the animal's digestive system. The weakening of the seed coat through digestion means that the seed is ready to sprout when it gets deposited (along with a bit of fertilizer) by the disperser somewhere new in the environment. Yet other seeds require a brief exposure to fire to scarify the seed coat.

In species with thin seed coats, light may be able to penetrate into the dormant embryo. The embryo may then either use the presence of light or the absence of light to trigger its germination process. Small seeds with thin seed coats are likely to use light as a signal for germination. If these are buried shallowly enough for light to hit the seed, then its germination will initiate and before it runs out of reserves, its cotyledons will be doing photosynthesis. If the small seed is deeply buried it does not germinate; it has no idea how deep it is buried and may not have sufficient reserves on board to make it to the surface before they run out. Evolution favors shutting down germination in the dark for those species. For large-seeded species, the opposite may be true. There is an advantage to having your reserves buried before starting to germinate...otherwise the sprout just acts as a flag to say..."come and eat my nutritious seeds here on the surface." Evolution favors a large seed with lots of investment in storage to wait until it is buried before sending up its shoot. Such a seed will key in on darkness rather than light. In laboratory you will see an example of each of these kinds of evolutionary adaptations!

In yet other species, the embryo is fully developed when the seeds are dispersed. They need to be deposited in soil that possesses fungi that serve as host to the seed and seedling until the development is completed and the seedling is big enough to grow on its own. Trying to sprout many orchid seeds in common garden soil is futile...the required fungal hosts are not leaving wild orchids where they naturally grow is the best idea.

In most species of wild plants the mother invests the seed with abscisic acid. This hormone makes the embryo dormant until environmental conditions and native enzymes permit the abscisic acid to be broken down inside the embryo. The enzyme required is present in the embryo in the fall, but it is inactive until activated by low temperature. About four weeks at 4°C is required to activate the enzyme and degrade all the abscisic acid in the embryo. This cold treatment is often called stratification. Then the seed is ready to germinate, but it is too cold to do so. Vernalization or bringing the cold-treated seed into warm spring-like conditions will then allow the germination process to begin. This way seeds that have evolved in areas with cold winters avoid germinating until spring!

For desert plants, the role of abscisic acid it taken instead by phenolic compounds. These chemicals inhibit seed germination too, but they are not broken down by cold weather. Instead phenolics are water soluble and need to be leached out of the seeds by repeated washings to initiate germination. For a desert plant, this mechanism has evolved to ensure that seeds do not begin to germinate until the wet season has certainly arrived. A single passing shower in the dry season will not takes several soaking rains to do it.

Examples of various mechanisms are summarized in the figure below.

Also shown above is a mechanism for seed germination in two species: Barley and lettuce. That is a bit misleading as the sketch clearly shows barley (a monocot) and not lettuce (a dicot) so read carefully below:

The barley seed shown is a typical monocot. It has a seed coat (fused to fruit coat), a large endosperm area filled with starch, and an embryo (or germ). Barley lacks any special dormancy...germination is initiated by water and reasonably warm temperature. The seed takes up water from the environment in the process known as imbibition. The water passes through the embryo, picking up the germination signal: the hormone Gibberellic Acid. The water moves the hormone from the embryo to the aleurone layer of the endosperm. This layer of cells stores much protein and is the "brown" of "brown rice." When cooked, this protein-rich layer gives brown rice its chewiness. The water activates hydrolysis enzymes that degrade the storage protein into amino acids. The gibberellic acid activates the DNA gene coding for the enzyme amylase in the aleurone cells. Transcription in the nucleus and translation by ribosomes in the cytosol results in the production of amylase inside the aleurone cells. The amino acids from hydrolysis of storage protein are used in the translation of amylase. The amylase is shipped by ER into the Golgi, sorted and packaged into vesicles, and exported through the cell membrane by exocytosis. The amylase is thus dumped into the endosperm interior. There the amylase catalyzes the hydrolysis of starch into sugar. The sugar happens to be maltose, which is transported to the embryo. The sugar fuels respiration in the embryo so it can grow. The radicle protrudes from the seed coat, and germination is accomplished in barley.

A similar mechanism exists in lettuce. The picture above is only symbolic. Lettuce is a dicot, it has no aleurone layer, and the starch is mostly found in the embryo rather than in endosperm. Rather than gibberellic acid, the activating signal is a pigment called phytochrome. This pigment exists in two different chemical forms: Pr and Pfr. How a lettuce seed responds depends on how much of each of these two forms is present in each cell. The active form of phytochrome is Pfr. If there is sufficient Pfr in the seed when the imbition takes place, it will photoactivate the genes for amylase and the seed will sprout using the mechanisms shown in the diagram above.

Typical lettuce seed batches germinate between 5% and 20% if placed in darkness because at least this many seeds have enough Pfr to stimulate germination without any help from humants. If, however, you put the lettuce seeds in red light (660 nm), the red light causes all the Pr to change into Pfr. Now 85-95% of the seeds can sprout because they all have an abundance of Pfr inside. On the other hand, if you put lettuce seeds in far-red (730 nm) light, the far-red light causes all the Pfr to change into Pr. In far-red light, then, all the seeds have essentially no Pfr and so very few (0-5%) actually sprout. You carried out these experiments in lab, or soon will.


This page © Ross E. Koning 1994.



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