Download Plant Growth and Development: Seed Germination OVERVIEW

Plant Growth and Development: Seed Germination
OVERVIEW
INSTRUCTOR:
UNIT: Performance of Technical Skills Related to Plant and Soil Science and Technology
LESSON: Plant Growth and Development: Seed Germination
IMS REFERENCE: IMS #8385
TOPIC NOTES
Have you ever thought of how most plants come from only a small seed? Seeds* are essential for
the survival and continued existence of many plant species. The life cycle of a plant species
begins with the seed.
A plant’s seed contains the genetic material to produce another plant with identical, similar, or
unlike characteristics of the parent plant. A plant produced from a single seed may produce
hundreds or thousands of seeds. For example, one wheat grain or seed produces a plant capable of
producing a thousand grains. A single tobacco seed can develop a plant capable of producing
another million seeds. Seed producers plant special seed crops each year just to furnish seeds to
food crop producers.
Seeds are an important source of nutrition for humans and animals. Cereal grains, feed grains,
nuts, beans, peas, sunflower seeds, and cottonseeds are examples of seeds used as food and feed.
Cereals, beverages, breads, pastries, syrups, cooking oils, and salad dressings are examples of
foods produced from seeds. Examples of livestock feeds produced from seeds include cottonseed
cake, brewers’ grains, flaxseed, oat middlings, and rice bran.
Seeds are a source of raw materials for the development of many chemical products used in
homes, businesses, and industries. Medicinals, pesticides, fuels, and construction materials (i.e.,
paints, oils, and plastics) are examples of products containing seed by-products.
These important seed benefits would be impossible without the plant’s first life process, which is
seed germination. A seed is packed with enough stored energy to power the young seed until it
can capture its own energy, as a plant, from the sun. Because of its importance to world crop
production, seed germination is discussed in this topic.
STRUCTURAL PARTS OF A SEED
All seeds contain an embryo and have their own food supply. The embryo consists of a plumule,
epicotyl, cotyledons, hypocotyl, and a radicle. The plumule includes the young primordial leaves
and growing point of the stem. The epicotyl is the portion of the stem above the cotyledon. The
cotyledons are the seed leaves used for food storage. The hypocotyl is the portion of the stem
below the cotyledons. The radicle is the young embryonic root and root tip.
Most field crops are either dicotyledonous (dicots) or monocotyledonous (monocots). Garden
beans and leguminous crops (i.e., alfalfa, soybeans, and cowpeas) are typical dicots. Corn and
small grains (i.e., wheat, rice, and oats) are typical monocots. This classification is based on the
number of cotyledons present in the embryo of the plant.
* Underlined words are defined in the Glossary of Terms.
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Plant Growth and Development: Seed Germination
Figure 1. Seeds are classified as either dicots or monocots.
Monocot and dicot seeds have many similarities and differences. Both types of seeds are similar
in that they contain an embryonic plant and stored food reserves that are protected by an outer
seed coat (see Figure 1). One difference between monocot and dicot seeds is the location of their
stored energy supply. In dicots, the stored energy exists in the cotyledons. The endosperm is the
source of stored energy in monocot seeds.
The seeds of dicotyledonous plants have two cotyledons. For this reason, dicot seeds have a
larger embryo than monocots. Stored food for the embryos of dicot plants are in the cotyledons.
The plumule of a dicot is large with folded leaves. The seed coat (testa) of a dicot seed has a
hilum and a micropyle.
The seeds of monocotyledonous plants have only a single cotyledon. For this reason, a monocot
seed has a smaller embryo. Stored food for the embryos of monocot plants is in the large
endosperm. Dicot seeds do not have a separate endosperm. The small plumule of a monocot has
rolled leaves. The plumule is enclosed in a tubular sheath called the coleoptile. The testa of a
monocot corn seed has a silk scar and a point of attachment.
FACTORS AFFECTING SEED GERMINATION
In order for a seed to germinate, it must be viable. That is, the seed embryo must be alive. Several
factors affect whether a seed will germinate or not. These factors include environmental
conditions, seed dormancy, and any unique seed characteristics or adverse conditions.
Environmental Factors
Moisture, temperature, oxygen, and light are four environmental factors that affect seed
germination. These environmental conditions must be favorable for chemical processes to occur
within the seed that allow germination to occur.
Moisture
The first step in the germination process is the absorption of water into the seed. A seed must
have an ample supply of moisture for germination to occur. Seeds fail to germinate if chemical
processes that change insoluble starches to soluble sugars do not occur. Moisture content needed
for seed germination to occur ranges from 25% to 75%. For example, small grains germinate
when their seed moisture content is about 50%. Grain sorghum and sudangrass seeds germinate if
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Plant Growth and Development: Seed Germination
their moisture content is approximately 26%. Soybeans will not germinate unless their moisture
content is about 75%. Once the germination process begins, a dry period or lack of water will
cause the death of the developing embryo.
Temperature
Temperatures affect both the germination percentage and the germination rate. Germination rate
is lower at low temperatures. Most plant seeds germinate at an optimum temperature range of
68oF to 85oF. However, most field crop seeds can germinate at temperatures ranging from 32oF
to 120oF. For example, wheat, rye, barley, and oat seeds germinate at temperatures slightly above
32oF. Field pea, alfalfa, soybean, flax, and some clover seeds germinate at 40oF. Sorghum grain
and corn seeds have a minimum germination temperature of approximately 48oF. Germination
percentage may remain relatively constant if sufficient time is allowed for germination to occur.
Oxygen
Oxygen is necessary for respiration to occur within a seed. Respiration converts the stored food in
the seed into energy for germination. Some seeds require less oxygen than others. For example,
rice seeds germinate when covered with water, although little free oxygen is present. Other small
grains and cottonseed germinate only if a large amount of oxygen is available. Oxygen deficiency
occurs if seeds are planted in flooded or compacted soil. For this reason, the medium in which the
seed is placed should be well aerated and loose.
Light
The presence or absence of light may or may not have an effect on germination. However, the
light factor is not as important as a viable seed, germination medium, water, optimum
temperature, and oxygen. Seeds of some cultivars of tobacco and grass require a certain amount
of light for germination. Bermudagrass, Kentucky grass, bentgrass, slender wheatgrass, and
Canada grass are examples of grass seeds that require adequate light for germination. Seeds of
most American cultivars of tobacco germinate in the absence of light. Seeds of pigweed and thorn
apple also germinate in total darkness.
Seed Dormancy
Even if all favorable environmental conditions needed for germination are present, seed
germination may still not occur. Most seeds produced by mature plants pass through a period of
inactivity prior to germination. During this period of inactivity or dormancy, seeds remain viable.
Wheat, barley, oats, and some wild plant seeds are examples of seeds that must pass through a
dormant stage before germination will take place. Dormancy may be internal, external, or a
combination of both.
Embryo (Internal) Dormancy
Internal factors of dormancy occur when a seed is otherwise mature, but may contain an
underdeveloped or immature embryo. Another internal dormancy of most plants involves a period
of after-ripening. After- ripening occurs when the seed will not or is not ready to germinate until
after a certain stage of development occurs. Some small grain seeds must pass through an afterripening period before germination occurs. Some seeds mature in the fruit but do not germinate
until released from the fruit. For example, a tomato seed matures in the fruit but remains dormant
until extracted and placed in a favorable environment for germination.
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Plant Growth and Development: Seed Germination
Stratification is the process of overcoming internal factors of dormancy. Stratification is the
process of overcoming embryo rest or dormancy by satisfying a seed’s chilling requirements.
With this technique, seeds require exposure to cold (35oF to 40oF), moist conditions for 6 to 12
weeks for the completion of embryo development. In nature, the cold of the winter and the
moisture in the soil act as a natural method of stratification. Some underdeveloped embryos may
be stratified under warm conditions (above 45oF) for completion of embryo development.
Seedcoat (External) Dormancy
The external factors of dormancy preventing germination refer to the seed’s coat and its
surrounding environment. As discussed previously, a seed may require a certain amount of light
to germinate causing the seed to be dormant until exposure to light. The seed coat may be hard
and/or thick, preventing the absorption of water, intake of oxygen, or physically preventing the
expansion of the embryo. Many legumes have hard seed coats that require a period of natural
scarification before germination occurs. A hard seed coat may also physically restrict the
expansion of the embryo such as fruit or nut tree seeds.
Scarification is the process of chemically or mechanically treating seeds to break or soften their
hard seed coats. Seed coats are scratched or cracked by blowing and/or rolling seeds against an
abrasive surface. Natural methods of scarification may include factors such as physical abrasion,
fire, alternate freezing and thawing, attack by microorganisms such as fungi and bacteria, and
passing through the digestive tract of an animal. Artificial methods include mechanical, or
physical, abrasion, hot water soaking, and acid scarification techniques.
Some seeds may have a double dormancy because of a hard seed coat and a dormant embryo. A
combination of scarification followed by stratification may be necessary to overcome the double
dormancy and allow the seeds to germinate.
Adverse Conditions Affecting Germination
Many different adverse conditions may also affect the viability and germination of seeds. Some
adverse conditions preventing seed germination include mechanical injury, diseases, improper
storage conditions, old seeds, and inadequate growing medium.
Mechanical Injury - Seed coats that are mechanically injured may cause seeds to either germinate
pre-maturely or not germinate. Pests, heat, and mechanical harvesting can damage the seed coat.
Diseases - Many diseases caused by fungi or bacteria affect plants. Air, water, and insects
carrying spores and bacteria to plants contaminate the seeds.
Improper Storage Conditions - Seeds become dehydrated if stored in locations having high
summer temperatures or excessive artificial winter heating. Damp storage causes mold and other
fungal conditions to develop on seeds. Most seeds store best in a cool, dry location with a low
relative humidity.
Old Seeds - The viability of some seeds decreases with age. They fail to germinate after a certain
time has passed. Most seeds over a year old do not germinate. However, one-year old or older
seeds of some plant cultivars germinate if favorable moisture and temperature conditions exist.
The maximum period those seeds remain viable ranges from one year to ten years.
Inadequate Growing Medium - Seeds fail to germinate properly in an inadequate growing
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medium. An excessively dry or wet growing medium seriously hinders seed germination. In
addition, seeds planted too deep in a growing medium may germinate, but the resulting young
plants exhaust their food supplies and die before reaching the soil surface.
THE GERMINATION PROCESS
Germination occurs when a seed’s embryo begins to grow. Steps in the germination process are
the absorption of water, radicle emergence, plant emergence, leaf formation, and photosynthesis.
Figure 2 illustrates the germination of monocots and dicots.
Water Absorption
First, the seed absorbs water and oxygen. Absorbed water and oxygen cause the seed to swell and
increase in size. The seed secretes enzymes that convert insoluble starches into soluble sugars.
Soluble sugars dissolve in the absorbed water and are used as food by the plant embryo.
Emergence of Radicle
The seed coat ruptures permitting the young root (radicle) to emerge and grow downward to
anchor the plant. In a dicot, the seed coat (testa) splits near the hilum, and the young root
becomes the primary root from which branching roots form. In a monocot, the young root breaks
through the coleorhiza (sheath). The primary root system that develops from the radicle is
temporary and is replaced later with a fibrous root system.
Plant Emergence
The above-soil surface portion of the plant emerges as the radicle develops into the plant’s root
system. In a dicot, the hypocotyl elongates, forming an arch and pulling the cotyledons upward.
The hypocotyl arch straightens to a vertical position after passing through the soil surface.
Monocots emerge through the soil surface differently. In a germinating monocot seed, no
hypocotyl arch exists to push the leaf portions through the soil. Instead, the coleoptile covering
the plumule (tight roll of leaves) pierces the soil surface exposing the developing plant to
sunlight.
Germination differs among dicots in this stage of the process. Two types of seed germination in
dicots based on how the seedling emerges are epigeous and hypogeous germination (see Figure
2). In epigeous germination, the hypocotyl of the embryo elongates and raises the plumule,
epicotyl, and cotyledons through the soil surface and above the ground. Garden beans have an
epigeous type of germination.
In hypogeous germination, the epicotyl elongates and raises the plumule above the ground. The
cotyledons, which are usually still enclosed by the seed coats, and the hypocotyl never emerge
and remain below the surface of the soil. Peas have a hypogeous type of germination.
Leaf Formation and Photosynthesis
After emerging through the soil, new leaves form and photosynthesis begins. In a dicot, as the
hypocotyl arch straightens, the plumule is shed. The cotyledons spread apart to serve as first
leaves to transfer food to other parts of the plant. After it is exposed to air and light, the epicotyl
begins to develop into the stem and true leaves are formed. The cotyledons shrivel and die as the
seedling plant uses their stored food supply. The developing true leaves continue to
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Plant Growth and Development: Seed Germination
photosynthesize and produce a constant supply of food reserves. Hypocotyl elongation is
restrained by growth hormones.
In a monocot, after the coleoptile and plumule emerge, the first true leaves begin to form. At this
point, the food supply in the endosperm is used up and photosynthesis begins in the true leaves as
they develop. Growth hormones prevent further development of the coleoptile and plumule.
About the time the coleoptile appears above the soil surface, a second root system begins to
develop at the base of the coleoptile known as nodal or adventitious roots.
Figure 2. Examples of epigeous, hypogeous, and monocot germination.
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Plant Growth and Development: Seed Germination
ACKNOWLEDGEMENTS
Shannon Houy, Graduate Technician, Department of Agricultural Education, Texas A&M
University, researched and developed this topic.
Keith Zamzow, Curriculum Specialist, Instructional Materials Service, Texas A&M University,
edited and reviewed this topic.
Vickie Marriott, Office Software Associate, Instructional Materials Service, Texas A&M
University, edited and prepared the layout and design for this topic.
Christine Stetter, Artist, Instructional Materials Service, Texas A&M University,
prepared the illustrations for this topic.
REFERENCES
Christensen, Norman L. (1998). Plant: Seed Germination. [Online]. Available:
http://www.comptons.com/ encyclopedia/ARTICLES/0125/01453312_A.html#P128 [2001,
March 22]
Hartmann, Hudson T., et al. Plant Propagation: Principles and Practices. 6th ed. Upper
Saddle River, NJ: Prentice Hall, Inc., 1997.
Texas Agricultural Extension Service. Master Gardner Handbook. 3rd ed. College
Station, TX, 1995.
GLOSSARY OF TERMS
Dicotyledonous - A plant seed having two seed leaves or cotyledons.
Embryo - An undeveloped plant inside a seed.
Endosperm - Nutritive tissue contained inside a monocot seed.
Germination - The renewal of a rapid rate of cell division in the embryo and growth and
differentiation of cells into various plant tissues; process causing a seed to sprout and grow.
Hilum - A scar on a dicot seed marking the point of attachment to the ovule of a flower’s ovary.
Micropyle - A minute opening in the seed that is a remnant of the pollen tube’s penetration into
the embryo sac of the flower’s ovary.
Monocotyledonous - A plant seed having only one seed leaf or cotyledon.
Optimum temperature - Most favorable temperature.
Scarification - Any process of breaking, scratching, or mechanically altering the seed coat to
make it permeable to water and gases.
Seed - A living young plant existing in an arrested state of growth.
Stratification - Process of exposing the seed to a particular duration of moist-prechilling and/or
moist- warm periods.
SELECTED STUDENT ACTIVITIES
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