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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. Instructional Materials Service 1 www.myimsservices.com 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 Instructional Materials Service 2 www.myimsservices.com 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. Instructional Materials Service 3 www.myimsservices.com 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 Instructional Materials Service 4 www.myimsservices.com Plant Growth and Development: Seed Germination 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 Instructional Materials Service 5 www.myimsservices.com 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. Instructional Materials Service 6 www.myimsservices.com 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 Instructional Materials Service 7 www.myimsservices.com