OVERVIEW INSTRUCTOR: UNIT: Performance of Technical Skills Related to Plant and Soil Science and Technology LESSON: Sexual and Asexual Reproduction of Plants IMS REFERENCE: IMS #8388 TOPIC NOTES Plant reproduction is necessary for the survival and perpetuation of plant species that are such an important part of our daily lives. Plants have the capability to propagate or multiply themselves by means of sexual and/or asexual reproduction. Both methods of reproduction are important to the agricultural industry, specifically the commercial production of horticultural, agronomic, and forestry crops. This topic presents an overview of the sexual and asexual reproduction of plants. Each method of reproduction has advantages as well as limitations when compared with the other. Seed or sexual reproduction involves using seeds to produce new plants. Vegetative or asexual reproduction includes using plant parts other than seeds to produce new plants. Seeds are the primary means by which annuals such as field crops, flowering bedding plants, and vegetables reproduce. Many perennials such as landscape plants, turfgrasses, and fruit crops reproduce quicker and more efficiently by vegetative means. SEXUAL REPRODUCTION Seeds are the means by which plants sexually reproduce. They contain all the genetic characteristics of the parent plant. However, the characteristics of plants grown from seeds may vary from the parent plant. Seed propagation is the most economical and widely used method of producing new plants, provided that plants of the desired type can be reproduced by this method. Reproducing plants by seeds allows a grower to produce a large number of plants. * Underlined words are defined in the Glossary of Terms. 1
Pollination and fertilization are part of the sexual process that occurs within the flowers or reproductive structures of a plant. Pollination and fertilization result in the formation and development of seeds. Each seed produced contains a tiny plant and a supply of stored food, surrounded by a protective covering. Pollination When a flower opens, the stamens mature and become fully developed. The anthers on the stamen then release hundreds of pollen grains. The pollen grains are the male sex cells. Each pollen grain contains genetic characteristics of the plant. The flower ovary contains one or more ovules, which are the female sex cells. Each ovule contains genetic characteristics of the plant. As the pistil in the flower matures, its stigma develops a sticky surface. As the anthers ripen and split open, gravity, insects, wind, or animals transfer the pollen grains to the stigmas. Pollination occurs when the pollen grains come into contact with the moist, sticky surface of a stigma. Self-pollination occurs when pollen comes into contact with a stigma within the same flower or other flowers on the same plant (e.g., self-pollinating corn). Flowers cross-pollinate when pollen from the flower of one plant transfers to stigmas of flowers on another plant. Alfalfa and many clovers require cross- pollination for reproduction to occur. Corn plants may self-pollinate or cross-pollinate. Pollen from staminate flowers on the tassel transfers to pistillate flowers on the silk of the same plant to self-pollinate. Pollen may transfer to the pistillate flowers of other plants to cross-pollinate. 2
The staminate flowers in one corn plant tassel may produce as many as 15 million pollen grains. Pollen grains do not remain viable much longer than 24 hours after leaving the tassel. Fertilization After a pollen grain becomes anchored on the moist, sticky surface of the stigma, it germinates and produces a pollen tube. The pollen tube grows down through the style and into the ovary where it eventually reaches an ovule. Fertilization occurs when the male sex cell in the pollen tube unites with the female sex cell in the ovule. For each seed a plant produces, a separate grain of pollen must reach and unite with an ovule. During the fertilization process, one sperm* cell from the pollen tube unites with the egg cell in the ovule to form a zygote. The second sperm cell unites with two separate polar nuclei to form a primary endosperm nucleus. Double fertilization results from the unions of the two male sex cells. The zygote proceeds through cell division and develops into a seed embryo. The primary endosperm nucleus develops to form a food source to be used by the embryonic plant during seed germination. In corn, the zygote and primary endosperm nucleus develop within the same corn kernel. The mature kernel contains both endosperm and embryo. Fertilization in corn plants begins within 24 hours after pollen reaches the stigmas of the pistillate flowers. Pollen grains germinate and send pollen tubes through styles (silk) to blister-like ovules on the young corncob. Pistillate flowers number approximately 800 per corn plant. One pollen grain is needed for the development of each corn kernel. Research data shows that about 95% of the kernels are fertilized through cross-pollination. The remaining kernels are fertilized as a result of self-pollination. 3
ASEXUAL REPRODUCTION Asexual or vegetative reproduction involves the production of new plants by use of vegetative parts from an existing plant. Vegetative plant parts used in asexual reproduction include stems, buds, leaves, and roots. The Importance of Plant Production by Vegetative Parts The vegetative parts of many plants have the ability to produce new roots and/or shoots, depending on the type of vegetative plant part. The vegetative parts of two different, but closely related woody plants can be united with each other through budding and grafting techniques to form a vascular connection. Asexual or vegetative reproduction offers growers several important advantages over sexual reproduction: Plants reproduce and mature quicker because of the elimination of the dormancy period required of some seeds and the juvenile stages of plant growth. Disease-free stock plants are produced under controlled environmental conditions. Reproduction is possible for plants that do not develop reproductive parts or viable seeds. Sugarcane, seedless grapes, navel oranges, bananas, and certain sterile hybrid grasses are examples of crops that do not produce viable seeds. Such crop species must be reproduced vegetatively. If no means for vegetatively reproducing these plants existed, they would be lost entirely. Plant selection for desired characteristics is more reliable. Asexual reproduction of plants transmits identical characteristics from parent to offspring. Selection of mutations (changes) by vegetative reproduction improves many plant cultivars. New plants produced are genetically identical to the parent plant. A cutting from a parent plant has the same genetic make-up as the parent plant. Genetic variation occurs with sexual reproduction. Plants grown from seeds do not have the same genetic make-up as either parent. They have the characteristics of both parent plants. As a result, the new plants vary somewhat from either parent plant. Plant Parts Used in Asexual Reproduction Roots, buds, leaves, and stems are vegetative plant parts used in asexual reproduction. Roots such as sweet potato and cassava are underground plant parts without nodes. Buds from woody plants such as peaches, pecans, and roses are above-ground stem parts cut from a plant and placed in close contact on another closely related plant. Leaves are above-ground plant parts cut from a plant and placed into the soil for the purpose of producing new plants. Stems may either be taken from above or below-ground parts of plants. The following paragraphs describe the similarities and differences between above-ground stems and below-ground stems. Above-ground Stems Above-ground stems may stand erect or grow along the soil surface. Stems that grow horizontally along the soil surface are stolons or runners. Portions of above-ground stems produce adventitious roots when placed in the soil. The rooted stems may then be separated from the parent plant to grow as new plants. 4
Examples of agricultural crops reproduced asexually from above-ground stems include sugarcane, sweet potato, clover, alfalfa, and lespedeza. Additional examples are strawberries, bermudagrass, and buffalograss. Below-ground Stems Below-ground stems are similar to above-ground stems in that they also have nodes and internodes. How- ever, below-ground stems are modified structures that grow primarily beneath the soil surface. Examples of below-ground stems are tubers, rhizomes, bulbs, and corms. Tubers are short, thickened underground stems that contain eyes. The eyes are nodes that are capable of producing shoots of new plants. Examples of tubers are the Irish potato and artichoke. Rhizomes are underground, horizontal growing plant stems that produce new plants at each of their nodes. Examples of plants with rhizomes are johnsongrass, bermudagrass, Aloe vera, and asparagus. Bulbs are underground stems with scale-like, fleshy leaves. Onion, garlic, and tulip are examples of plants that grow from bulbs. Corms are short, underground stems similar to bulbs, however, they do not contain fleshy leaves. Gladioli and water chestnut are examples of plants grown from corms. Methods of Vegetative Plant Reproduction Propagation by cuttings is the most widely used method of vegetative or asexual reproduction. Other methods include propagation by layering, separation and division, grafting and budding, and tissue culture. Cuttings A cutting is any plant part severed from the parent plant. Cuttings used in asexual reproduction include those taken from stems, roots, and leaves. For successful reproduction to occur, the severed plant part must develop those parts to make it a complete plant. Stem cuttings must develop a new root system. Root cuttings must develop a new shoot system. Cuttings from leaves must produce both new root and shoot systems. Stem cuttings are those taken from the stems of a plant. For this method of asexual reproduction, a grower inserts a portion of a stem, including a node, into a rooting medium. After several weeks, root formation occurs along the portion of stem in the rooting medium. Example of a stem, root and leaf cutting. The use of stem cuttings is a successful method for reproducing many types of ornamental plants, as well as several horticultural crops such as sweet potatoes, grapes, citrus, and figs. 5
Stem cuttings classified according to their stage of growth are herbaceous, softwood, semi hardwood, and hardwood. Growers primarily use four types of stem cuttings to asexually reproduce plants. These include tip cuttings, medial cuttings, single-eye cuttings, and cane cuttings. Modified stem cuttings taken from tubers and rhizomes are important for reproducing white potatoes, cassava, and certain grasses. Each cutting requires an eye or thickened area for root and shoot development to occur. Root cuttings consist of sections of thickened roots cut into 2- to 3-inch sections and placed in a rooting medium. They do not have any leaves or stems attached. Plants asexually reproduced from root cuttings are those that naturally produce sprouts or suckers from the root system. Pecan, apple, pear, blackberries, sweet potato, and horseradish are examples of nuts, fruits, and vegetables reproduced by root cuttings. Leaf cuttings consist of a leaf blade and petiole, only a leaf blade, or a section of a leaf blade. They do not include any stem tissue or lateral buds. Growers use leaf cuttings to asexually reproduce herbaceous plants that have a basal rosette arrangement of leaves. This method is common for reproducing many ornamental and greenhouse plants. Layering Layering involves forcing a part of a plant to form roots while it remains attached to the parent plant. Air layering and ground layering are two common layering techniques. Air layering involves forcing roots to form on a stem, outside the soil. The grower makes a girdle cut or a cleft cut on an above-ground section of a stem and then covers it with a moisture-holding media. Root formation occurs on the plant stem, in the area of the cut, while the stem remains attached to the parent plant. Ground layering involves extending part of a plant into the ground, covering it with soil, and allowing it to root. To encourage root formation on the stem, a cut is made on the section of stem covered with soil. Ground layering methods include simple layering, trench layering, and mound layering. After roots develop on the stem in the area of the cut, the grower removes the rooted stem from the parent plant to form a new plant. Layering is a simple method of asexual plant reproduction and effective for propagating some plants that are difficult to reproduce by other methods. It is also a cost-effective method of plant propagation that allows growers to produce large plants in a relatively short period. 6
Separation and Division Many plants have specialized vegetative structures. Bulbs, corms, tubers, runners, rhizomes, and stolons are examples of specialized vegetative structures. These specialized vegetative structures contain vegetative buds that are capable of producing new shoots. The new shoots eventually form roots and develop into entire plants. Separation and division are two methods used for producing new plants from their specialized vegetative structures. Separation involves removing new plants formed on specialized stems and separating them from the parent plant. Division is the technique of cutting specialized plant structures into sections and forcing each section to grow into a new plant. Grafting and Budding Grafting is the oldest method of asexual reproduction. It was practiced over two thousand years ago. Grafting consists of joining a hardwood scion from a plant to the rootstock of another hardwood plant. 7
In most cases, scion and rootstock wood of the same plant type is used. However, grafting of wood between two closely related plants is also possible. An example of the former is the grafting of papershell pecan wood onto a native pecan rootstock. The grafting of plum scions to peach rootstock is an example of the latter. The size of the scion and the rootstock determine the grafting method used. Whip-and-tongue, splice, wedge, and approach grafting are used when both the scion and rootstock are similar in size. Side, cleft, notch, and bark inlay grafting are used when the scion is smaller in diameter than the rootstock. Side grafting is used to add branches to bare areas of trees. Whip-and-tongue grafting is used to reproduce nursery stock. Budding is a method of propagation similar to grafting. It is a popular and successful method used to reproduce roses and many fruit and nut trees. Budding consists of removing buds from one plant and placing them on stems of other closely related plants. Normal buds (buds developed normally on the plant at a leaf axis or terminally on the stem) and adventitious buds (buds developed in positions other than where normal buds are found) are used in budding. Plant breeders use budding as a means of changing cultivars of plant species. Patch budding, T- budding, and chip budding are common budding methods. Tissue Culture 8
Tissue culture, or micropropagation, is a special type of asexual reproduction. It involves placing a very small piece of plant tissue on a specialized culture medium in a test tube. The plant tissue multiplies and grows into new plants under sterile conditions in the test tube. Plant tissues used in tissue culture include the tips of shoots and roots, and sections of leaves and stems. Other tissues include embryos, seeds, and pollen grains. Many advantages exist for using micropropagation: 1. Tissue culture allows large numbers of offspring to be produced quickly. 2. Tissue culture allows growers to produce plants that are free of disease. 3. Tissue culture is a cost-efficient method of reproducing plants. 4. Plants reproduced by tissue culture have the same characteristics as the parent plant. Disadvantages of tissue culture include: Tissue Culture 1. The equipment required for tissue culture is expensive. 2. Tissue culture requires exact conditions. 3. More time and labor is required. SUMMARY Growers use several methods to multiply or increase the numbers of plant species. These methods include both sexual and asexual methods of reproduction. Sexual plant reproduction involves reproducing plants from seeds. Seed formation occurs as a result of the pollination and fertilization processes that occur in the flowers of a plant. Asexual or vegetative reproduction involves growing new plants from the vegetative parts of existing plants. These vegetative parts include leaves, stems, and roots. Asexual reproduction by cuttings is the most widely used method of asexual propagation. Other methods include propagation by layering, separation and division, grafting and budding, and tissue culture. ACKNOWLEDGEMENTS Shannon Houy, Graduate Technician, Department of Agricultural Education, Texas A&M University, researched and developed this topic. Keith W. Zamzow, Curriculum Specialist,, Texas A&M University, edited and reviewed this topic. Vickie Marriott, Office Software Associate,, Texas A&M University, edited and prepared the layout and design for this topic. Christine Stetter, Artist,, Texas A&M University, prepared the illustrations for this topic. 9
REFERENCES Christensen, Norman L. (1998). Plant: Plant Reproduction and Growth, [Online]. Available: http:// www.comptons.com/encyclopedia/articles/0125/01453312_a.html#p124 [2001, April 13] Poincelot, Raymond P. Horticulture: Principles and Practical Applications. Englewood Cliffs, NJ: Prentice-Hall, Inc., 1980. Texas Agricultural Extension Service. Master Gardner Handbook. 3rd ed. College Station, TX, 1995. Egg - Ovule; female reproductive body. GLOSSARY OF TERMS Eye - In reference to plants, an undeveloped bud. Rootstock - A plant with an established root system that receives a scion; it is also referred to as stock. Scion - A severed plant part that is joined to a rootstock; also referred to as graftwood. Sperm - Male reproductive body. Zygote - A new developing individual produced by union of egg and sperm (a cell formed from the fusion of male and female gametes). TOPIC ACTIVITIES FILL-IN-THE-BLANK: Complete the following statement. 1. is the process in sexual reproduction occurring first where pollen grains come in contact with the stigma. 2. is the process in seed formation when the male sex cell in the pollen tube unites with the female sex cell in the ovule. 3. A develops from the union of sperm and egg cells. 4. Plants reproduce sexually by, which contain all the genetic characteristics of that plant. 5. contain the male sex cells of the flower. SHORT ANSWER/LISTING: Answer the following questions or statements. 6. List the four vegetative parts of a plant used in asexual reproduction. 10
7. List the above- and below-ground stems of a plant used in asexual reproduction. 8. Explain the difference between self-pollinated and cross-pollinated flowers in sexual reproduction. 9. Explain the difference in the genetic characteristics of plants grown sexually vs. asexually. 10. Explain some important reasons agriculturists reproduce plants sexually and asexually. TRUE/FALSE: Circle the "True" if the statement is true or the "False" if it is false. 11. Perennials including landscape plants, turfgrasses, and fruit crops are reproduced quicker and more efficiently from seeds. 12. After a pollen grain germinates on the stigma, it releases two sperm into the pollen tube; one unites with the egg and the other unites with the polar nuclei. 13. Budding, a form of grafting, is a popular and successful method used to reproduce roses and many fruit and nut trees. 14. Tubers and rhizomes are used in leaf cuttings. 15. When grafting hardwood plants, usually the scion and rootstock wood of the same plant type are used, but grafting between two closely related plants is possible. 16. Stem cuttings are used successfully to reproduce sweet potatoes. 17. Many species of berries are reproduced by some method of air layering. 18. Sugarcane, seedless grapes, navel oranges, bananas, and certain sterile hybrid grasses produce a viable seed. 11
MATCHING: Match the term in the left column with its definition in the right column. 19. Rhizomes A. The reproduction of plants by use of vegetative parts to reproduce new plants. 20. Tissue culture B. The reproduction of plants by use of seeds to reproduce new plants. 21. Asexual reproduction C. Underground, horizontal growing plant stems that produce new plants at the nodes. 22. Stolons D. Above-ground, horizontal growing stems (runners) form new plants at the nodes. 23. Sexual reproduction E. The reproduction of a plant using plant tissue and growing it in a test tube. 24. Grafting F. Involves forcing a part of a plant to form roots while it remains attached to the parent plant. 25. Layering G. Joining a hardwood scion from one plant to the rootstock of another. ADVANCED ACTIVITIES 1. Visit the Web site: http://aggie-horticulture.tamu.edu/propagation/propagation.html from Texas A&M University's horticulture page on plant propagation and choose one of the procedures to perform. These procedures may be time-consuming; therefore, it is best to work in groups or as a class. 2. Many times agriculturists raising fruit and nut crops, such as pecans, apples, peaches, and plums, use grafting techniques to reproduce their crops. Why might you think such crops are usually reproduced asexually vs. sexually? What are reasons for using grafting to reproduce such crops as opposed to other methods of asexual reproduction? (Research may be found on the Internet to explain some reasons.) 12