Propagating Plants Asexually
HERE is much discussion in the news today about
the cloning of people. That is the creation of genetically identical people. While that involves cutting-edge technology, the cloning of plants has been taking place with little technology for thousands of years. In fact, some plants reproduce entirely by cloning. In this E-unit, methods of cloning plants will be addressed.
Explain asexual propagation of plants. (Courtesy, Don Farrall / Getty Images)
adventitious roots asexual propagation budding bulbils bulblets clones cormels
cuttings division grafting hardwood cuttings layering leaf cuttings leaf-bud cuttings
leaf-petiole cuttings rootstock scion semi-hardwood cuttings softwood cuttings stem cuttings tissue culture
Asexual Propagation Asexual propagation involves the vegetative parts of a plant. Leaves, stems, and roots are used for asexual propagation purposes. Unlike sexual reproduction, asexual reproduction results in offspring that are the genetic duplicates, or clones, of the parent plant.
FIGURE 1. After a cutting is planted, it will grow on its own and become a clone of the parent.
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Some advantages are associated with asexual reproduction. Since recombination of genes does not occur, plants with outstanding characteristics can be reproduced without the risk of losing the desired characteristics. Examples are the asexual reproduction of a red delicious apple tree and the asexual reproduction of an ornamental plant that has desired flower color or leaf variegation. Some plants are difficult to reproduce sexually. They may produce few seeds, or the seeds they produce may result in low germination rates. With asexual reproduction, huge numbers of genetically identical plants can be produced. Also, mature plants can be obtained more rapidly than those grown from seed.
METHODS OF ASEXUAL REPRODUCTION Asexual reproduction most often concerns the regeneration FIGURE 2. The proper environment is needed for asexual reproduction to be of roots on stems or leaves. Root- successful. ing is a complex physiological process. The speed of root development is influenced by a number of factors, including the plant species or variety, the age of the plant, the type and location of the cutting, the absence or presence of leaves, and the nutritional status of the plant. Woody plants tend to take longer to propagate than herbaceous (nonwoody, soft-stem) plants. Environmental conditions critical for successful rooting include proper temperature, high humidity, and sufficient light. The rooting medium must be free of disease organisms. It should also have good water-holding ability and good aeration. There are many methods of reproducing plants asexually. Cutting propagation, grafting, layering, division, and tissue culture are types of asexual propagation methods. Some plants are easily propagated in a number of ways. Other plants respond best to a particular method. One of the simplest and most common methods of asexual propagation involves the use of cuttings. Cuttings may be made from portions of stems, leaves, or roots. Once taken, cuttings are given optimal environmental conditions to promote FIGURE 3. The use of cuttings is a the regeneration of the roots. major asexual propagation method.
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Stem Cuttings Stem cuttings involve the stem and leaf portions of a plant. There are three basic types of stem cuttings. Softwood cuttings are taken from new growth at the succulent stage. Semi-hardwood cuttings are the most common cuttings and are taken from first-year branches of woody plants during the growing season. Hardwood cuttings are taken from one-year-old growth of deciduous or evergreen plants. The tissue has become woody, and the plant is dormant. Stem cuttings, usually 2 to 5 inches in length, are taken in the morning when the tissues are turgid, or full of water. A sharp knife or razor is used for the cuts. Disinfecting the knife between cuts is advisable to avoid spreading disease. Any flowers on the stem should be removed to direct energy to the production of roots. The distal end of each cutting, or the end closest to the root system, is usually dipped in a 4. A stem cutting involves the stem and rooting hormone to hasten the formation of adventi- FIGURE leaves. tious roots. Adventitious roots are roots that form in unusual places, such as along the stem. The cuttings are then placed in rooting cubes or directly into a rooting medium. After the cuttings are stuck, they are placed under an intermittent mist system. The mist is applied between dawn and sunset to reduce water loss from plant tissues, or transpiration. Misting is continued until the roots form and can absorb moisture for the plant. The propagation area is often shaded to reduce the intensity of the sun. Intense sunlight increases the level of transpiration and puts the cuttings under stress. Bottom heat is also provided to maintain a medium temperature between 75° and 80°F (23.89° and 26.67°C). Most greenhouse crops are propagated by herbaceous, or soft-stem, cuttings.
EXPLORING OUR WORLD… SCIENCE CONNECTION: Banana Production Bananas are grown in humid, tropical regions. They rank as the fourth largest fruit crop in the world. The fruit of the banana is technically a berry. It forms parthenocarpically, or without pollination. On rare occasions, pollen from wild varieties will pollinate cultivated bananas and seeds will form, but generally seeds are not produced. Since most bananas fail to produce seeds, they are propagated asexually. The method used is division. Rhizomes grow as suckers off the main plant. These suckers are removed with a clump of roots and planted.
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Leaf Cuttings A relatively small number of plants have the ability to produce plantlets on their leaves. Entire leaves or portions of leaves are removed from the parent plant for use as leaf cuttings. Healthy leaves that have just reached maturity should be used. Healthy, mature leaves are at a stage when food production and the capacity to produce new plantlets are highest.
Leaf-Petiole Cuttings Some plants, including African violets, are easily propagated by leaf-petiole cuttings. For a leaf-petiole cutting, a leaf blade is taken from the parent plant, along with the petiole. Under proper conditions, a cluster of plantlets develops where the petiole was cut. When the plantlets have grown large enough to handle, they are separated and potted in individual containers.
Leaf-Bud Cuttings Many plants cannot be propagated by leaf or leaf-petiole cuttings. However, propagation can be accomplished with leaf-bud cuttings. A leaf-bud cutting is composed of the leaf blade, the petiole, a bud at the base of the petiole, and a portion of the stem. The leaf provides the energy for the development of roots. The roots sprout from the stem and are often concentrated at the node of the stem. The bud develops into the stem of the new plant.
Grafting A method of asexual reproduction common with woody plants is grafting. Grafting is the process in which the stem of one plant is made to grow on the roots of another plant. The portion of the graft that is to become the stem is the scion. The lower portion of the plant that includes the root system is called the rootstock or the understock. Budding is a form of grafting in which the scion consists of a single bud. Grafting involves a stem being placed onto another stem or individual 1 2 3 4 5 buds being placed into a stem. In FIGURE 5. Budding—a form of grafting. either case, it is important that the cambium wood of both the scion and the rootstock line up. Then, the union must be protected from moisture loss. It is also important that the scion and the rootstock materials be related and capable of growing together. In time, the conductive tissues grow as one. E-unit: Propagating Plants Asexually Page 4 u www.MyCAERT.com Copyright © by CAERT, Inc. — Reproduction by subscription only.
The fruit industry employs grafting techniques for a number of reasons. One is so it can produce genetically identical fruit, such as Bartlett pears or navel oranges. Also, the time for harvesting the first crop is attained more quickly than with some other propagation techniques. Another reason is the rootstock selected can influence the growth of the scion. Scions grafted to vigorously growing rootstocks grow more vigorously. Sometimes scions are grafted to rootstocks that inhibit growth, resulting in dwarf trees. In addition, the hardiness or cold tolerance of a plant is often related to its root system. Using a hardy rootstock allows the growth of certain plants in colder climates.
Layering Layering is a method of asexual reproduction in which roots form on a stem while the stem is still attached to the parent plant. The advantage of layering is that the parent plant provides the plant-to-be with water and minerals until the plant-to-be produces its own roots. Layering is sometimes used with brambles, woody shrubs, and select greenhouse plants. FIGURE 6. Layering is achieved by burying an attached stem so that it will begin to grow its own roots.
Division Propagation of some plants can be done by division, in which plant roots or an entire plant may be cut into sections to make two or more plants from the original plant. The sections are then planted. This is a common method used with herbaceous perennial plants and houseplants.
FIGURE 7. Using the division technique, a plant can be cut into sections. The new sections can then be planted as multiple plants.
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A few plants, including the piggyback plant, kalanchoe, and the strawberry geranium, produce foliar embryos. Through a complex process, cells in small areas of a leaf develop into plantlets. The plantlets can be separated and planted. Several methods are used to propagate bulbs and corms. Tulips and daffodils reproduce by natural division. Bulbs are produced off the main bulb. These are removed and planted. Some species of lilies produce bulbils, or tiny aboveground bulbs, in the axils of their leaves. These can also be removed and planted. Lilies may produce tiny bulbs below the ground called bulblets. Some lilies and fritillaries can be propagated by removing bulb scales and placing them in moist medium. In time, they root and produce bulblets that can be separated and planted. Corms, including crocuses and gladiola, develop small corms called cormels. These miniature corms can be separated and planted.
Tissue Culture A very technical method of asexual propagation is tissue culture. Tissue culture involves the culture, or growing, of small pieces of plant tissue. It is performed on an artificial medium under sterile conditions.
Asexual propagation involves leaves, stems, FIGURE 8. Tissue culture is a method of asexual propagation conducted in a controlled environment. (Courtesy, Agricultural Research Service, USDA) and roots of a plant. Unlike sexual reproduction, asexual reproduction results in offspring that are the genetic duplicates, or clones, of the parent plant. Asexual reproduction most often concerns the regeneration of roots on stems or leaves. Environmental conditions critical for successful rooting include proper temperature, high humidity, and sufficient light. The rooting medium must be free of disease organisms. There are many methods of reproducing plants asexually. Cutting propagation, grafting, layering, division, and tissue culture are types of asexual propagation methods.
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Checking Your Knowledge:
1. What is asexual reproduction? 2. What conditions must be addressed to achieve success in asexual reproduction? 3. What are the major methods of asexual reproduction? 4. Name four different types of cuttings used in asexual reproduction. 5. What is a primary difference between a bulbil and a bulblet?
Expanding Your Knowledge:
Practice different asexual propagation techniques using house plants. Which techniques were successful? Which failed?
Asexual Propagation of Plants http://www.hcs.ohio-state.edu/mg/manual/prop2.htm Asexual Reproduction http://home.comcast.net/~john.kimball1/BiologyPages/A/ AsexualReproduction.html Agricultural Career Profiles http://www.mycaert.com/career-profiles
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