Asexual Plant Propagation

In recent years, we have heard amazing stories about scientists who have cloned animals such as sheep and cows. Botanists are far ahead in that arena. They have been cloning plants for centuries! Many plants are easy to propagate asexually, using vegetative parts rather than seeds. For example, crown division is a simple technique in which a plant is separated into several pieces, each of which contains a crown and a portion of the root system (Fig. 14.17). This is commonly used for many ornamental perennial plants. A breeder who has identified a superior plant of an asexually propagated species can perpetuate that genotype indefinitely. This avoids the genetic variability and consequent unpredictability that results from seed propagation.

Cuttings

We know that nothing lives forever. Or do we? If you are particularly fond of your grandmother's African violet plant, you can use a leaf to propagate a new plant. As that plant begins to age, you can propagate a new one again from a leaf and eventually pass the plant on to your grandchildren. This plant could be propagated indefinitely and would essentially live forever.

Sansevieria Vegetative Propagation
Figure 14.17 Asexual reproduction of a daylily by crown division.

Propagation of plants from parts such as leaves is called propagation by cuttings. If a stem is used as a cutting, it needs to be coaxed into producing adventitious roots (roots produced on internodes or other parts of plant organs). Buds on the stem will grow into the shoot system. Leaf and root cuttings, though, must develop both adventitious roots and shoots (Fig. 14.18). The formation of adventitious structures requires plant cells near the wound to dedifferentiate (become less specialized) and create a new meristematic region.

After a cutting is made, it must be kept in an environment that will allow wound healing and development of new organs. The most critical step usually is prevention of water loss. Because a stem or leaf cutting has lost its root system, its tissues must be kept moist until roots can once again supply water. This is easily done by keeping the cuttings in an enclosed container to maintain high humidity and to reduce transpiration. In commercial production systems, frequent misting or constant fog is used to keep cuttings from drying out. Sometimes, rooting is stimulated by application of a rooting powder containing a growth regulator such as auxin. Rooting powders also often contain fungicides to prevent pathogens from entering wounds. The cuttings must be kept in a potting mix such as perlite or vermiculite, which holds water but also drains well. This is preferable to rooting cuttings in water, because oxygen levels in water are low and are quickly depleted as young cells in developing roots undergo respiration. Temperature is also a factor in successfully generating cuttings. Ideally, young roots should be warm enough to enhance growth, while the above-ground parts should be cool enough to minimize growth and transpiration. For this reason, cuttings are often grown on heating pads or in heated beds (Fig. 14.19). Once roots are established, fertilizer helps to stimulate growth.

Many house plants are easily propagated by cuttings. These include African violet (Saintpaulia), snake plant (Sansevieria), Begonia, Coleus, and Kalanchoe. Outdoor

Plant Propagation Area
Figure 14.18 Asexual reproduction by cuttings. Left: Stem cuttings of an ornamental fig (Ficus). Right: Leaf cuttings of Sansevieria. Note the adventitious roots developing from both types of cuttings.

Plant Breed ing and Propagation

Asexual Propagation Types Images

Figure 14.19 Cuttings are often grown on a heated bed to stimulate root development.

mound of soil

Tip Layering

Figure 14.20 Tip layering. The tips of canes are bent to the ground and covered with a small mound of soil. When a new plant has developed at the tip, it can be cut from the parent plant and grown independently.

mound of soil

Figure 14.19 Cuttings are often grown on a heated bed to stimulate root development.

plants propagated by cuttings include Forsythia, Geranium, rose (Rosa), Spiraea, raspberry (Rubus), and juniper (Juniperus). The main advantage of propagation by cuttings is that identical copies of a valuable plant can be made. A major disadvantage is that diseases carried by the mother plant, including those caused by viruses, fungi, and bacteria, are also propagated.

Layering

Layering is a modified form of cutting propagation and works well for some plants that are not easy to propagate by cuttings. This procedure allows the adventitious root system to develop before the new plant part is severed from the parent plant.

Tip layering is used with blackberries, boysenberries, and other plants with flexible stems. The canes are bent over until the tips touch the ground; the tips are then covered with a small mound of soil. Roots form on the portion of buried stem, and eventually, shoots will also appear (Fig. 14.20). These new plants can then be separated from the parent stems. Variations of tip layering include forcing a stem to lie horizontally and covering it with small mounds of soil at intervals, or heaping soil around the base of a plant so that the individual stems produce roots there. Once roots have been established, the individual plantlets or pieces of stem can be cut off from the original parent and grown independently.

Air layering is sometimes used to propagate tropical trees and shrubs. It is useful for producing a few large plants from a single plant with a rigid stem. A branch or main stem is wounded with a sterile knife and then may be dusted with a rooting powder. The wound is produced by gashing the stem or, alternatively, by girdling. Girdling is the removal of a ring of bark around the stem. Damp sphagnum moss is wrapped around the wound, and the area is covered with clear plastic film to retain moisture. Then, aluminum foil is placed over the plastic film to reflect sunlight and prevent overheating (Fig. 14.21). When roots are observed in the moss through the plastic film, the layer is removed from the parent plant and transplanted.

Figure 14.20 Tip layering. The tips of canes are bent to the ground and covered with a small mound of soil. When a new plant has developed at the tip, it can be cut from the parent plant and grown independently.

Citrus Fruit Tree Air Layering

Figure 14.21 Steps in air layering. A. Cuts are made at an angle to the axis of a stem of a rooted plant. B. Damp sphagnum moss is wrapped around the cut area. C. Polyethylene film is wrapped around the moss, and the ends of the film are taped shut. D. Aluminum foil is wrapped around the film-covered moss. Within a few weeks, adventitious roots should develop and protrude through the moss. After the roots have appeared, the rooted portion can be cut off and planted.

Figure 14.21 Steps in air layering. A. Cuts are made at an angle to the axis of a stem of a rooted plant. B. Damp sphagnum moss is wrapped around the cut area. C. Polyethylene film is wrapped around the moss, and the ends of the film are taped shut. D. Aluminum foil is wrapped around the film-covered moss. Within a few weeks, adventitious roots should develop and protrude through the moss. After the roots have appeared, the rooted portion can be cut off and planted.

Grafting

Imagine walking out into your yard in the morning and picking a fresh grapefruit for your breakfast. Then, you go to the same tree at lunchtime and harvest an orange. Later in the day, you pick a lemon off the same tree to squeeze on your fish dinner. Sound impossible? It is not, if you are growing a tree that was created by grafting.

Grafting is a process by which segments of different plants are connected and induced to grow together as one plant. It has been performed as a horticultural art for thousands of years, dating back to around 1560 B.C. Historically,

Chapter 14

How Conduct Asexual PropagationPlant That Graft Togethar
Figure 14.23 A simple graft. The root portion (stock) and portion to be grafted onto the stock (scion) are cut so that the two parts will fit together with the cambium of both portions in close contact.

Figure 14.22 A brightly colored cactus stem has been grafted on to a green cactus plant.

grafting has been performed mainly to clone plants that are difficult to propagate as cuttings. Today, many fruit and nut trees are grafted. Trees are bred for high-quality fruits or nuts. Then, they are grafted onto root systems selected for traits such as winter hardiness, dwarfing, and disease resistance. The top part of the graft is called the scion. The bottom portion, that forms the root system, is the rootstock. To take grafting a step farther, though, it is possible to graft several different but related plants onto the same rootstock. You could, for example, have grapefruit, orange, and lemon grafted onto the same Citrus rootstock. Almond, plum, and apricot can be grafted onto a peach rootstock. Many nurseries sell grafted apple trees containing two or more varieties. Brightly colored novelty cactus plants, which are not capable of photosynthesis, survive because they are grafted onto normal plants (Fig. 14.22).

To describe how grafting is performed, assume we have bred a new extra-juicy variety of apple and would like to graft it onto a young native tree with a hardy root system. In late spring, we would collect some young branches from the tree with the juicy apples (the scions) and keep them in a cooler to inhibit the buds from growing. Then, later in the spring when the rootstock begins to grow, we would bring the scion branches out of the cooler. Using sterilized instruments, we would cut the top of the rootstock diagonally several inches above the soil line. At the same time we would make a diagonal cut in a scion branch that is similar in diameter to that of the rootstock. Successful grafting depends on good contact between the vascular cambium of the scion and that of the rootstock. Therefore, we align the scion and rootstock so their cambia are in contact and hold them together with tape or string (Fig. 14.23). The graft union is often also sealed with wax to prevent the wounded tissues from drying out. If the graft is successful, several weeks later, the vascular cambia of the scion and rootstock will have grown together. The tree now consists of a single plant with one variety of rootstock below and a different fruiting variety above.

Propagation of Specialized Stems and Roots

If you would like to create a bed of tulips in your garden, you would not plant seeds or rooted cuttings. Instead, you would rely on bulbs, which are natural propagules produced by tulip plants. Herbaceous perennials produce shoots that die during the winter, but in the spring, they regrow from underground storage structures. The underground structures that survive over winter may be bulbs, corms, tuberous roots, or rhizomes (discussed in Chapter 6) (Fig. 14.24). These structures are typically fleshy, with abundant food-storage tissue, and contain buds that will sprout in the spring.

Micropropagation

One of the major disadvantages of most asexual propagation techniques is that they also propagate pathogens. Once a plant is diseased, there is no immune system to activate or

Plant Breeding and Propagation 269

Plant Propagation Crown Division
Figure 14.24A Propagation of specialized stems.

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Responses

  • catriona
    Which indoor plants is used division Propagation?
    8 years ago
  • Yorda Omar
    How to do division of plant propagation?
    8 years ago
  • almaz
    How to propagate plant division?
    8 years ago
  • MATHILDA
    How to air layer citrus?
    8 years ago
  • kimberly
    What are examples of plants that uses asexual methods of propagating plants?
    7 years ago
  • MARIA PAGE
    What plants reproduce by division?
    7 years ago
  • fergus
    How to do a "leaf cutting" with begonias?
    4 years ago
  • marcos
    What are the different types of specialaize stem photo?
    3 years ago
  • james
    How to propagated asexually a flower?
    2 years ago
  • jose
    How to propagate insulin plant?
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