ASEXUAL PROPAGATION OF INTRODUCTION - ASEXUAL PROPAGATION OF PLANTS

Asexual propagation has been widely used within the sciences as a method of maintaining genetic purity. Most of the uses have been within , although we do have examples in both forages and agronomic cropping systems. We have also found this to be an efficient method of maintaining some botanical varieties.

When using asexual propagation, we are essentially producing clones. Clones are groups of plants that are identical to their parent (or donor) plant. We also use this process for plant types that can only be propagated asexually. Several classic examples of these are the Bartlett pear and the Red Delicious apple. The specific genetic composition has been asexually propagated for many years.

Major methods of asexual propagation of stems and are cuttings, layering, division, and grafting. We also find the production of specialized plant parts that allow for asexual propagation. One of the widely used plant parts are the production of modified stems. These can be found in the form of tubers, bulbs, rhizome , corms and stolons. Although less frequent, we do find the use of root tissue for . LEARNING OUTCOMES 1. Understand the importance of economically important, asexually propagated species and the unique characteristics or each type of classification. 2. Be able to correctly use the terminology and identify the structures of the asexually propagated plant parts that are commonly found in horticultural species. 3. Learn the proper methods associated with asexually propagating economically important plant species.

Advantages of vegetative reproduction

Since higher levels of stored reserves are available throughout the year and the parent plant with its root system can absorb water from quite a wide area, two of the hazards of germination are reduced. Buds are produced in an environment where the parent is able to flourish, but many dispersed from plants never reach a suitable situation for effective germination.

Vegetative reproduction does not usually result in rapid and widespread distribution of offspring in the same way as , but tends to produce a dense clump of plants with little room for competitors between them. Such groups of plants are very persistent and, because of their buds and underground food stores, can still grow after their foliage has been destroyed by insects, fire, or cultivation. Those species regarded as weeds are difficult to eradicate, since even a small piece of an asexual propagule bearing a bud can give rise to a new colony (clone). The following sections will describe the diversity of methods and/or structures that can be used for the process of .

SEPERATION

Separation is the propagation method in which these natural structures are simply removed from the parent plant and planted to grow on their own. Crown division is one method and one of the best methods to propagate herbaceous perennial plants (Figure 1). With this process, the plant is simply cut into sections with a knife. Individual growing points (shoots) that contain roots may then be used be used to form new plants. Some woody perennials that branch or sucker (adventitious shoots) from the main plant can also be reproduced in this way.

A B C

Figure 1. Example of an asexual propagation called separation: (A) Large, mature plant that can be separated into multiple shoots with adventitious roots, (B) several stems with roots that have been removed, and (C) new plant is potted. PROPAGATING PLANTS BY GRAFTING (OR BUDDING)

Grafting and budding are used as one of the primary methods of propagating certain plants such as fruit (illustrated in Figure 2). Grafting is a horticultural technique whereby plant parts from one plant are attached onto those of another so that the two plant types join together. In most cases, one plant is selected for its root characteristics and this is called the stock or rootstock The other plant is selected for its vegetative characteristics such as stem, , flower or fruit and is called the scion.

LAYERING

A simple method where the potential cutting(s) is (are) attached to the plant until the adventitious roots form is called layering (Figure 3). This is done by covering the developing rooting areas with some form of media such as soil , sand, or a potting mix. This allows for nutrients from the stock plant to supplement growth until the ‘cutting’ can be detached producing functional plants. A variation of this procedure is air layering (Figure 4). This is done by taking a stem and covering 1 – several nodes with a media until such a time that we get the formation of adventitious roots. The stem can then be severed from the stock plant and establish a new cutting.

Figure 2. Illustration of the grafting process with a separate rootstock and a scion.

Figure 4. Illustration of the air layering process.

Figure 3. Illustration of the layering process. CUTTINGS

This method of asexual propagation is one of the most popular techniques due to the limited resources that are needed and it’s efficient use on various landscaping, fruit, and vegetable plants. A cutting is a vegetative plant part which is severed from the parent plant in order to produce a plant that is genetically identical to the donor (Figure 5). There are a number of various methods that fall under this classification. Cuttings may be taken at any time on herbaceous plants but may vary with perennial plants. At least one node should be below the media surface since many plant types will produce roots with this plant part, but some plants can root at the internodes. Some of the variation within this procedure are the tip cuttings (stem cutting which includes the terminal bud), cane cuttings (stems are cut into sections containing one or two nodes), or leaf cuttings (may be a whole leaf with or without the petiole or leaf sections).

VEGETATIVE PLANT STRUCTURES

Another major area is the natural production of vegetative plant structures that have the capacity to produce new plants. Asexual propagation, as mentioned earlier, is the best way to maintain some species, particularly an individual that best represents that species. Many of these vegetative structures are modified stems and include structures produced below the soil line (bulbs, corms, rhizomes, and tubers) and above the soil (stolons). Many of these serve as food storage areas for the plant. Each of the major types of vegetative plant structures will be described.

STOLONS

A stolon is a horizontal stem that can root and produce new adventitious roots (Figure 6). A term that is also used is a runner . Plants that produce stolons or runners are propagated by initiating the production of adventitious roots and then removing the plantlet from the maternal plant. Plantlets can also be initiated at the tips of stolons and form roots while still attached to the parent (spider plant) or detached and placed in a rooting medium. Examples: strawberry, spider plant, bermudagrass.

BULBS

Bulbs consist of very short stems with closely packed leaves arranged in concentric circles round the stem. The leaves are swollen with stored food e.g. onion (Figure 7). A terminal bud will produce the next A B

C D E

Figure 5. Examples of cuttings: stem cutting (A, B, &D) and a leaf cutting (C & E). A

B C

Figure 6. Examples of several plant types that reproduce asexually by means of a modified stem called a stolon: (A) strawberries showing a plant producing stolons and inset a stolon without the production of adventitious roots and a mature plantlet with adventitious roots. (B&C) A plantlet and a mature spider plant. year’s flowering shoot and the lateral (axillary) buds will produce new plants. There are also a number of flowering bulbs that are used in the horticultural industry.

RHIZOMES

Rhizomes are stems which grow horizontally under the ground. In some cases, the underground stems are swollen with food reserves. The terminal bud turns upwards to produce the vegetative shoot and

A B

A

C D

Figure 7. Example of a simple bulb (Allium cepa). A mature bulb (A) and a X-section (B) showing the modified leaves. An immature bulb (C) showing the production of adventitious roots and a X-section of the immature growth (D). the lateral buds may grow out to form new rhizomes. This tissue typically has stored reserves. At each node we find an axillary bud covered by a leaf scale. This tissue has the ability to produce a shoot and adventitious roots. If a rhizome is cut into pieces with at least one node, this may allow for the production of a new plant.

A B

C

Figure 8. Examples of a rhizome producing plant. (A) A sedge (Cyperus spp.) producing a plant from the activity of a rhizome. (B) A rhizome from a johnsongrass plant (Sorghum halepense) with the individual rhizome with a functional plant with leaves and adventitious roots (C) and rhizomes with developing shoots prior to emergence. A

B C

Figure 9. An example of a bermudagrass plant that produces both a stolon (A) and a rhizome (B&C). TUBERS

Tubers are thick, underground stems that develop at the end of rhizomes (Figure 10). The tuber has all the parts of a normal stem including nodes and internodes. The nodes are the ‘eyes’, and they are arranged around the tuber in a spiral fashion beginning on the end opposite the attachment point to the rhizome. A tuber will typically have a form of stored reserves in cells classified as parenchyma. The tuber is produced in one growing season and is a means of propagation. One of the best examples of a tuber is the potato (Solanum tuberosum).

CORMS

Corms are a shortened stem, but it is the stem itself which swells and stores food. The circular leaves typically form papery scales. As with bulbs, the terminal bud grows into a flowering shoot, and the lateral buds produce new plants. Corms are sometimes confused with bulbs as they are similar in appearance externally. Corms are stems that are internally structured with solid tissues, which distinguishes them from bulbs which are mostly made up of layered fleshy scales that are modified leaves. As a result, when a corm is cut in half it is solid, but when a true bulb is cut in half it is made up of layers (modified leaves). Examples are taro (Colocasia esculenta) and crocus (Crocus spp.).

TUBEROUS ROOT

A tuberous root (or sometimes called a storage root) is a modified lateral root which is enlarged and will function as a means of storage for future vegetative development. It has the structure of a root since there are no nodes and internodes. Root tubers have one end called the proximal end which is attached to the old plant. The apical end has crown tissue that produces buds which grow into new stems. This makes this growth biennial in duration with the first year producing root tubers and the next growing season producing new shoots. Within this area, the best representative species is the sweet potato (Ipomoea batatas).

Another species that reproduces through a ‘true’ root system producing adventitious shoots is the Horsenettle (Solanum carolinense). It is in the nightshade (Solanaceae) family. New plants emerge from rootstocks buried below the soil surface. This characteristic is also found within the family asteraceae and typically in the thistle group.

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Figure 10. Examples of a tuber. (A) production of a new potato plant arising from an ‘eye’ located on the modified stem. The roots that develop will be adventitious. (B) A potato that has produced a tuber; they are still attached to the rhizome. (C) Production of tubers from a rhizome on a sedge plant. A

B C

Figure 11. Examples of corms. (A) a taro corm in which new growth has been initiated. (B) A crocus corm with the circular leaves forming papery scales. (C) A crocus corm where the scales have been removed and you can see the development of cormels. A

B C

Figure 12. Examples of roots that asexually propagate (A) Sweet potatoes propagate by means of a root tubers. These structure have one end that is called the proximal end which is attached to the old plant. The apical end has crown tissue that produces buds which grow into new stems. (B&C) Asexual reproduction through a ‘true’ root system is found in several plant types. These are examples of Horsenettle root producing a functional plant. The images demonstrate an early (B) and a later (C) stage of development. INTRODUCTION Many types of plants are propagated by cuttings. A cutting is a vegetative plant part which is cut from the stock plant in order to regenerate a genetically identical plant (clone). Both woody and herbaceous species can be regenerated in this way. LEARNING OUTCOMES 1. Understand the process of asexual propagation in respect to the maintenance of genetic purity. 2. Be able to successfully propagate a plant using proper techniques. MATERIALS – ASEXUAL PROPAGATION OF STEMS AND LEAVES 1. Plant specimens of various plant species of both monocots and dicots. The available plants will primarily be economically important horticultural species.. 2. Cutting instrument such as razor blades or scalpels. 3. Rooting media and rooting pots. 4. Alcohol or bleach solution for sterilization. METHODS 1. You will be taking cuttings from the plant types provided in class. 2. Take a sharp blade to remove the cutting from the parent plant (this will reduce injury to the parent plant) that has been dipped in either alcohol or a bleach mixture (one part bleach : nine parts water ). This is done to prevent transmitting diseases from infected plant parts to healthy ones. 3. Remove certain parts of the cutting, such as flowers and flower buds. This will allow the use of stored reserves for root and shoot formation. 4. For the stimulation of rooting or to potentially increase the number of roots, we may use a rooting hormone such as IBA or IAA. 5. Insert the cuttings into a rooting medium such as coarse sand, vermiculite, perlite, soil, water, or a mixture of peat and perlite. Moisten the media before inserting cuttings, and keep it evenly moist while cuttings are rooting and forming new shoots. 6. Place the stem and leaf cuttings into the growth chamber which will be set for an optimum temperature and light conditions for plant growth. 7. After approximately 10 – 14 days, check for the development of adventitious roots or shoot growth. A B

C D

E F

Figure 13. Methods for asexual propagation by cuttings: (A) Insure that your sharp blade has been dipped in either alcohol or a bleach mixture. (B) Make sure that your stem cutting has at least 1 viable node. (C) For more rapid results, dip the cut end in a rooting hormone. (D) Insert the cuttings into a rooting medium and keep it evenly moist while cuttings are rooting. (E&F) Cuttings that have rooted. INTRODUCTION Asexual propagation of many types of plants is accomplished through the used of specialized vegetative structures. This will produce a vegetative plant that will regenerate a genetically identical plant (clone). This laboratory exercise will expose you to the different types of structures available for propagation. LEARNING OUTCOMES 1. Understand the process of asexual propagation in respect to the maintenance of genetic purity using modified stems or roots. 2. Be able to successfully propagate a plant using these modified plant structures. MATERIALS – ASEXUAL PROPAGATION USING STEMS AND LEAVES 1. Plant structures including bulbs, rhizomes, stolons, corms, and tubers will be made available from various plant species. These will primarily be from economically important horticultural species or weed species. 2. Cutting instrument such as razor blades or scalpels. 3. Rooting media and rooting pots.

PROCEDURE 1. Take the various types of asexual propagule available and place into a pot with a growing media that has been moistened. 2. Label and place these pots into a growing environment suitable for growth. 3. Once growth has be reinitiated, observe the specific types of growth produced by the developing root and shoot system. Primary emphasis should be placed on the terminology of the developing plant.

REFERENCES

Forbes, J.C., and K.D.Watson. 1992. Plants in Agriculture. University Press, Cambridge.

Parker, R. 2004. Introduction to Plant Science. Thomson, Delmar Learning.

McMahon, M.J., A.M. Kofranek, and V.E. Rubatzky. 2011. Plant Science: Growth, Development, and Utilization of Cultivated Plants, Fifth Edition. Prentice Hall.