2.3 Artificial Methods of Asexual Reproduction
2_Vegetative-Propagation-in-Plants
Vegetative Propagation in Plants
Overview
Adventitious roots of Magnolia cutting Pistoia - Baldacci Vivai (25.06.1980, photo: Mihailo Grbić)
The original uploader was Gmihail at Serbian Wikipedia., CC BY-SA 3.0 RS <https://creativecommons.org/licenses/by-sa/3.0/rs/deed.en>, via Wikimedia Commons
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Introduction
Learning Objectives
Describe how plants use corm, rhizome, tuber, bulbs, stolons, or runners as methods of natural asexual reproduction.
Describe apomixis.
List and describe how grafting, cutting, layering, and micropropagation are used as the artificial methods of asexual reproduction in plants.
Describe the advantages and disadvantages of asexual reproduction.
Key Terms
apomixis - a process by which seeds are produced without fertilization of sperm and egg
bulb - modified stem used for propagation
corm - modified stem with scales used for propagation
cutting - method of asexual reproduction where a portion of the stem, containing nodes and internodes, is placed in moist soil and allowed to root
grafting - method of asexual reproduction where the stem from one plant species is spliced into a different plant
layering - method of propagating plants by bending a stem under the soil
micropropagation - propagation of desirable plants from a plant part; carried out in a laboratory
rhizome - modified stem grows underground and produces roots and shoots from its nodes
runner/stolon - a modified stem that grows horizontally on the soil surface and gives rise to new plants
tuber - a stem modified as a storage organ also used in vegetative propagation
Introduction
Many plants propagate themselves asexually using vegetative parts like stems, roots, leaves, or apomixis. Asexual reproduction is cost-effective since it does not require the plant to produce a flower, attract pollinators, or find a means of seed dispersal.
An advantage of asexual reproduction is that the resulting plant will reach maturity faster. Since the new plant is arising from an adult plant or plant parts, it will also be sturdier than a seedling. New cells are formed via mitosis and undergo differentiation to produce different parts of a plant. And asexual reproduction can take place by natural or artificial (assisted by humans) means.
Natural Methods of Asexual Reproduction
Many plants use stems or roots to propagate. These vegetative structures are identified by the presence or absence of scales, if the structure is a storage organ or not, and called
Corm: solid fleshy stems that look like bulbs. For example, garlic (Figure 3.2.1) and gladiolus (Figure 3.2.2).
Rhizome: is a subterranean stem that produces roots and shoots from its nodes; this is apparent with ginger and iris plants (Figure 3.2.1c). Rhizomes give rise to multiple plants.
Tuber: Potato form fleshy stem tubers. Each eye in the stem tuber can give rise to a new plant. A potato is a stem tuber but with an enlarged structure for storage of food (Figure 2.3.4).
Bulb: stout stem covered with scales. The scales can be fleshy (non-tunicate bulbs, ex. lilies) or dry (tunicate bulbs, ex. Onion, daffodil) (Figure 3.2.3).
Stolon/runner: stems that grow at the soil surface or just below ground and can give rise to new plants. A stem tuber is usually a stolon, also called a runner; this can be found with strawberries (Figure 3.2.1e; 3.2.5)., In sweet potatoes, adventitious roots come out of the nodes of the stem to give rise to a new plant (Figure 3.2.5).
Parsnip propagates from a taproot, while Ivy uses an adventitious root—a root arising from a plant part other than the primary root.
In Bryophyllum and kalanchoe, the leaves have small plantlets on their margins. When these plantlets drop off the mother plant, they grow into independent plants; or they may start growing into independent plants if the leaf touches the soil (Figure 3.2.6).
Some plants can produce seeds without fertilization. Either the ovule or part of the ovary, which is diploid in nature, gives rise to a new seed. This method of reproduction is known as apomixis. Seeds are produced in either of the two ways:
- In one form, the egg is formed with 2n chromosomes and develops without ever being fertilized.
- In another version, the cells of the ovule (2n) develop into an embryo instead of - or in addition to - the fertilized egg.
Hybridization between different species often yields infertile offspring. But in plants, this does not necessarily doom the offspring. Many such hybrids use apomixis to propagate themselves.
The many races of Kentucky bluegrass growing in lawns across North America and the many races of blackberries are two examples of sterile hybrids that propagate successfully by apomixis.
Access for free at https://openstax.org/books/biology-2e/pages/32-3-asexual-reproduction
Artificial Methods of Asexual Reproduction
Artificial methods are frequently employed to give rise to new, and sometimes novel, plants. They include grafting, cutting, layering, and micropropagation.
Grafting
Grafting has long been used to produce novel varieties of roses, citrus species, and other plants, and it is widely used in viticulture (grape growing) and the citrus industry In grafting, two plant species are used; part of the stem of the desirable plant is grafted onto a rooted plant called the stock. The part that is grafted or attached is called the scion. Both are cut at an oblique angle (any angle other than a right angle), placed in close contact with each other, and are then held together (Figure 3.2.7). Matching up these two surfaces as closely as possible is extremely important because these will be holding the plant together. The vascular systems of the two plants grow and fuse, forming a graft. After some time, the scion starts producing shoots and eventually starts bearing flowers and fruits. Scions capable of producing a particular fruit variety are grafted onto rootstock with specific resistance to disease.
Cutting
Plants such as coleus and money plant are propagated through stem cuttings, where a portion of the stem containing nodes and internodes is placed in moist soil and allowed to root. In some species, stems can start producing a root even when placed only in water. For example, leaves of the African violet will root if kept in water undisturbed for several weeks. Similarly, many indoor ornamental plants, such as rubber plants, poinsettia, and pothos, also propagate through cutting.
Layering
Layering is a method in which a stem attached to the plant is bent and covered with soil. Young stems that can be bent easily without any injury are preferred. Jasmine and bougainvillea (paper flower) can be propagated this way (Figure 3.2.8). In some plants, a modified form of layering known as air layering is employed. A portion of the bark or outermost covering of the stem is removed and covered with moss, which is then taped. Some gardeners also apply rooting hormones (unit 2 lesson 5). After some time, roots will appear, and this portion of the plant can be removed and transplanted into a separate pot.
Micropropagation
Micropropagation (also called plant tissue culture) is a method of propagating multiple plants from a single plant in a short time under laboratory conditions (Figure 3.2.9). This method allows the propagation of rare, endangered species that may be difficult to grow under natural conditions, are economically important or are in demand as disease-free plants. To start plant tissue culture, a part of the plant such as a stem, leaf, embryo, anther, or seed can be used. The plant material is thoroughly sterilized using a combination of chemical treatments standardized for that species. Under sterile conditions, the plant material is placed on a plant tissue culture medium that contains all the minerals, vitamins, and hormones required by the plant. The plant part often gives rise to an undifferentiated mass known as callus, from which individual plantlets begin to grow after a period of time. These can be separated and are first grown under greenhouse conditions before they are moved to field conditions.
Asexual reproduction produces new plants faster and more adapted to survive well under stable environmental conditions when compared with plants produced from sexual reproduction. This method produces plants that are genetically identical to their parents. Such populations are less likely to survive if the environmental conditions change.
Access for free at https://openstax.org/books/biology-2e/pages/32-3-asexual-reproduction
Attributions
Adventitious roots of Magnolia cutting Pistoia
The original uploader was Gmihail at Serbian Wikipedia., CC BY-SA 3.0 RS <https://creativecommons.org/licenses/by-sa/3.0/rs/deed.en>, via Wikimedia Commons
International potato center https://cipotato.org/