Asexual Reproduction

Asexual Reproduction in Plants

Plants can reproduce asexually, without the fertilization of gametes, by either vegetative reproduction or apomixis.

Learning Objectives

Summarize methods of asexual reproduction in plants

Key Takeaways

Key Points

  • Asexual reproduction produces individuals that are genetically identical to the parent plant.
  • Roots such as corms, stem tubers, rhizomes, and stolon undergo vegetative reproduction.
  • Some plants can produce seeds without fertilization via apomixis where the ovule or ovary gives rise to new seeds.
  • Advantages of asexual reproduction include an increased rate of maturity and a sturdier adult plant.
  • Asexual reproduction can take place by natural or artificial means.

Key Terms

  • stolon: a shoot that grows along the ground and produces roots at its nodes; a runner
  • apomixis: process of reproduction in which plants produce seeds without fertilization

Asexual Reproduction

Many plants are able to propagate themselves using asexual reproduction. This method does not require the investment required to produce a flower, attract pollinators, or find a means of seed dispersal. Asexual reproduction produces plants that are genetically identical to the parent plant because no mixing of male and female gametes takes place. Traditionally, these plants survive well under stable environmental conditions when compared with plants produced from sexual reproduction because they carry genes identical to those of their parents.

Plants have two main types of asexual reproduction: vegetative reproduction and apomixis. Vegetative reproduction results in new plant individuals without the production of seeds or spores. Many different types of roots exhibit vegetative reproduction. The corm is used by gladiolus and garlic. Bulbs, such as a scaly bulb in lilies and a tunicate bulb in daffodils, are other common examples of this type of reproduction. A potato is a stem tuber, while parsnip propagates from a taproot. Ginger and iris produce rhizomes, while ivy uses an adventitious root (a root arising from a plant part other than the main or primary root), and the strawberry plant has a stolon, which is also called a runner.

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Roots: Different types of stems allow for asexual reproduction. (a) The corm of a garlic plant looks similar to (b) a tulip bulb, but the corm is solid tissue, while the bulb consists of layers of modified leaves that surround an underground stem. Both corms and bulbs can self-propagate, giving rise to new plants. (c) Ginger forms masses of stems called rhizomes that can give rise to multiple plants. (d) Potato plants form fleshy stem tubers. Each eye in the stem tuber can give rise to a new plant. (e) Strawberry plants form stolons: stems that grow at the soil surface or just below ground and can give rise to new plants

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.

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. Asexual reproduction can take place by natural or artificial (assisted by humans) means.

Natural and Artificial Methods of Asexual Reproduction in Plants

Plants can undergo natural methods of asexual reproduction, performed by the plant itself, or artificial methods, aided by humans.

Learning Objectives

Distinguish between natural and artificial methods of asexual reproduction in plants

Key Takeaways

Key Points

  • In natural asexual reproduction, roots can give rise to new plants, or plants can propagate using budding or cutting.
  • In grafting, part of a plant is attached to the root system of another plant; the two unite to form a new plant containing the roots of one and the stem and leaf structure of the other.
  • Cutting is the process in which the stem of a plant is placed in moist soil or water to generate a new root system.
  • In layering, part of the plant’s stem is bent down and covered with soil; this stem can generate a new root system and, therefore, an entirely new plant.
  • Micropropagation is the process in which part of a plant is placed in plant culture medium and provided with all the hormones and nutrients it needs in order to generate new plants.
  • When part of a plant is placed in plant culture medium and provided with all the hormones and nutrients it needs, it can generate new plants; this is known as micropropagation.

Key Terms

  • layering: a method of plant propagation in which a bent stem is covered with soil in order to generate new roots
  • grafting: process of attaching part of a stem from one plant onto the root of another plant
  • micropropagation: practice of rapidly multiplying plant material to produce a large number of progeny plants using plant tissue culture methods
  • cutting: placing part of a stem containing nodes or internodes in water or moist soil in order to produce new plants

Natural Methods of Asexual Reproduction

Natural methods of asexual reproduction include strategies that plants have developed to self-propagate. Many plants, such as ginger, onion, gladioli, and dahlia, continue to grow from buds that are present on the surface of the stem. In some plants, such as the sweet potato, adventitious roots or runners (stolons) can give rise to new plants. In Bryophyllum and kalanchoe, the leaves have small buds on their margins. When these are detached from the plant, they grow into independent plants; they may also start growing into independent plants if the leaf touches the soil. Some plants can be propagated through cuttings alone.

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Runners: asexual reproduction: A stolon, or runner, is a stem that runs along the ground. At the nodes, it forms adventitious roots and buds that grow into a new plant.

Artificial Methods of Asexual Reproduction

Artificial methods of asexual reproduction 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. 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. 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 a period of time, the scion starts producing shoots, eventually bearing flowers and fruits. Grafting is widely used in viticulture (grape growing) and the citrus industry. Scions capable of producing a particular fruit variety are grafted onto root stock with specific resistance to disease.

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Grafting: Grafting is an artificial method of asexual reproduction used to produce plants combining favorable stem characteristics with favorable root characteristics. The stem of the plant to be grafted is known as the scion, and the root is called the stock.

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 undisturbed in water for several weeks.

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 the preferred plant for this method. Jasmine and bougainvillea (paper flower) can be propagated this way. 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 hormone. After some time, roots will appear; this portion of the plant can be removed and transplanted into a separate pot.

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Layering: In layering, a part of the stem is buried so that it forms a new plant.

Micropropagation

Micropropagation (also called plant tissue culture) is a method of propagating a large number of plants from a single plant in a short time under laboratory conditions. This method allows 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 a callus, from which, after a period of time, individual plantlets begin to grow. These can be separated; they are first grown under greenhouse conditions before they are moved to field conditions.

Plant Life Spans

The life cycles and life spans of plants vary and are affected by environmental and genetic factors.

Learning Objectives

Explain the process of aging in plants

Key Takeaways

Key Points

  • The life span of a plant is the length of time it takes from the beginning of development until death, while the life cycle is the series of stages between the germination of the seed until the plant produces its own seeds.
  • Annuals complete their life cycle in one season; biennials complete their life cycle in two seasons; and perennials complete their life cycle in more than two seasons.
  • Monocarpic plants flower only once in their lifetime, while polycarpic plants flower more than once.
  • Plant survival depends on changing environmental conditions, drought, cold, and competition.
  • Senescence refers to aging of the plant, during which components of the plant cells are broken down and used to support the growth of other plant tissues.

Key Terms

  • annual: a plant which naturally germinates, flowers, and dies in one year
  • biennial: a plant that requires two years to complete its life cycle
  • perennial: a plant that is active throughout the year or survives for more than two growing seasons
  • monocarpic: a plant that flowers and bears fruit only once before dying
  • polycarpic: bearing fruit repeatedly, or year after year
  • senescence: aging of a plant; accumulated damage to macromolecules, cells, tissues, and organs with the passage of time

Plant Life Spans

The length of time from the beginning of development to the death of a plant is called its life span. The life cycle, on the other hand, is the sequence of stages a plant goes through from seed germination to seed production of the mature plant. Some plants, such as annuals, only need a few weeks to grow, produce seeds, and die. Other plants, such as the bristlecone pine, live for thousands of years. Some bristlecone pines have a documented age of 4,500 years. Even as some parts of a plant, such as regions containing meristematic tissue (the area of active plant growth consisting of undifferentiated cells capable of cell division) continue to grow, some parts undergo programmed cell death (apoptosis). The cork found on stems and the water-conducting tissue of the xylem, for example, are composed of dead cells.

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Plant life spans: The bristlecone pine, shown here in the Ancient Bristlecone Pine Forest in the White Mountains of eastern California, has been known to live for 4,500 years.

Annuals, Biennial, and Perennials

Plant species that complete their life cycle in one season are known as annuals, an example of which is Arabidopsis, or mouse-ear cress. Biennials, such as carrots, complete their life cycle in two seasons. In a biennial’s first season, the plant has a vegetative phase, whereas in the next season, it completes its reproductive phase. Commercial growers harvest the carrot roots after the first year of growth and do not allow the plants to flower. Perennials, such as the magnolia, complete their life cycle in two years or more.

Monocarpic and Polycarpic Plants

In another classification based on flowering frequency, monocarpic plants flower only once in their lifetime; examples of monocarpic plants include bamboo and yucca. During the vegetative period of their life cycle (which may be as long as 120 years in some bamboo species), these plants may reproduce asexually, accumulating a great deal of food material that will be required during their once-in-a-lifetime flowering and setting of seed after fertilization. Soon after flowering, these plants die. Polycarpic plants form flowers many times during their lifetime. Fruit trees, such as apple and orange trees, are polycarpic; they flower every year. Other polycarpic species, such as perennials, flower several times during their life span, but not each year. By this method, the plant does not require all its nutrients to be channeled towards flowering each year.

Genetics and Environmental Conditions

As is the case with all living organisms, genetics and environmental conditions have a role to play in determining how long a plant will live. Susceptibility to disease, changing environmental conditions, drought, cold, and competition for nutrients are some of the factors that determine the survival of a plant. Plants continue to grow, despite the presence of dead tissue, such as cork. Individual parts of plants, such as flowers and leaves, have different rates of survival. In many trees, the older leaves turn yellow and eventually fall from the tree. Leaf fall is triggered by factors such as a decrease in photosynthetic efficiency due to shading by upper leaves or oxidative damage incurred as a result of photosynthetic reactions. The components of the part to be shed are recycled by the plant for use in other processes, such as development of seed and storage. This process is known as nutrient recycling. However, the complex pathways of nutrient recycling within a plant are not well understood

The aging of a plant and all the associated processes is known as senescence, which is marked by several complex biochemical changes. One of the characteristics of senescence is the breakdown of chloroplasts, which is characterized by the yellowing of leaves. The chloroplasts contain components of photosynthetic machinery, such as membranes and proteins. Chloroplasts also contain DNA. The proteins, lipids, and nucleic acids are broken down by specific enzymes into smaller molecules and salvaged by the plant to support the growth of other plant tissues. Hormones are known to play a role in senescence. Applications of cytokinins and ethylene delay or prevent senescence; in contrast, abscissic acid causes premature onset of senescence.

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Plant senescence: The autumn color of these Oregon Grape leaves is an example of programmed plant senescence.