Download Sexual and Asexual Reproduction of Plants OVERVIEW

Sexual and Asexual Reproduction of Plants
OVERVIEW
INSTRUCTOR:
UNIT: Performance of Technical Skills Related to Plant and Soil Science and Technology
LESSON:
Sexual and Asexual Reproduction of Plants
IMS REFERENCE: IMS #8388
TOPIC NOTES
Plant reproduction is necessary for the survival and perpetuation of plant species that are such an
important part of our daily lives. Plants have the capability to propagate or multiply themselves
by means of sexual and/or asexual reproduction. Both methods of reproduction are important to
the agricultural industry, specifically the commercial production of horticultural, agronomic, and
forestry crops. This topic presents an overview of the sexual and asexual reproduction of plants.
Each method of reproduction has advantages as well as limitations when compared with the
other.
Seed or sexual reproduction involves using seeds to produce new plants. Vegetative or asexual
reproduction includes using plant parts other than seeds to produce new plants.
Seeds are the primary means by which annuals such as field crops, flowering bedding plants, and
vegetables reproduce. Many perennials such as landscape plants, turfgrasses, and fruit crops
reproduce quicker and more efficiently by vegetative means.
SEXUAL REPRODUCTION
Seeds are the means by which plants sexually reproduce. They contain all the genetic
characteristics of the parent plant. However, the characteristics of plants grown from seeds may
vary from the parent plant.
Seed propagation is the most economical and widely used method of producing new plants,
provided that plants of the desired type can be reproduced by this method. Reproducing plants by
seeds allows a grower to produce a large number of plants.
* Underlined words are defined in the Glossary of Terms.
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Sexual and Asexual Reproduction of Plants
Pollination and fertilization are part of the sexual process that occurs within the flowers or
reproductive structures of a plant. Pollination and fertilization result in the formation and
development of seeds. Each seed produced contains a tiny plant and a supply of stored food,
surrounded by a protective covering.
Pollination
When a flower opens, the
stamens mature and become
fully developed. The anthers on
the stamen then release
hundreds of pollen grains. The
pollen grains are the male sex
cells. Each pollen grain contains
genetic characteristics of the
plant.
The flower ovary contains one
or more ovules, which are the
female sex cells. Each ovule
contains genetic characteristics
of the plant. As the pistil in the
flower matures, its stigma
develops a sticky surface.
As the anthers ripen and split
open, gravity, insects, wind, or
animals transfer the pollen grains to the stigmas. Pollination occurs when the pollen grains come
into contact with the moist, sticky surface of a stigma.
Self-pollination occurs when pollen comes into contact with a stigma within the same flower or
other flowers on the same plant (e.g., self-pollinating corn). Flowers cross-pollinate when pollen
from the flower of one plant transfers to stigmas of flowers on another plant. Alfalfa and many
clovers require cross- pollination for reproduction to occur.
Corn plants may self-pollinate or cross-pollinate. Pollen from staminate flowers on the tassel
transfers to pistillate flowers on the silk of the same plant to self-pollinate. Pollen may transfer to
the pistillate flowers of other plants to cross-pollinate.
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Sexual and Asexual Reproduction of Plants
The staminate flowers in one corn plant tassel may produce as many as 15 million pollen grains.
Pollen grains do not remain viable much longer than 24 hours after leaving the tassel.
Fertilization
After a pollen grain becomes anchored on the moist, sticky surface of the stigma, it germinates
and produces a pollen tube. The pollen tube grows down through the style and into the ovary
where it eventually reaches an ovule. Fertilization occurs when the male sex cell in the pollen
tube unites with the female sex cell in the ovule. For each seed a plant produces, a separate grain
of pollen must reach and unite with an ovule.
During the fertilization process, one sperm* cell from the pollen tube unites with the egg cell in
the ovule to form a zygote. The second sperm cell unites with two separate polar nuclei to form a
primary endosperm nucleus. Double fertilization results from the unions of the two male sex
cells.
The zygote proceeds through cell division and develops into a seed embryo. The primary
endosperm nucleus develops to form a food source to be used by the embryonic plant during seed
germination.
In corn, the zygote and primary endosperm nucleus develop within the same corn kernel. The
mature kernel contains both endosperm and embryo.
Fertilization in corn plants begins within 24 hours after pollen reaches the stigmas of the pistillate
flowers. Pollen grains germinate and send pollen tubes through styles (silk) to blister-like ovules
on the young corncob.
Pistillate flowers number approximately 800 per corn plant. One pollen grain is needed for the
development of each corn kernel. Research data shows that about 95% of the kernels are fertilized
through cross-pollination. The remaining kernels are fertilized as a result of self-pollination.
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Sexual and Asexual Reproduction of Plants
ASEXUAL REPRODUCTION
Asexual or vegetative reproduction involves the production of new plants by use of vegetative
parts from an existing plant. Vegetative plant parts used in asexual reproduction include stems,
buds, leaves, and roots.
The Importance of Plant Production by Vegetative Parts
The vegetative parts of many plants have the ability to produce new roots and/or shoots,
depending on the type of vegetative plant part. The vegetative parts of two different, but closely
related woody plants can be united with each other through budding and grafting techniques to
form a vascular connection.
Asexual or vegetative reproduction offers growers several important advantages over sexual
reproduction:
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Plants reproduce and mature quicker because of the elimination of the dormancy period
required of some seeds and the juvenile stages of plant growth.
Disease-free stock plants are produced under controlled environmental conditions.
Reproduction is possible for plants that do not develop reproductive parts or viable seeds.
Sugarcane, seedless grapes, navel oranges, bananas, and certain sterile hybrid grasses are
examples of crops that do not produce viable seeds. Such crop species must be
reproduced vegetatively. If no means for vegetatively reproducing these plants existed,
they would be lost entirely.
Plant selection for desired characteristics is more reliable. Asexual reproduction of plants
transmits identical characteristics from parent to offspring. Selection of mutations
(changes) by vegetative reproduction improves many plant cultivars.
New plants produced are genetically identical to the parent plant. A cutting from a parent
plant has the same genetic make-up as the parent plant. Genetic variation occurs with
sexual reproduction. Plants grown from seeds do not have the same genetic make-up as
either parent. They have the characteristics of both parent plants. As a result, the new
plants vary somewhat from either parent plant.
Plant Parts Used in Asexual Reproduction
Roots, buds, leaves, and stems are vegetative plant parts used in asexual reproduction. Roots such
as sweet potato and cassava are underground plant parts without nodes. Buds from woody plants
such as peaches, pecans, and roses are above-ground stem parts cut from a plant and placed in
close contact on another closely related plant. Leaves are above-ground plant parts cut from a
plant and placed into the soil for the purpose of producing new plants.
Stems may either be taken from above or below-ground parts of plants. The following paragraphs
describe the similarities and differences between above-ground stems and below-ground stems.
Above-ground Stems
Above-ground stems may stand erect or grow along the soil surface. Stems that grow horizontally
along the soil surface are stolons or runners. Portions of above-ground stems produce adventitious
roots when placed in the soil. The rooted stems may then be separated from the parent plant to
grow as new plants.
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Sexual and Asexual Reproduction of Plants
Examples of agricultural crops reproduced asexually from above-ground stems include
sugarcane, sweet potato, clover, alfalfa, and lespedeza. Additional examples are strawberries,
bermudagrass, and buffalograss.
Below-ground Stems
Below-ground stems are similar to above-ground stems in that they also have nodes and
internodes. How- ever, below-ground stems are modified structures that grow primarily beneath
the soil surface. Examples of below-ground stems are tubers, rhizomes, bulbs, and corms.
Tubers are short, thickened underground stems that contain “eyes.” The eyes are nodes that are
capable of producing shoots of new plants. Examples of tubers are the Irish potato and artichoke.
Rhizomes are underground, horizontal growing plant stems that produce new plants at each of
their nodes. Examples of plants with rhizomes are johnsongrass, bermudagrass, Aloe vera, and
asparagus.
Bulbs are underground stems with scale-like, fleshy leaves. Onion, garlic, and tulip are examples
of plants that grow from bulbs. Corms are short, underground stems similar to bulbs, however,
they do not contain fleshy leaves. Gladioli and water chestnut are examples of plants grown from
corms.
Methods of Vegetative Plant Reproduction
Propagation by cuttings is the most widely used method of vegetative or asexual reproduction.
Other methods include propagation by layering, separation and division, grafting and budding,
and tissue culture.
Cuttings
A cutting is any plant part severed from the parent plant. Cuttings used in asexual reproduction
include those taken from stems, roots, and leaves.
For successful reproduction to occur, the severed plant
part must develop those parts to make it a complete
plant. Stem cuttings must develop a new root system.
Root cuttings must develop a new shoot system.
Cuttings from leaves must produce both new root and
shoot systems.
Stem cuttings are those taken from the stems of a plant.
For this method of asexual reproduction, a grower
inserts a portion of a stem, including a node, into a
rooting medium. After several weeks, root formation
occurs along the portion of stem in the rooting medium.
Example of a stem, root and leaf cutting.
The use of stem cuttings is a successful method for reproducing many types of ornamental plants,
as well as several horticultural crops such as sweet potatoes, grapes, citrus, and figs.
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Sexual and Asexual Reproduction of Plants
Stem cuttings classified according to their stage of growth are herbaceous, softwood, semi
hardwood, and hardwood. Growers primarily use four types of stem cuttings to asexually
reproduce plants. These include tip cuttings, medial cuttings, single-eye cuttings, and cane
cuttings.
Modified stem cuttings taken from tubers and rhizomes are important for reproducing white
potatoes, cassava, and certain grasses. Each cutting requires an “eye” or thickened area for root
and shoot development to occur.
Root cuttings consist of sections of thickened roots cut into 2- to 3-inch sections and placed in a
rooting medium. They do not have any leaves or stems attached. Plants asexually reproduced
from root cuttings are those that naturally produce sprouts or suckers from the root system. Pecan,
apple, pear, blackberries, sweet potato, and horseradish are examples of nuts, fruits, and
vegetables reproduced by root cuttings.
Leaf cuttings consist of a leaf blade and petiole, only a leaf blade, or a section of a leaf blade.
They do not include any stem tissue or lateral buds. Growers use leaf cuttings to asexually
reproduce herbaceous plants that have a basal rosette arrangement of leaves. This method is
common for reproducing many ornamental and greenhouse plants.
Layering
Layering involves forcing a part of a plant to form roots while it remains attached to the parent
plant. Air layering and ground layering are two common layering techniques.
Air layering involves forcing roots to form on a stem, outside the soil. The grower makes a girdle
cut or a cleft cut on an above-ground section of a stem and then covers it with a moisture-holding
media. Root formation occurs on the plant stem, in the area of the cut, while the stem remains
attached to the parent plant.
Ground layering involves extending part of a plant into the ground, covering it with soil, and
allowing it to root. To encourage root formation on the stem, a cut is made on the section of stem
covered with soil. Ground layering methods include simple layering, trench layering, and mound
layering.
After roots develop on the stem in the area of the cut, the grower removes the rooted stem from
the parent plant to form a new plant.
Layering is a simple method of asexual plant reproduction and effective for propagating some
plants that are difficult to reproduce by other methods. It is also a cost-effective method of plant
propagation that allows growers to produce large plants in a relatively short period.
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Sexual and Asexual Reproduction of Plants
Separation and Division
Many plants have specialized vegetative structures. Bulbs, corms, tubers, runners, rhizomes, and
stolons are examples of specialized vegetative structures. These specialized vegetative structures
contain vegetative buds that are capable of producing new shoots. The new shoots eventually
form roots and develop into entire plants. Separation and division are two methods used for
producing new plants from their specialized vegetative structures.
Separation involves removing new plants formed on specialized stems and separating them from
the parent plant. Division is the technique of cutting specialized plant structures into sections and
forcing each section to grow into a new plant.
Grafting and Budding
Grafting is the oldest method of asexual reproduction. It was practiced over two thousand years
ago. Grafting consists of joining a hardwood scion from a plant to the rootstock of another
hardwood plant.
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Sexual and Asexual Reproduction of Plants
In most cases, scion and rootstock wood of the same plant type is used. However, grafting of
wood between two closely related plants is also possible. An example of the former is the grafting
of papershell pecan wood onto a native pecan rootstock. The grafting of plum scions to peach
rootstock is an example of the latter.
The size of the scion and the rootstock determine the grafting method used. Whip-and-tongue,
splice, wedge, and approach grafting are used when both the scion and rootstock are similar in
size. Side, cleft, notch, and bark inlay grafting are used when the scion is smaller in diameter than
the rootstock. Side grafting is used to add branches to bare areas of trees. Whip-and-tongue
grafting is used to reproduce nursery stock.
Budding is a method of propagation similar to grafting. It is a popular and successful method
used to reproduce roses and many fruit and nut trees. Budding consists of removing buds from
one plant and placing them on stems of other closely related plants. Normal buds (buds developed
normally on the plant at a leaf axis or terminally on the stem) and adventitious buds (buds
developed in positions other than where normal buds are found) are used in budding.
Plant breeders use budding as a means of changing cultivars of plant species. Patch budding, Tbudding, and chip budding are common budding methods.
Tissue Culture
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Sexual and Asexual Reproduction of Plants
Tissue culture, or micropropagation, is a special type of asexual reproduction. It
involves placing a very small piece of plant tissue on a specialized culture medium
in a test tube. The plant tissue multiplies and grows into new plants under sterile
conditions in the test tube. Plant tissues used in tissue culture include the tips of
shoots and roots, and sections of leaves and stems. Other tissues include embryos,
seeds, and pollen grains. Many advantages exist for using micropropagation:
1.
2.
3.
4.
Tissue culture allows large numbers of offspring to be produced quickly.
Tissue culture allows growers to produce plants that are free of disease.
Tissue culture is a cost-efficient method of reproducing plants.
Plants reproduced by tissue culture have the same characteristics as the
parent plant.
Tissue Culture
Disadvantages of tissue culture include:
1. The equipment required for tissue culture is expensive.
2. Tissue culture requires exact conditions.
3. More time and labor is required.
SUMMARY
Growers use several methods to multiply or increase the numbers of plant species. These methods
include both sexual and asexual methods of reproduction.
Sexual plant reproduction involves reproducing plants from seeds. Seed formation occurs as a
result of the pollination and fertilization processes that occur in the flowers of a plant.
Asexual or vegetative reproduction involves growing new plants from the vegetative parts of
existing plants. These vegetative parts include leaves, stems, and roots.
Asexual reproduction by cuttings is the most widely used method of asexual propagation. Other
methods include propagation by layering, separation and division, grafting and budding, and
tissue culture.
ACKNOWLEDGEMENTS
Shannon Houy, Graduate Technician, Department of Agricultural Education, Texas A&M
University, researched and developed this topic.
Keith W. Zamzow, Curriculum Specialist, Instructional Materials Service, Texas A&M
University, edited and reviewed this topic.
Vickie Marriott, Office Software Associate, Instructional Materials Service, Texas A&M
University, edited and prepared the layout and design for this topic.
Christine Stetter, Artist, Instructional Materials Service, Texas A&M University, prepared the
illustrations for this topic.
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Sexual and Asexual Reproduction of Plants
REFERENCES
Christensen, Norman L. (1998). Plant: Plant Reproduction and Growth, [Online]. Available:
http:// www.comptons.com/encyclopedia/ARTICLES/0125/01453312_A.html#P124 [2001, April
13]
Poincelot, Raymond P. Horticulture: Principles and Practical Applications. Englewood Cliffs,
NJ: Prentice-Hall, Inc., 1980.
Texas Agricultural Extension Service. Master Gardner Handbook. 3rd ed. College Station, TX,
1995.
GLOSSARY OF TERMS
Egg - Ovule; female reproductive body.
Eye - In reference to plants, an undeveloped bud.
Rootstock - A plant with an established root system that receives a scion; it is also referred to as
stock.
Scion - A severed plant part that is joined to a rootstock; also referred to as graftwood.
Sperm - Male reproductive body.
Zygote - A new developing individual produced by union of egg and sperm (a cell formed from
the fusion of male and female gametes).
TOPIC ACTIVITIES
FILL-IN-THE-BLANK: Complete the following statement.
1.
___________is the process in sexual reproduction occurring first where pollen grains
come in contact with the stigma.
2.
__________ is the process in seed formation when the male sex cell in the pollen tube
unites with the female sex cell in the ovule.
3.
A __________ develops from the union of sperm and egg cells.
4.
Plants reproduce sexually by_____, which contain all the genetic characteristics of that
plant.
5.
__________ contain the male sex cells of the flower.
SHORT ANSWER/LISTING: Answer the following questions or statements.
6.
List the four vegetative parts of a plant used in asexual reproduction.
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Sexual and Asexual Reproduction of Plants
7.
List the above- and below-ground stems of a plant used in asexual reproduction.
8.
Explain the difference between self-pollinated and cross-pollinated flowers in sexual
reproduction.
9.
Explain the difference in the genetic characteristics of plants grown sexually vs.
asexually.
10. Explain some important reasons agriculturists reproduce plants sexually and asexually.
TRUE/FALSE: Circle the "True" if the statement is true or the "False" if it is false.
11. Perennials including landscape plants, turfgrasses, and fruit crops are reproduced quicker
and more efficiently from seeds.
a. True
b. False
12. After a pollen grain germinates on the stigma, it releases two sperm into the pollen tube;
one unites with the egg and the other unites with the polar nuclei.
a. True
b. False
13. Budding, a form of grafting, is a popular and successful method used to reproduce roses and
many fruit and nut trees.
a. True
b. False
14. Tubers and rhizomes are used in leaf cuttings.
a. True
b. False
15. When grafting hardwood plants, usually the scion and rootstock wood of the same plant
type are used, but grafting between two closely related plants is possible.
a. True
b. False
16. Stem cuttings are used successfully to reproduce sweet potatoes.
a. True
b. False
17. Many species of berries are reproduced by some method of air layering.
a. True
b. False
18. Sugarcane, seedless grapes, navel oranges, bananas, and certain sterile hybrid grasses
produce a viable seed.
a. True
b. False
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Sexual and Asexual Reproduction of Plants
MATCHING: Match the term in the left column with its definition in the right column.
_____
19. Rhizomes
A.
The reproduction of plants
by use of vegetative parts to
reproduce new plants.
_____
20. Tissue culture
B.
The reproduction of plants
by use of seeds to reproduce
new plants.
_____
21. Asexual reproduction
C.
Underground, horizontal
growing plant stems that
produce new plants at the
nodes.
_____
22. Stolons
D.
Above-ground, horizontal
growing stems (runners)
form new plants at the
nodes.
_____
23. Sexual reproduction
E.
The reproduction of a plant
using plant tissue and
growing it in a test tube.
_____
24. Grafting
F.
Involves forcing a part of a
plant to form roots while it
remains attached to the
parent plant.
_____
25. Layering
G.
Joining a hardwood scion
from one plant to the
rootstock of another.
ADVANCED ACTIVITIES
1.
Visit the Web site: http://aggie-horticulture.tamu.edu/propagation/propagation.html from
Texas A&M University's horticulture page on plant propagation and choose one of the
procedures to perform. These procedures may be time-consuming; therefore, it is best to
work in groups or as a class.
2.
Many times agriculturists raising fruit and nut crops, such as pecans, apples, peaches, and
plums, use grafting techniques to reproduce their crops. Why might you think such crops
are usually reproduced asexually vs. sexually? What are reasons for using grafting to
reproduce such crops as opposed to other methods of asexual reproduction? (Research
may be found on the Internet to explain some reasons.)
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