Plant Breeding
Objective
This section will focus on genetics and breeding providing a brief overview
of them in an attempt to show you how science is working to improve the world in
which humans live.
Introduction
From the beginning of time humans have recognized the importance of
plants to their survival. Virtually every aspect of our lives is connected to plant
materials in some way or another. Food, clothing, building material, and
medicines are only a few ways in which we make use of plants.
Given the importance of plants it is not at all surprising that humans
learned to manipulate them and make them more suitable for human use. We
have learned to increase crop yields, adapt crops to areas they would not
normally grow in and create resistance to insects and disease. These changes
have been helpful in efforts to feed an increasing global population.
Plant Breeding and Genetics
Breeding, the art and science of changing and improving the heredity of
plants, has been practiced by humans since plants were first cultivated.
Selection, the process of choosing those plants considered superior and
harvesting them for seed, was one of the earliest techniques. This ability of the
breeder to select plants for economically valuable characteristics is the art of
breeding.
The scientific aspect comes with knowledge of genetics and other plant
sciences. Genetics can be defined as the science that studies heredity and
variation or those things that make individuals similar (heredity) and different
(variation). Genetics seeks to explain how and why variation and heredity
occur. This knowledge has allowed breeders to manipulate the heredity of plants
to produce higher yields, resistance to insects and disease and tolerance to
drought. The changes that are made are not temporary so the benefits can be
reaped over and over.
First published in 1866, Gregor Mendel proposed that hereditary units,
now called genes, controlled the expression of an individual’s characteristics or
traits. For instance, Mendel proposed that if you have two apple trees and one
has a higher yield, the tree with the higher yield possessed genes for higher
yield. He went on the explain that each individual inherits an equal number of
genes from each parent and the expression of a trait is determined by the
combination of genes inherited from both parents. This concept was more fully
developed by Walter S. Sutton who in 1902, proposed the chromosome theory
of inheritance. This stated that the hereditary factors Mendel wrote of, genes,
were located on chromosomes which split during meiosis.
Chromosomes are located in the nucleus of a cell and carry and transmit
genetic information of an individual from one generation to the next. In plants the
number of chromosomes in vegetative cells is the same for an entire species.
This number is referred to as the 2n or diploid number. In alfalfa it is 32 and
corn 20. In the sex cells, gametes, the number is one-half of the diploid and is
known as the 1n or haploid chromosome number. This information is invaluable
to the breeder working to develop better cultivars.
So far, the only method or breeding technique mentioned has been
selection. Planned hybridization between carefully selected parents is now one
of the dominant methods available today. The breeder must find two parents that
will combine to produce a superior first generation offspring or F1 hybrid plant.
The F1 should be superior to both of its parents measured in terms of increased
vegetative growth, cell size, plant height, etc.
Types of Hybrids
1. Singe cross (A x B)
Cross between two unrelated inbred lines A and B.
2. Modified single cross (A1 x A) x B
A1 and A are two closely related inbred lines which are crossed to produce
an F1 which is then crossed with B, an unrelated inbred.
3. Three-way cross (A x B) x C
Cross between two unrelated inbred lines A and B to produce and F1
which is then crossed with C, an unrelated inbred.
4. Modified three way cross (A x B) (C1 x C1)
Cross between two unrelated inbred lines, A and B, to produce an F1;
cross between two closely related inbreds, C1 and C, to produce and F 1;
cross between the two F1 hybrids.
5. Double cross (A x B) (C x D)
Cross between two unrelated inbreds, A and B, the produce an F 1; cross
between two unrelated inbreds, C and D, to product an F1; Cross the two
F1 hybrids.
The single cross is generally preferred for greater expression of hybrid
vigor measured in higher yields, greater uniformity of plant height, fruit size, etc.
The other crosses are used as a way of reducing seed cost by using a more
productive non-inbred female in seed field.
Breeding and genetics have dramatically influenced agricultural
production. World wide, crop productivity has increased substantially. Countries
which once imported basic foodstuffs have become exporters. Plant breeding
has enabled farmers to expand cultivating regions, meaning that basic crops are
no longer as limited in terms of where they can be cultivated. The development
of hybrid varieties has contributed significantly to higher yields. Resistance to
disease and insects has helped reduce economically devastating crop damage
and decrease the reliance on pesticides. The breeder today is involved in
improving the quality of goods demanded by consumers such as more durable
cotton and blemish-free apples. Finally, many crops have been adapted to
mechanical harvesting which increases overall productivity of agricultural
production. These are only a few ways that breeding and genetics have
influenced agriculture.
Laboratory Exercise
Objective
This laboratory exercise is designed to provide the student with practical
experience in the breeding method known as hybridization. The student will
learn how to emasculate the female plant and cross pollinate it with pollen from
the male plant. The parent plants which students will use will be provided by the
instructor from a population which he has developed. The plants used in this
exercise are alfalfa. They were chosen because they grow and flower quickly.
Materials








Tweezers
Scissors
Vacuum or alcohol
Cotton swabs (alcohol)
Paper “boats”
Tags
Pencil
Two plants
Methods
Step 1: Obtain 2 plants from the instructor. Designate one plant as a female
and the other male.
Step 2: On the female plant choose a raceme composed of 8-10 flowers. Trim
the raceme by removing 4-5 flowers at their bases (see picture #1).
Step 3: The outer petals of each flower should be removed using the scissors.
The inner petals should be gently trimmed so as to allow the sexual
column containing the ovary and staminal column to be exposed.
WARNING—Failure to be gently in trimming the inner petals could result
in cutting off the staminal column.
Step 4: You are now ready to begin the process of emasculating the flowers
which is done be remove the pollen. Take the tweezers and gently
squeeze the base of the flower to trip or release the sexual column. You
should be able to see the pollen as tiny yellow grains on the top of the
column.
Step 5: Gently use the vacuum to remove the pollen from the sexual column.
DO NOT allow the vacuum to directly tough the column, it may kill the
flower. If a vacuum is unavailable alcohol swabbed on the pollen will kill
it. Emasculation is complete.
Step 6: Choose a raceme from the male plant and remove it at the base of the
stem. Gently remove or trim the outer and inner petals being careful not
to trip or cut-off the sexual column.
Step 7: Gently use the tweezers to trip the sexual column to expose the pollen.
Using a paper “boat” collect the pollen by using the pointed end of the
“boat” to pick-out the pollen.
Step 8: Take the “boat” filled with pollen and carefully deposit the pollen grains
onto the flowers which were emasculated on the female plant (See
picture #2).
Step 9: Tag the newly pollinated flower with a tag upon which you have noted
the female parent, male parent, and the date of the cross. (See picture
#3). Attach to the stem of the raceme.
Step 10: Within a few days you should be able to see if the cross was
successful. If it was you will notice the beginning of the seed pod
formation.
Figure 36. Equipment commonly used in emasculating and crossing small
grains.
Name:_______________________________________Date:________________
Plant Breeding Worksheet and Questions
Part I Worksheet
Female
Male
Date
Florets
Pods
# Of
Crosses
Time
Comments
Part II Questions
Answer the following questions.
1. Why is plant breeding considered an art and a science?
2. Define plant breeding and genetics.
3. List three benefits that genetics and breeding have brought to agriculture.
4. Define emasculation. Describe the method of emasculation you used in
this exercise.
5. Describe your observations of your female flowers one, two, and three
weeks after the hybridization exercise.