Section 1: Plant Breeding and Propagation

Glossary

Lesson 2: Conventional Breeding Methods

Within the cells of any living organism, there are special molecular structures called chromosomes. These chromosomes contain genes that control all the characteristics and qualities of a plant.

Conventional methods of plant breeding consist of combining chromosomes containing the genes of one plant with those of another plant. The idea is to inherit the best traits of both plants, thereby obtaining an offspring plant with superior qualities.

There are three main procedures that have conventionally been used to accomplish the combining of plant chromosomes from different plants. These are selection, hybridization, and induced mutation.

A microscopic image of chromosomes.

Selection

Selection is the most ancient and basic procedure in plant breeding. A large number of plants from the same plant line are selected for their desirable traits. The offspring from these selected plants are examined, and again the best ones are chosen to produce further offspring. This procedure can be repeated any number of times. Since these successive selections involve plants from the same plant line, this is a form of plant inbreeding.

It is believed that this procedure actually occurs in nature, and is referred to as natural selection. It is thought that the plants with the best and strongest qualities are more likely to produce offspring, which results in greater propagation of the stronger plants. Plant breeders that employ this method of selection are merely speeding up this natural process of selection.

Hybridization

Hybridization is the most frequently employed plant breeding technique. The aim of hybridization is to bring together desirable traits, found in different plant lines, into one plant line, by means of cross-pollination. The procedure generally involves three steps:

  • The first step in this technique is to produce plants with a desired trait within one plant line. This can be done by a process of repeated selection, or by self-pollinating plants where pollen from male flowers pollinates female flowers from the same plants. This results in a pure inbred line.

  • Once this pure inbred line is generated, it is outcrossed (i.e., it is combined with another pure inbred line). From the resulting offspring, the plants with the desired traits from the two parents are selected.

  • From the plants with the desired traits, any undesirable traits must be eliminated. This is done by backcrossing (i.e., repeated crossing with the parent plants) until the repeatedly selected plants no longer contain the undesirable traits.

Cobs of sweet Taber (Alberta) corn! What is known in many English-speaking countries as corn, is really a type of maize. Maize is the most widely grown grain crop in the Americas. Early breeding efforts were based on mass selection, but later efforts created hybrids from selected inbred lines. The latest are double-cross hybrids using four inbred lines.

The inbreeding of most plants results in a significant reduction of plant vigour or size. However, when such highly inbred lines of plants are crossed with other such inbred lines, very vigorous, large-sized, and large-fruited plants may result. This is known as hybrid vigour, and the scientific name for this phenomenon is "heterosis".

Induced Mutation

Mutations are changes in the genetic make-up of organisms that occur naturally. They are sudden and spontaneous and can be caused by radiation, viruses, and various chemicals. Some mutations are called hereditary mutations because the new genetic make-up can be passed on from parent to offspring.

Plant breeders can make use of this phenomenon by inducing mutations in a plant. They use chemicals or radiation to achieve this. The resultant mutant plants are then tested and further selected for desired traits, usually through repeated generations of offspring.

However, the great majority of mutant plants carry undesirable traits. For that reason this method has not been widely used in breeding programs.

A double red pepper is a mutation!

The "Green Revolution", occurring from the 1940's to the 1970's, consisted of initiatives that increased agricultural production around the world, much of it due to plant breeding that produced high-yielding varieties of cereal grains.

Norman Borlaug, the "Father of the Green Revolution," is credited with saving over a billion people from starvation!