Every average farmer is concerned with how to maintain food supply despite any sharp increase or decrease in population, constant reduction in agricultural land, and unfavourable environmental conditions that may emerge intermittently. It is with this concern that plant breeding was introduced.
Plant breeding is the art and science of altering the genetic pattern of a plant’s genome to increase its value. It is a branch of agriculture specifically concerned with manipulating plant heredity to produce new and improved plant types for use by people. Plant breeding has been practiced for many centuries, since near the beginning of human civilization, and has been believed that breeding new crops is important for establishing and maintaining food security by developing new crop varieties that are better in traits like high-yielding, resistant to pests and diseases, drought-resistant or regional adaptation to different environments and growing conditions. So, numerous methods are employed to develop new varieties, but their primary techniques of development are artificial selection, hybridization, and the use of induced mutations. Others include tissue culture and genetic engineering.
METHODS OF PLANT BREEDING
- Artificial selection: this is the most ancient and basic procedure in plant breeding. It could be stated to have existed as early as cultivation, being that the choice of plant to cultivate is plant breeding. Even the primitive farmers in the village use this skill in checking which crop to cultivate, this is the reason a particular genotype known to be cultivated by a particular village can still be observed planted by the same people across decades. In practicing selection, plant breeders select plants that possess desirable features to be taken to the next generation for propagation and discard plants that are inferior for those features of interest. Following this procedure, plant breeders can select many times for the trait across successive generations, shifting the population in the desired direction.
- Hybridization: here, different plant strains are crossed to join in one progeny with the desirable traits of both parents that were crossed. Hybridization aims to bring together desired traits found in different plant lines via cross-pollination. Undesirable traits also enter the combination. However, hybridization is usually followed by several generations of selection. This allows breeders to discard plants without desired features, choosing for further propagation only those plants with the desired combination of traits. The first step in hybridization is to generate homozygous inbred lines by employing self-pollinated plants. The inbred line is crossed with another inbred line and the resulting progeny with the desired trait is selected. If the progeny has a lot of undesired traits, those traits are removed by backcrossing which is crossing repeatedly with the crop’s parent. When desirable characteristics are fully developed in a hybrid plant, it is often more vigorous than either parent.
- Induced mutation: Mutation induction has become a proven way of creating variation within a crop variety. It has become an alternative in the introduction of variability through mutations induced by mutagens, instead of relying solely on wild species gene pool and other cultivars. It brings to chance inducing desired traits that either cannot be found in nature or have been lost during evolution. The mutants are examined and then selected further for desired traits. The site of the mutation cannot be controlled when chemicals or radiation are used as agents of mutagenesis. This method has not been generally used in breeding programs. This is because the great majority of mutants carry undesirable traits. Also, there is a concomitant increase in plant injury, especially with increasing mutagen dose.
- Tissue culture: This is a practical means for accelerating success in plant breeding and improving economically important crops. It involves the culture of detached fragments of plant tissue or isolated cells on a nutrient medium under aseptic conditions. Subsequently, these cultures are regenerated into functional plant.
- Genetic engineering: This is the use of recombinant or non-recombinant DNA technology to study, categorize, and manipulate genetic material. Here, genes or specific sequences for traits are used to manipulate specific regions to achieve the desired result. The gene transferred is called transgene and when this is successfully carried out on a plant, the plant is called a transgenic or genetically modified plant. Therefore, genetically modified plants are plants that have been made to carry and express genes from other organisms to ensure superior performance and with the potential for commercial exploitation.
This is a type of breeding whereby parents with a particular trait produce offspring of that particular trait only. In simple form, it is simply explained as a kind of breeding where organisms breed true for their genotype and phenotype. In plants, it is seen when an individual homozygous for a trait is allowed to self-fertilize. For instance, a plant that has a milky flower color and the alleles for the trait are either homozygous recessive or homozygous dominant, if the seeds collected from the plant are planted, it will produce a plant with the same flower color.
Plant breeding true for flower colour.
GOAL OF PLANT BREEDING
As a science, plant breeding is faced with following goals.
1. Providing variety that has generally greater physiological efficiency. Example is seen in sugar beets (this plant is important as it is the source of about two-fifths of the world supply of sugar) whereby less than 7% sugar is produced in the variety available then before it underwent improvement, but after 175 years, 15-18% sugar was gotten from the developed variety.
2. Development of better variety for new agricultural area. This is achieved by adjusting the growth cycle of the variety to suit better the growing season or climatic changes of the new location.
3. Improvement of plants in agronomic or horticultural characteristics. The grain sorghum makes a good example because the type originally introduced grew taller than a man’s head and it was a bit difficult to harvest because it had to be harvested by hand, the breeding of dwarf variety which grows to a height of 3 to 4 feet and has made combine harvesting a practical undertaking.
4. Development of crop variety that is resistant to diseases and insects. Example is seen in stem rust of wheat, the most important features of resistant variety are the stabilizing effect they have on production. With resistant variety, farmers will not or will apply small number of pesticides, this will lead to less environmental pollution from agricultural sources and high yield consequently.
5. Improving quality of agricultural products or developing food with higher nutritional value there by reducing illness in the society. Example is seen in breeding of tomatoes with increased vitamin content which reduces night blindness.
6. Providing plants with resistance to drought and other climatic stress.
7. Increment in yield. This is where most plant breeder centre on, plant with high nutritional value but very minute yield still needs improvement on yield trait to draw interest from farmers.