Plant Breeding Methods: In Brief for Students

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Published on International Journal of Agriculture & Agribusiness
Publication Date: May 13, 2019

Itefa Degefa
MSc. In Genetics
Dambi Dollo University, Faculty of Natural and Computational Sciences
Department of Biotechnology
Dambi Dollo, Ethiopia

Journal Full Text PDF: Plant Breeding Methods: In Brief for Students.

Abstract
Science of breeding is playing great role from the earlier to current in field of agriculture to keep up the accessibility of food in the world. It is an art and a science maybe which ought to also be added a business. Modern plant breeding is a discipline that is firmly rooted in the science of genetics. As an applied science, breeders are offered opportunities to apply principles and technologies from several scientific disciplines to manipulate plants for specific purposes. This short note written on the title of “Plant Breeding methods: In Brief for students” is most importantly used for under graduating students taking the course plant breeding and others those interested to know the since of plant breeding since there is brief and precise idea about the concept of plant breeding. The material contains twelve chapters which are arranged fluently as the flow of the theories and advancement of plant breeding. Basically the material focused on the objectives and methods required to make improved crops. In each chapter there is unit review and review questions for the understanding of the readers. At the end, there are somehow discussed modern techniques used in the plant breeding programmes.

Keywords: Plants, Breeding, Methods, Genetic.

1. CHAPTER I
1.1 Introduction
Many people define plant breeding in various ways, and some of them are given below. According to Nikolai I. Vavilov, plant breeding is plant evolution directed by the will of human. N.W. Simmonds defined it as an applied evolution, working towards defined objectives by tolerably well-understood methods. I.M. Poehlman designated it as the art and the science of improving the heredity of plants for the benefits of humankind.
Hence, plant breeding is a science based on principles and practices of genetics and cytogenetic i.e., it is applied plant genetics. It focuses on improving the genetic makeup of the plants to develop improved varieties. In other words, it is manipulation of plant genetics to produce more desirable products. Plant breeding is selection made possible by the existence of variability. In short, plant breeding is the manipulation of a biological system that requires many generations to achieve results which is a dynamic, exciting and challenging profession operating under continually changing conditions.

1.2. Evolution of plant breeding
Plant breeding has begun when humans first chose certain plants for cultivation. This is known as domestication, the process of bringing a wild species under human management. There was some selection during domestication to give rise to better types than the wild ones. Hence, domestication that has been in practice since ancient times can be regarded as a method and the beginning of plant breeding. Some examples of these are food crops, timber trees, medicinal plants and microbes that used to meet special requirements. During these practices both natural and artificial selections have definitely been acted upon the domesticated plant species. Some of the major historical events in plant breeding are summarized in (Table.1).

1.3. Roles of scientific methods in plant breeding
Scientific methods are the solid bases for plant breeding as modern plant breeding which is based on the scientific principles of genetic and cytogenetic since the days of Gregor Mendel. Science by itself is the knowledge gathered through scientific methods. The scientific methods are consisted of observation, formulation of hypothesis, experimentation and drawing conclusion either to accept or to reject the hypothesis. In early days, plant breeding was largely an art; people depended on their skill in selecting better plants. Their knowledge about plants was very limited. They hardly knew about inheritance of characters, role of environment in producing them and the basis of variation in various plant characters. However, the current plant breeding methods are entirely based on scientific principles of plant sciences. Thus plant breeding today is largely a science with little involvement of art. Consequently, a modern plant breeder should have a through knowledge of the scientific methods and other related disciplines. In short, breeders must know fundamentals of scientific principles, plant reproduction, function and culture, and be able to use proper experimental techniques to insure a reasonable probability of making correct breeding decisions.

1.4. Main achievements
There are great changes and important achievements as a result of plant breeding both on global, regional and local levels. There are significant variability in productivity and quality between the cultivated plants and their wild relatives as a result of plant breeding. Plant breeding has been offering enormous opportunities that have ranged from backyard hobbies to multimillion corporate. One of the outstanding successes was the discovery of dwarf wheat and rice varieties, which were high yielding, resistance to lodging, rusts and other major diseases, responsive to fertilizers, photo-insensitive and thus suitable to many localities. Hence, plant breeding has played a remarkable role in helping to reduce massive hunger and malnutrition by improving food supplies. The Noble Peace Prize awarded to Dr. N.E. Borlaug in 1970 is a good testimony to the importance of plant breeding to human welfare. Dr. Borlaug’s intense efforts in developing improved dwarf wheat varieties have led to the extensive production increases in many calorie-deficient areas of the world. Nevertheless, breeding efforts must be supported with many other sectors besides production and marketing improvements. Moreover, adequate population control is also required, if food needs are to be properly addressed.
Production efficiency is the result of increased output per units of inputs. Improved varieties contain in-built production stability factors provided at a minimum financial expense for both the producer and consumer, and help in insuring a steady food, feed, clothe and shelter supply. Our total environment, including the plant species around us has a direct effect on our psychological well-being and happiness. In plant species, the genetic manipulation of plant characteristics that provide pleasure and enjoyment is an important function of plant breeders (Table 2).

1.5. Plant breeding and its allied disciplines
For successful outcomes, plant breeders should know the following disciplines. Botany: study of plants, about taxonomic classification, anatomy, morphology, reproductive mechanism and cellular structure of plants. Genetics and Cytogenetic: study of cells, chromosomes, genes, mechanisms of heredity and molecular genetics. Plant Physiology: is study of internal function of plants, response of plants to environmental conditions, soil nutrients, moisture, temperature and light. Plant Protection (pathology, entomology and weed science): is a study on health of plants and how to protect them from harmful crop enemies. Plant Biochemistry: Are studies of chemical structures and their functions, (e.g., nutritional values or chemical compositions of carbohydrates, proteins and oils). Soil Science and Agronomy: study of soils, husbandry and management of crops (e.g., soil types, structure, fertility/ fertilizers, planting, seed rates, etc.). Statistics: study of numerical facts, measurements and data. Agricultural economics or engineering: study of costs and benefits or farm machineries, effectiveness, efficiencies, etc. Agro-Meteorology: study of weather conditions, climates, forecasting. Computer Science: knowledge of computer (helpful and efficient machine) in planning, recording, analysing and interpreting of data.

1.6 Unit Summary
Plant breeding is a science based on principles and practices of genetics and cytogenetic i.e., it is applied plant genetics which focuses on improving the genetic makeup of the plants to develop improved varieties. Plant breeding has begun when humans first chose certain plants for cultivation. The scientific methods are consisted of observation, formulation of hypothesis, experimentation and drawing conclusion either to accept or to reject the hypothesis which is very important in the science of breeding. There are significant variability in productivity and quality between the cultivated plants and their wild relatives as a result of plant breeding as a result plant breeding has played a remarkable role in helping to reduce massive hunger and malnutrition by improving food supplies.

1.7 Review questions
Write the correct answer for the following questions
1. What are disciplines highly important while studying plant breeding course?
2. What does it mean domestication and how human being started breeding in the earliest time?
3. What do you thing a significant changes from the earliest to recent time due to the advancement of breeding?
4. Why scientific method is important in the breeding techniques?

2. CHAPTER II
2.1 Objectives and Strategies of Plant Breeding
Plant breeding programs have to set clear breeding objectives based on plant species and the growing environment for efficient use of resources. In fact, breeders have to know what to achieve at the end, pure lines, hybrids, synthetics, composites and clones. Generally, breeders aim to improve the characteristics of plants to make them more desirable agronomical and economically. The most common breeding objectives are listed below.

2.2. Improving Yield
Yield improvement is the ultimate goal of all plant breeding programs. It could be expressed in tons/ha of food, feed or number of blooms per plant. Breeders often look for an increased quantity of usable or marketable products. Improved yield can generally be attributed to two major causes. Firstly, every plant contains an inherent physiological production capacity that operates on energy, nutrients, water, and other natural resources required for normal plant performance. All varieties do not have the same inherent physiological production capacity to yield. For example in rice, the introduction of semi-dwarf genes reduced plant height and improved the plant’s capacity more efficiently use the natural resources at its disposal. Generally, breeders regard yield as a very complex array of plant component interactions, and by manipulating these genetic systems yield is improved as the result of plant efficiency improvement. The second major area of yield improvement is the protection against biological and environmental hazards, such as diseases, insects, drought, frost, saline, etc. Yield is a highly complex trait, resulting from many interacting factors and its heritability is low as large number of genes involved on top of high level of environmental interactions.

2.3. Improving Quality
Quality is a component that adds value the crop. Quality may mean the physical characters such as grain size, colour, taste/ flavour, odour and texture or chemical compositions that attributes to these and other nutritional and utilization values. For example, grain size, colour, milling and baking or cooking qualities are important in cereals, pulses, oil crops and vegetables. More specifically, lower protein in malting barley, higher lysine in cereals, more methionine and tryptophan amino acids in legumes, better keeping qualities in edible oils and vegetables, and improved size, colour and flavours in fruits are some of the desirable quality parameters in different breeding programs. As quality parameters determine the suitability and profitability of plants, they remain as important aspects in plant breeding programs.

2.4. Improving Disease or Pest Resistance
Almost all plants are exposed to various biological pests (weeds, insects and diseases) during their life cycles. Resistant varieties offer the cheapest method of pest control. Such resistant varieties not only increase production, but also stabilize it.

2.5. Environmental Stress Tolerance
The common environmental stresses include drought, water logging, frost, acidity, salinity, hail damages, etc. These problems are generally very difficult to deal with by breeding because their complex nature requires a wide array of genetic response mechanisms. The plant response to drought, for example, is not well understood physiologically but is the net result of several systems including the coating on leaf surfaces, the number, size and response of stomata and the ability of the root system to function efficiently during periods of moisture deficiency. Despite the difficulties, plant breeders accepted improved response to stresses as achievable plant breeding goals. For instance, different varieties of barley and linseed were developed for frost-prone highlands of Ethiopia, while that of tee, noug and faba bean were bred for waterlogged black soils (vertisols).

2.6. Improved Adaptation
All plants have adaptive mechanisms allowing them to exist in a complementary manner with environment. The major environmental factors that determine the adaptation of plants include temperature, moisture, photoperiod, and wind and soil conditions. These conditions limit the viability and productivity of many crops. Two approaches can be taken in the improvement of plant adaptation. The first approach is modifying the environment so that the plant will not subject to abnormal stresses. The second approach is to alter physiological mechanisms associated with adaptation. Adaptation tests of different varieties across multi-locations in Ethiopia could be the best example for the latter case. Through such practices, well-adapted cultivars will be identified whether they are widely or narrowly adapted certain localities. The other good example of improved adaptation is the acclimatization of maize to the highland areas of Ethiopia in recent years.

2.7. Suitability to Mechanization
With the increasing production costs, producers are continually seeking ways to produce high quality crops more efficiently. Genetic variability has been demonstrated for a number of cost-saving traits. Mechanical harvesting equipment has been developed or is being considered for several crops. Traits that can be altered by breeding include constant head size, medium height, uniform ripening, resistance to mechanical injury and ease of harvesting. The basic elements of plant breeding strategy are: To identify morphological, physiological and pathological traits in cultivated plants; to search out new genes that encode for desired traits in different species of plants; to combine genes for the desired traits into an improved cultivar; to assess the performance of improved lines in local environment in comparison with the present cultivars; and to release a new variety.

2.8 Unit summary
In plant breeders, the aim is to improve the characteristics of plants to make them more desirable agronomical and economically. Yield improvement: is the ultimate goal of all plant breeding programs. Quality improvement: may mean the physical characters such as grain size, colour, taste/ flavour, odour and texture or chemical compositions that attributes to these and other nutritional and utilization values. Resistant varieties improvement: it offers the cheapest method of pest control. Improving adapted varieties: All plants have adaptive mechanisms allowing them to exist in a complementary manner with environment. Suitable mechanization: With the increasing production costs, producers are continually seeking ways to produce high quality crops more efficiently.

2.9 Review questions
Explain the following terms
1. Resistant variety
2. Quality
3. Environmental stress
4. List another important objectives of breeding if you know
5. How do you think improvement of stress resistant crops?
6. What types of factors are regarded as stresses?

3. CHAPTER III
3.1Variation In Plant Populations
Genetic variation is a genetically and morphological varying of plants which can be detected at molecular and morphological level. Breeders all ways change the phenotype of their plant by changing the genotype (genes) that encode it. Visible variation in morphological traits (e.g., height, colour, size), manifests genetic variation, while compositional or chemical traits (e.g., protein content, sugar content of a plant part) require various tests or devices for evaluating them. Phenotype is the addition of the genotype and the environment (P = G + E). So, the phenotype may be altered by altering genotype, environment or both.
In plant breeding, an increased phenotypic variability is tried to be achieved through recombination of genomes, traditionally in particular via crossing of different phenotypes (i.e. hybridization), more recently through more directed changes at molecular level resulting from advances in molecular biology. After this step of creating genetic variability, it is necessary to select those phenotypes which display the desired traits.
There are three ways in which genetic or heritable variability originated in a plant population: gene recombination, modifications in chromosome number and mutations. Recombination applies only to sexually reproducing species through physical exchange of parts of homologous chromosomes. It represents the primary source of variability for plant breeders in those species which generates variability by assembling new combinations of genes from different parents. Through modifications, new variability may arise naturally in chromosome number as a result of hybridization (between un identical genotypes) or abnormalities in the nuclear division processes (spindle malfunction). Failure of the spindle mechanism, during karyo kinesis, can lead to errors in chromosome numbers transmitted to cells, such as polyploidy (individuals with multiples of the basic set of chromosomes for the species in their cells). In another case, mutation is ultimate source of biological variation important in biological evolution as sources of heritable variation. It cause changes in genes them-selves in order to generate variability which is arise spontaneously in nature as a result of errors in cellular processes such as DNA replication (or duplication) and chromosomal aberrations (deletion, duplication, inversion, translocation).