Plant breeding: Introduction, Breeding methods in self and cross pollinated and vegetatively propagated crops

Introduction of Plant breeding:-

·   Plant reproduction is a natural process while plant breeding is an artificial process.

·   Plant Breeding:- This is an art and science by which genetic improvements are made in plants to make them economically more beneficial.

·   Genetic Improvements:-

i. Vigorous in Growth

ii. Lodging Resistant

iii. Insect Resistant

iv. Disease Resistant

v. Cold Resistant

vi. Drought Resistant

Nature of Plant Breeding:-

·  Initially the nature of plant breeding was artistic.

·  As with time the knowledge of plant breeders grew up, the use of science get increased and the use of art get decreased in plant breeding.

·  Thus at present the nature of plant breeding is artistic as well as scientific.

Objectives of Plant Breeding:- The main objective of plant breeding is to make stable genetic changes in plants to make them economically more beneficial.

·  More Yield:- More income

·  More Quality:- More income

·   Resistant Varieties:- 

i. Insect Resistance

ii. Disease Resistant

iii. Cold Resistance

iv. Drought Resistance

·   More Efficient Varieties:- So that plants can utilize more manure, fertilizers and irrigation.

·   Early Maturation:- Plants get protected from late season insects and diseases.

·   Synchronous Growth:- The problem of shattering get overcomed so that crop yield get increased.

Role of Plant Breeding:-

1. Nobilization of Indian Sugarcane:- Indian sugarcane is genetically improved by crossing Saccharum barberi (Indian sugarcane) with Saccharum officinarum (tropical sugarcane) at Sugarcane Breeding Center, Coimbatore. Indian sugarcane have very hard stem, less yield and less juice content while tropical sugarcane have very soft stem, more yield, more juice content, very less fiber content, thick stem and soft peel. But tropical sugarcane has very less yield in Indian climate. Hence superior traits of tropical sugarcane are transferred into Indian sugarcane by hybridization. This process is called as nobilization of sugarcane.

2. Development of semi-dwarf varieties:-

i. In Rice:- Dee – geo – woo – gen, Taching Native 1, IR 8

Following superior traits are found in above 3 varieties:-

·  Lodging Resistant

·  High Yielding

·  Fertilizer Responsive

·  Photo insensitive

ii. In Wheat:- Kalyan Sona,  Sonalika. 

Following superior traits are found in above 2 varieties:-

·  Lodging Resistant

·  High Yielding

·  Fertilizer Responsive

·  Rust Resistant

·   Photo insensitive

3. Development of hybrid varieties in Cotton:- H4 hybrid variety of cotton is developed by C. T. Patel at Surat center of Gujarat Agriculture University. Some other hybrid varieties of cotton are - Godavari, Savitri, Varalakshmi, Jayalakshmi.

4. Development of hybrid varieties in Sorghum, Bajra and Maize:-

i. In Sorghum:- CSH 1 से CSH 11 (CSH 1 to CSH 11)

ii. In Bajra:- PHB 10, PHB 14, BJ 104, BK 506

iii. In maize:- Manjri, Vikram, Sona, Vijay, Kisan

Breeding methods in self pollinated crops:-

•   Genetic Basis:- Due to self-pollination, heterozygosity in each generation is reduced to half of the previous generation and homozygosity increases in the same proportion.

•    Homozygosity:- The percent frequency of genes present in the homozygous state of any plant population is called the homozygosity of that population.

• The following results are obtained by self-pollination the Aa genotype plants -

• Homozygosity is inversely proportional to crossing over.

• Homozygosity is directly proportional to linkage.

• 2 main breeding methods are -

1. Mass Selection:- When superior and uniform plants are selected from a diverse plant population, and their seeds are mixed together in a group, then it is called mass selection.

2. Pure line Selection:- When superior plants are selected from a diverse plant population and their seeds are collected separately to grow individual plant progenies, it is called pure line selection.

Breeding methods in cross pollinated crops:-

Genetic Basis:- 

•    Due to cross pollination, heterozygosity increases generation by generation and homozygosity decreases.

•     Heterozygosity:- The percent frequency of genes present in the heterozygous condition in a plant population is called heterozygosity of that population.

• Heterogeneity is directly proportional to crossing over.

• Heterogeneity is inversely proportional to linkage.

Breeding Methods:-

1. Mass Selection

2. Ear to Row Method

3. Recurrent Selection

1. Mass Selection:- When superior and uniform plants are selected from a diverse population, and their seeds are mixed together as a mass, this method is called mass selection. It is the oldest method of improvement in cross pollinated crops.

The seeds produced by open pollination in plants with good and desirable traits, are collected and mixed from which the next generation is grown.

2. Ear to Row Method:-

•    This method was developed by Hopkins in 1908.

•    This method is much used in maize.

•    Definition:- When superior plants are selected from a diverse population and their seeds are collected separately to grow individual plant progenies, it is called the Ear to Row method.

Many superior plants are selected from a plant population. Open pollination is allowed in these plants. Collect the seeds of each plant separately. A row of 10–50 plants are grown from the seeds of each plant. Select superior rows. Now select 5-10 superior plants from each superior row. Collect the seeds of each plant separately and repeat the method by growing them again in rows. As shown in the diagram below.

3. Recurrent Selection:- When hybridization is done in all possible combinations and repetitive yield trials are performed, it is called recurrent selection. This method is the modification of Ear to Row method.

Source Populations:- The populations from which the inbreds are obtained are called source populations. Generally, the probability of obtaining a superior inbred from a population depends on the frequency of superior genotypes present in that population.

4 types:-

a. Simple Recurrent Selection

b. Recurrent Selection for GCA

c. Recurrent Selection for SCA

d. Reciprocal Recurrent Selection

a. Simple Recurrent Selection:- When individual plant progenies are grown from seeds obtained by self-pollination of all the selected plants without testing, it is called simple recurrent selection. The process given in the diagram below is called a selection cycle. This type of 2 or more selection cycles are completed.

b. Recurrent Selection for GCA:- When GCA test is performed for selected plants and individual plant progenies are grown from seeds obtained from only high GCA plants, it is called recurrent selection for GCA.

Combining Ability:- The ability to produce suoerior hybrids as a result of hybridization is called combining ability. It is of 2 types -

i. General Combining Ability or GCA:- The ability to produce superior hybrids when a plant is hybridized with an open-pollinated variety, is called GCA.

ii. Specific Combining Ability or SCA:- The ability of a plant to produce superior hybrids when it is hybridized with a superior inbred, is called SCA.

In this method individual plant progenies are grown from seeds obtained from self-pollinated of plants that exhibit high GCA. As shown in the diagram below.

c. Recurrent Selection for SCA:- When SCA testing of selected plants is performed and individual plant progenies are grown from seeds obtained from self-pollination of only high SCA plants, it is called recurrent selection for SCA. It was proposed by Hull in 1945. In this selection, the yield of the population increases on average 5.5% per cycle.

d. Reciprocal Recurrent Selection:- When recurrent selection is done simultaneous in 2 plant populations A and B and improve both the populations simultaneously, it is called reciprocal recurrent selection. To improve population A, use population B as a tester and to improve population B, use population A as a tester. This method was suggested by 3 scientists in 1949 -

i. Comstock

ii. Robinson

iii. Harvey

Breeding methods in vegetatively propagated crops:-

Asexual Propagation:-

•    The crops in which the crop is developed by growing vegetative parts are called asexually propagated crops.

•    In many Field and horticultural crops, vegetative propagation is done like in sugarcane, potato, sweet potato, Arabi, ginger, turmeric, banana, almost all fruit trees.

•    Each variety of these crops are asexual progenies of the same plant. All these plants have the same genotype. Hence, they are also called clones. Such crops are called clonal crops.

•    In clonal crops, flowers and seeds are reduced like sugarcane, potato, sweet potato etc. Or flowers and seeds are not formed like banana which is a triploid plant.

•    Clonal crops have a specific character that a new variety can be developed by vegetative propagation from any homozygous or heterozygous plant.

•    The genotype of progeny plants in clonal crops is 100% similar to the parent plants. That is, progeny plants are clones of their parent plant.

•    The phenotype of a clone plant is determined by the effect of the environment on its genotype. This can be demonstrated by the following formula -

•    Clones are generally more heterozygous and exhibit high inbreeding depression.

•    Genetic diversity in clonal crops can arise due to 3 reasons: -

i. Mutation

ii. Mechanical mixture

iii. Sexual Reproduction

•    Clonal Degeneration:- The vigour and yield of clones of most crops decreases over time. This is called clonal degeneration. There can be three reasons for this: -

 i. Mutation

ii. Viral diseases

iii. Bacterial diseases

•    The breeding methods of clonal crops have 3 main stages -

i. To develop genetic diversity

ii. Selection of superior plants

iii. Asexual propagation

•    There are 2 breeding methods depending on the source of genetic diversity: -

i. Clonal selection     

ii. Hybridization