OrganiC Farmer'S GrouP AZ

What is Plant Breeding – Definition?

Plant breeding is an art and science which deals with changing genotypes of the plants, to adapt them to the service of human being.

(genotypes  Assembly of genes possessed by an individual (organism) is called as its genotype.)

Objectives or Aims of Plant Breeding

Plant breeding deals with the improvement of various characters. While implying breeding methods breeder always considers multiple characters, even in conditions where any single character is dominant. Characters are considered depending on various factors like crop, growing area etc.

•              High yields

•              Improving quality characters

•              Disease resistance

•              Insect or pest resistance

•              Broadening growing area

•              Modifying agronomic characteristics [dwarfing (height), determinate growth (branching), Tillering etc.]

•              Varieties suitable for new season

•              Changing maturity duration

•              Non-shattering characteristics

•              Uniformity in product

•              Synchronous Maturity

•              Dormancy

•              Salt Tolerance

•              Cold tolerance

•              Drought or moisture stress tolerance

•              Modification to make better use of inputs

•              Elimination of unwanted ones, like toxic substances, prickles or irritant hairs

Pollination

What is Pollination? – Definition

Pollination is the transfer of pollens from anthers (stamens) to stigma of the flower.

Pollination can be of following types

•              Self-pollination – Read page to get details

•              Cross-pollination – Read page to get details

•              Often cross pollination – Read page to get details

Self-pollination

What is Self-pollination? – Definition

Self pollination is the phenomenon in which pollen grains are transferred from anthers to stigma of the same flower or another flower of same plant (geitonogamy).

The another terms used for self-pollination are autogamy or self pollenizing.The basic requirement for this type of pollination is a flower should has both stamen and stigma, and should be able to contact each other for actual self-pollination. There are various mechanisms to promote self pollination. Generally these mechanisms are more efficient than those, which promote cross pollination.

Mechanisms facilitating self pollination

•              Cleistogamy

•              Chasmogamy

•              Hidden Stamen and Stigma

•              Homogamy

•              Bisexuality

•              Anther position

Cleistogamy

In this case flowers never open. This prevents foreign pollens to reach the stigma of flower with cleistogamy mechanism. Hence, facilitates self-pollination.

Examples – Some varieties of wheat, barley and oat, some grasses etc. have cleistogamy.

Chasmogamy

In Chasmogamy, flowers remain close still self-pollination occurs, once it over, they open. No doubt, this opening could allow some cross-pollination too.

Examples – rice, wheat, barley, oat etc.

Hidden Stamen and Stigma

In this mechanism, some kind of the floral organs ( as keel in case of legumes) do the job to hide or cover the reproductive organs (stamens and stigmas), to avoid cross-pollination.

Examples – legumes like pea, gram, black gram, soybean etc.

Anther position

Here, stigmas remain densely and closely surrounded by anthers. This much close position of anthers promotes self pollination, though cross pollination may occur too as flowers are open.

Examples – tomato, brinjal etc.

Homogamy

In this kind of mechanism, anthers and stigma of a flower mature at the same time. It is very important for occurrence of self pollination.

In some species, stigmas become receptive and elongate through staminal columns.

Bisexuality

It means presence of male and female reproductive organs in the same flower. A hermaphrodite flower is a characteristic feature of self pollinated plants.

Examples of Self Pollinated Crops

Group   Member

Legumes              Pea, gram, groundnut, cowpea, soybean, black gram, green gram, lentil, khesari, rajma, sunhemp, guar

Cereals and millets         Wheat, rice, barley, oat, ragi

Vegetables         Potato, tomato, brinjal, chillies, okra, lettuce

Forage crops      Wheat grass, burr clover, subterranean clover, velvet bean

Oilseeds              Sesamum, linseed

Fruit trees           Apricot, peach, citrus

Fiber crops         Jute (often cross pollinated)  

Genetic consequences of Self Pollination

Self pollination causes rapid increase in homozygosity, there is no incorporation of variation. Inbreeding depression is not observed in case of self pollinated species, but they may exhibit considerable heterosis.

Inbreeding – Inbreeding Depression

What is Inbreeding and Inbred? – Definition

Inbreeding is mating or crossing between two closely related i.e. genetically similar individuals.

Inbred line or Inbred – is the almost homozygous line obtained by continuous inbreeding.

(homozygous Individuals with like alleles at corresponding chromosomal loci i.e. for a single trait are called as homozygous.)

What is Inbreeding Depression? – Definition

Reduction in vigour or fertility because of inbreeding is called as inbreeding depression.

It was first noted by Darwin (1876). The main reason behind inbreeding depression is segregation. In F2 segregation occurs and results in the break down of heterotic gene combinations formed in F1.

Consequences / effects of Inbreeding

•              Yield reduction

•              Vigour reduction

•              Sub-lethal, lethal alleles may appear

•              Fertility reduction

•              Homozygosity increases, heterozygosity decreases because of gene fixation.

•              Fitness reduction

•              Continuous inbreeding leads to increase of total genotypic variance between the families and decrease of it within the families. It breaks the entire population into small, phenotypically distinct, non-interbreeding groups. This produces inbreds.

Heterosis

What is Heterosis? – Definition?

Heterosis is superiority of F1 (offspring from cross ) in one or more characters over its better parental or mid parental value.

Another term used for heterosis is hybrid vigour. Shull (1914) used this term for the first time.

Desirable heterosis can be

•              Positive – like in case of yield, quality, disease resistance

•              Negative – like in case of plant height, maturity duration

General Features of Heterosis

•              Wide occurrence in both plants and animals

•              In plants, more frequent in cross pollinated species as compared to self pollinated.

•              Superiority over parents with respect to many traits like yield, disease resistance etc.

•              Confined to F1 generation only, due to segregation and recombination declines and disappears in subsequent generations.

•              Mostly governed by nuclear genes, in some case by interaction between nuclear genes and cytoplasm.

•              Frequency of occurrence of desirable heterosis is very low.

•              Easily reproducible once identified

•              Positive association with specific combining ability.

•              Magnitude of heterosis is directly proportional with heterozygosity.

•              Can be exploited fully as in case of hybrids, or partially as in synthetic and composite varieties.

Types of heterosis

Types of heterosis based on origin and nature and type of estimation are given on the page. Visit to get details.

Causes or Basis of Heterosis

Various Causes or Basis for Heterosis like genetic, physiological are given, click to read these.

Factors affecting Heterosis

Some of the factors affecting heterosis are listed below

•              Parent genotype

•              Type of pollination followed

•              Genetic diversity present among parents

•              Parent’s adaptability

Causes – Basis of Heterosis

Various basis or causes of heterosis can be listed as follows

•              Genetic basis

o             Dominance hypothesis

o             Overdominance hypothesis

o             Epistasis

•              Physiological basis

•              Cytoplasmic basis

•              Biochemical basis

Dominance hypothesis

The dominance hypothesis was proposed by Charles Davenport (1908). Most widely accepted hypothesis among the other explanations for heterosis.

As per this hypothesis, heterosis results due to superiority of dominant alleles over the deleterious recessive alleles by masking their effect and heterosis is in direct proportion with number of dominant genes contributed by each the parent.

Overdominance hypothesis

The overdominance hypothesis was independently developed by Edward M. East (1908) and George Shull (1908).

As per this hypothesis, heterosis results due to superiority of heterozygote over its both homozygous parents, due to complementation between divergent alleles. Here, heterosis is in proportion with heterozygosis.

Types of heterosis

Types of heterosis on the basis of two different criteria are given below

On the basis of origin and nature

•              Euheterosis or true heterosis

o             Mutational heterosis

o             Balanced heterosis

•              Pseudoheterosis or luxuriance

On the basis of types of estimation

•              Average or Relative heterosis

•              Heterobeltiosis

•              Useful or standard or Economic heterosis

Mutational Heterosis

It is simplest type of heterosis.

Lethal (mostly), recessive, adaptively unfavorable mutants are either eliminated or sheltered by their non-lethal, dominant and adaptively superior alleles in cross pollinated crops. This is termed as mutational heterosis.

Naturally occurring mutants are generally of recessive and less adaptive to environmental conditions, hence risk of their elimination by natural selection process is higher. Its mutational heterosis which gives them to chance to be in population by sheltering them.

Balanced Heterosis

Well balanced gene combinations which are more adaptive to environmental conditions and useful from the agriculture point of view result in balanced heterosis.

It has application in hybrid production.

Pseudoheterosis

Also termed as luxuriance. Progeny possess superiority over parents is in vegetative growth, but not in yield and adaptation, usually sterile or poorly fertile. This concept cannot be utilized in hybrid varieties production.

Average or Relative Heterosis

When heterosis is estimated over mid parental value i.e. average of two parents it is referred as average or relative heterosis.

Formula

Average heterosis = [(F1 - MP) / MP] x 100

Where, F1 = value of F1, MP = mean value of two parents

Heterobeltiosis

When heterosis is estimated over better parent it is called as heterobeltiosis.

Formula

Heterobeltiosis = [F1 – BP] / BP x 100

Where, F1 = value of F1, BP = value of better parent

Standard Heterosis

When heterosis is estimated over standard commercial hybrid it is called as standard heterosis. It has practical importance in plant breeding. It is also referred as useful or economic heterosis.

Formula

Standard Heterosis = [(F1 - SH) / SH] x 100.