Polyploid breeding

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The source for plant breeding is variations in plants.. Heritable and
desirable variations occur in nature by mutation, polyploidy, recombination and chromosomal aberrations. A diploid plant has two sets of chromosomes but any organism in which the number of sets of chromosome is doubled is called a polyploid.

When chromosome number is doubled by itself in the same plant, it
is called autopolyploidy. For example, three sets of chromosomes i.e. a triploid condition in sugar beats, apples and pear has resulted in the increase in vigour and fruit size, large root size, large leaves, flower, more seeds and sugar content in them. Seedless tomato, apple, watermelon and orange are autopolyploids.

Polyploidy can be induced by the use of colchicine to double the
chromosome number. Allopolyploids are produced by multiplication of chromosome sets that are initially derived from two different species.
eg. Triticum × Secale gives Triticale.

The haploid individual plant will have only one set of chromosome.
Through the technique of anther and ovary culture, haploid plants can be modified to diploid ones by doubling their chromosomes.

Variations that are brought forth through plant tissue culture are called somoclonal variation. eg. disease resistant potato and rust resistant wheat.

Varieties of short duration sugarcanes are produced by polyploid breeding.

Mutation breeding

Radiation induces mutation to develop new variety of crops. Now
with newer and more powerful sources of radiations (UV shortwave, Xray, Alpha, Beta, Gamma waves) and many chemicals (mutagens) eg. Caesium, ethyl methane sulfonate, nitromethyl urea), we can increase the rates of mutation eg. Triple gene dwarf wheat with increase in yield and height. Atomita 2-rice with saline tolerance and pest resistance, groundnuts with thick shells are products of breeding methods through induced mutation.

Breeding for disease resistance

Many crop plants suffer from several diseases caused by pathogens
such as bacteria, fungi, viruses, nematodes, protozoa and mycoplasma. In vegetatively propagated plants like potato, cassava, sugarcane and dahlia, viral pathogens are transmitted through their roots, tubers, bulbs and rhizomes.

Disease free plants are obtained by shoot apical meristem culture technique. Plants raised through tissue culture are free from pathogens, which are widely cultivated.

Whenever, a trait that shows disease resistance in a plant is observed, the best way to transfer that trait to other useful crop is by the method of backcross. Repeated back crosses are attempted with the parent crop with more desirable characters and such a crop is known as recurrent parent.

For example, A is a non-recurrent parent and B* is a recurrent parent with desirable trait.

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Other links 

Plant physiology – photosynthesis and its significance

---

BIOLOGY IN HUMAN WELFARE Introduction & Food production

---

Aims of plant breeding

---

Aspects of plant breeding and Types

---

Hybridization in plant breeding

---

 

Genetic engineering, Improved varieties, Role of biofertilizers

---

Green manuring, Mycorrhiza as biofertilizer

---

Benefits from biofertilizers

---

Crop diseases and their control, Rice – Oryza sativa

---

Groundnut or peanut – Arachis hypogea

---

Citrus canker, Tungro disease of rice

---

Biocontrol of insect pests Bacterial pesticides

---

Genetically modified food

---

Bio war, Genetically Modified Organisms (GMO) in biological warfare

---

Biopiracy, Bioresources, Biomolecules, Biopatent, Biotechnology

---

Sustainable agriculture

---

Medicinal plants including microbes

---

Commonly Available Medicinal Plants

---

Microbes in medicine

---

Economic importance of Food plant Rice

---

Oil plant Groundnut Economic importance

---

Fibre plant – Cotton Economic importance

---

Timber yielding plant Teak Economic importance

---

Hybridization in plant breeding

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India has several varieties of crops such as maize, tobacco, tomato,
potato and brinjal which were introduced from countries such as America, China and Australia. Introduced varieties sometimes do not get adjusted easily with our local environment.

It takes some time for these introduced crops to settle. Sometimes, it is essential to select suitable and desirable variety from the introduced plants. For example, a mung Phaseolus mungo variety was introduced from China but was not giving good yield and produced dull coloured seeds. From amongst the introduced mung crop, a plant suddenly produced large and bright coloured seeds.

This aspect may be due to sudden mutation. This variant plant was selected and further subjected to inter or intra specific crosses with our native crop. In this way, new varieties were produced and released as newly developed mung variety. Such a mung No.1 variety is now being cultivated in Punjab.

Hybridization

Hybridization is a method in plant breeding to improve the native crops by obtaining diverse genotypes that can be used as a source material for collection of crop with desirable characters and genes obtained from many parts of the world.

It involves crossing of two varieties or species or genera having desirable genes and breeding them together of the desirable traits into one progeny, which is called the hybrid. Hybrids are the products of first generation obtained by crossing genetically unrelated parents. When two individuals of the same species are crossed, it is called inbreeding or selfing or self-pollination.

This results in the increase of homozygosity. Particularly homozygous recessive alleles develop loss of vigor in plants. By careful observation of morphological features, we can remove these deleterious and harmful alleles by selection.

Protoplasmic fusion or somatic hybridization

A hybrid produced from fusion of protoplasts of two different species is called somatic hybridization. Naked protoplasts are obtained through dissolution of their cell walls by the macerating enzymes such as pectinase and cellulase.

Fusion of protoplasts from two different varieties can be enhanced by treatment with the chemical called polyethylene glycol (PEG) in the presence of high voltage electric current on a suitable medium.

By this method somatic hybrid plants with desirable changes can be obtained.
This method in plant breeding is called protoplasmic fusion. This concept
had been studied by you already in the chapter four.

Heterosis

The superiority of the F1 hybrid in performance over its parents is called heterosis or hybrid vigour. Vigour refers to increase in growth, yield, resistance to diseases, pests and drought.

F1 hybrids of maize show 25% increase in yield when compared to their own parent crop. Vegetative propagation is the best suited measure for maintaining hybrid vigour, since the desired characters are not lost and can persist over a period of time.

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Other links 

 

Plant physiology – photosynthesis and its significance

---

BIOLOGY IN HUMAN WELFARE Introduction & Food production

---

Aims of plant breeding

---

Aspects of plant breeding and Types

---

 

Polyploid breeding, Mutation breeding, Breeding for disease resistance

---

Genetic engineering, Improved varieties, Role of biofertilizers

---

Green manuring, Mycorrhiza as biofertilizer

---

Benefits from biofertilizers

---

Crop diseases and their control, Rice – Oryza sativa

---

Groundnut or peanut – Arachis hypogea

---

Citrus canker, Tungro disease of rice

---

Biocontrol of insect pests Bacterial pesticides

---

Genetically modified food

---

Bio war, Genetically Modified Organisms (GMO) in biological warfare

---

Biopiracy, Bioresources, Biomolecules, Biopatent, Biotechnology

---

Sustainable agriculture

---

Medicinal plants including microbes

---

Commonly Available Medicinal Plants

---

Microbes in medicine

---

Economic importance of Food plant Rice

---

Oil plant Groundnut Economic importance

---

Fibre plant – Cotton Economic importance

---

Timber yielding plant Teak Economic importance

---

Aspects of plant breeding

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Present day crop plants are from wild species reared by careful
domestication, cultivation and management. We have several wild varieties
existing in nature just as in the case of salinity tolerant wild rice.

Through gene manipulation, the gene for salinity tolerance could be cloned in a
rice variety. In such of those areas where shortage of fresh water exists,
rice can be cultivated using seawater and can even be grown in extreme
saline soil.

Similarly, we need many more such wild plants showing increased capacity to extremes of climatic conditions and disease resistance for plant breeding programmes. In order to safeguard the biodiversity and certain important valuable crops from going into extinction, scientists are protecting these crops by establishing gene or germplasm banks by preserving their seeds.

Selection

It is one of the oldest procedures in which individual plants or group
of plants are sorted out from mixed population.

Thus eliminating undesirable ones. Selection methods are of two types – mass selection and pure line selection.

Mass selection

In this method, plants are selected based on their desirable morphological characters (phenotype). Their seeds are composite or mixed and the progenies are grown in masses. They are not individually tested.

After repeated selection for about five to six years, selected seeds are multiplied and distributed to farmers. The only disadvantage of mass selection is that it is difficult to distinguish hereditary variation from environmental variation.

Pure line selection

A pure line is a collection of plants obtained as a result of repeated
self-pollination from a single homozygous individual. Hence, a variety
formed by this method shows more homozygosity with respect to all genes.

One disadvantage is that new genotypes are never created by this method.
Genetic variability is essential for adaptations in different environmental
and seasonal conditions.

Clonal selection

Crops like sugarcane, potato, tea, banana and certain species of grasses
are asexually propagated and produce very poor seeds. Based on their
phenotypic appearance, the method of clonal selection is employed to
select improved variety from a mixed population (clones).

Selected plants are multiplied through vegetative propagation to give rise to a clone. The genotype of a clone remains unchanged for a long period of time.

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Other links 

 

Plant physiology – photosynthesis and its significance

---

BIOLOGY IN HUMAN WELFARE Introduction & Food production

---

Aims of plant breeding

---

Hybridization in plant breeding

---

Polyploid breeding, Mutation breeding, Breeding for disease resistance

---

Genetic engineering, Improved varieties, Role of biofertilizers

---

Green manuring, Mycorrhiza as biofertilizer

---

Benefits from biofertilizers

---

Crop diseases and their control, Rice – Oryza sativa

---

Groundnut or peanut – Arachis hypogea

---

Citrus canker, Tungro disease of rice

---

Biocontrol of insect pests Bacterial pesticides

---

Genetically modified food

---

Bio war, Genetically Modified Organisms (GMO) in biological warfare

---

Biopiracy, Bioresources, Biomolecules, Biopatent, Biotechnology

---

Sustainable agriculture

---

Medicinal plants including microbes

---

Commonly Available Medicinal Plants

---

Microbes in medicine

---

Economic importance of Food plant Rice

---

Oil plant Groundnut Economic importance

---

Fibre plant – Cotton Economic importance

---

Timber yielding plant Teak Economic importance

---

Aims of plant breeding

The first and foremost aim in plant breeding is to create useful variation in the crop plant. This can be achieved by the following measures.

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1. Bringing wild food crops to cultivation. (wheat, oats and many cereal crops were once wild plants which had now been domesticated).

2. Obtaining genes from desirable plants or related species (eg. as seeds from various parts of the world).

3. Introduction of plants from nearby regions or even from other
countries for improvement of the crop. (eg. cauliflower, tomato,
potato and soyabeans).

4. By employing certain plant breeding techniques, new varieties are
developed. eg. maize, sorghum, cotton and sunflower.

5. Auto and Allopolyploid breeding.

6. By inducing mutations using physical and chemical mutagens.

7. Production of haploids by the application of plant tissue culture of
anther and ovary.

8. Improvement of nutritional quality by genetic engineering (eg.
Fortified rice – iron rich rice and carotene rich rice).

9. Development of disease, drought and environmental stress resistant
varieties.

Back to botany topic list

Other links 

 

Plant physiology – photosynthesis and its significance

---

BIOLOGY IN HUMAN WELFARE Introduction & Food production

---

Aspects of plant breeding and Types

---

Hybridization in plant breeding

---

Polyploid breeding, Mutation breeding, Breeding for disease resistance

---

Genetic engineering, Improved varieties, Role of biofertilizers

---

Green manuring, Mycorrhiza as biofertilizer

---

Benefits from biofertilizers

---

Crop diseases and their control, Rice – Oryza sativa

---

Groundnut or peanut – Arachis hypogea

---

Citrus canker, Tungro disease of rice

---

Biocontrol of insect pests Bacterial pesticides

---

Genetically modified food

---

Bio war, Genetically Modified Organisms (GMO) in biological warfare

---

Biopiracy, Bioresources, Biomolecules, Biopatent, Biotechnology

---

Sustainable agriculture

---

Medicinal plants including microbes

---

Commonly Available Medicinal Plants

---

Microbes in medicine

---

Economic importance of Food plant Rice

---

Oil plant Groundnut Economic importance

---

Fibre plant – Cotton Economic importance

---

Timber yielding plant Teak Economic importance

---

BIOLOGY IN HUMAN WELFARE

The world’s human population, which was only one billion during
1850, had reached 6.1 billion around year 2000. This dramatic increase in population, otherwise called ‘population explosion’ has created its impact not only on the environment but also on food production.

Half of this 6.1 billion people live in poverty and one fifth of this
population suffer due to malnutrition. Increase in population, unplanned industrialization and migration towards cities and urban areas has resulted in the degradation of the environment. The present agricultural practices have polluted cultivable land physically, chemically and biologically.

The net productivity is gradually being reduced. These factors lead to the shrinkage of the agricultural lands and a fall in agricultural production.

Food production

In order to fight the menace of hunger and malnutrition, we need
crops with greater yield and better nutritive value. The quantity and
quality of crops can be improved by modern scientific methods through genetic manipulation called genetic engineering. However, the time old concept of breeding plants either interspecific or intraspecific to bring out the best hybrid plant in plant breeding programmes still remains in vogue even today. Efforts are being made by ICAR – Indian Council of Agricultural Research and other related organisations in our country to increase food production.

A plant breeder strives to get a group of plants or a variety with
suitable combination of genes that gives better yield and improved quality under a particular set of environmental conditions. A single species is a group of assemblage of innumerable number of genetic types such as lines, strains, etc. The strains are tested in various climatic conditions, successful ones are named, multiplied and distributed as a variety or cultivar eg. Oryza sativa Co.15, ADT. 16.

Breeding experiments

Increase in population has forced us to carry out continuous scientific experiments for the following reasons viz.

1. To develop more food crops.
2. For increase quality in food crops.
3. To have sustainable food quality in food crops and assured food
supply.

By introducing specialized technology, plant breeder are now able
to develop more crops, which they multiply and supply them to the growers. Improvement in the genetic make up of plants is called plant breeding.

Major aspects of plant breeding include

1. creation of useful variation in the cultivable crops.
2. selection of better crops.
3. conducting / carrying out breeding experiments to assess the quality of the crop and
4. release of a variety after their extensive multiplication.

Other links 

Plant physiology – photosynthesis and its significance

---

Aims of plant breeding

---

Aspects of plant breeding and Types

---

Hybridization in plant breeding

---

Polyploid breeding, Mutation breeding, Breeding for disease resistance

---

Genetic engineering, Improved varieties, Role of biofertilizers

---

Green manuring, Mycorrhiza as biofertilizer

---

Benefits from biofertilizers

---

Crop diseases and their control, Rice – Oryza sativa

---

Groundnut or peanut – Arachis hypogea

---

Citrus canker, Tungro disease of rice

---

Biocontrol of insect pests Bacterial pesticides

---

Genetically modified food

---

Bio war, Genetically Modified Organisms (GMO) in biological warfare

---

Biopiracy, Bioresources, Biomolecules, Biopatent, Biotechnology

---

Sustainable agriculture

---

Medicinal plants including microbes

---

Commonly Available Medicinal Plants

---

Microbes in medicine

---

Economic importance of Food plant Rice

---

Oil plant Groundnut Economic importance

---

Fibre plant – Cotton Economic importance

---

Timber yielding plant Teak Economic importance

---

Photoperiodism and vernalization

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The response of a plant to the relative lengths of light and dark periods is known as photoperiodism. In plants, most significant photoperiodic response is the initiation of flowering. It has been first observed in Maryland Mammoth variety of tobacco (Nicotiana tabacum). From the observation of Garner and Allard all the plants do not require the same length of light and dark periods for flowering. Plants require specific period of light and darkness for flowering. It is known as critical period.

Plants are classified into three classes

1. The plants requiring longer exposure to light than their critical period are known as long day plants eg. wheat and oats.
2. The plants requiring light for a shorter period than their critical period are known as short day plants eg. tobacco and Chrysanthemum.
3. The plants in which flowering is unaffected by the photoperiod are
known as day neutral plants eg. sunflower and maize.

Phytochromes and flowering

In 1959, Butler et al. were able to discover a photoreceptor flower
inducing pigment in plants which they name phytochromes. It is believed to be widely present in all green plants. Chemically, phytochrome is a biliprotein and exists in two forms. One form absorbs red with the wave length of 660 nm called Pr and the other form absorbs far red with the wave length of 730 nm called Pfr. The two forms of phytochrome are interconvertible as shown below:

Based on the absorption spectra, Pr is also called P 660 and Pfr is P 730. In short day plants, Pr promotes flowering while Pfr suppresses it , while it is viceversa in long day plants.

Vernalization

The term vernalization was first introduced by a Russian scientist
T.D. Lysenko in 1920. Many species, especially biennials and perennials are induced to flower at low temperature range of 1oC to 10oC. This is known as vernalization.

The response to the cold temperature stimulus is not uniform in all
plants. Plants, which are vernalized, are called inductive types. Those nonvernalized are called noninductive types.

Techniques of vernalization

The following are the steps to be taken to induce vernalization. Seeds are allowed to germinate and subjected to cold treatment for varying period of time depending on the species. Germinated seeds after this treatment are allowed to dry for sometime and then sown.

Devernalization

Reversal of the effect of vernalization is called Devernalization.
Subjecting the plants to higher temperature after a cold treatment brings about devernalization.

Practical application of vernalization

Russian scientists have used vernalization to shorten the time of crop maturity by hastening the flowering processes which are brought about by cold treatment.

Advantages

Crops can be produced earlier by vernalization. They can be cultivated in places where they naturally do not grow. Vernalization helps to accelerate the plant breeding.

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Other links 

Plant tissue culture – origin and techniques

---

Plant physiology – photosynthesis and its significance

---

Site of photosynthesis and Mechanism of photosynthesis

---

Electron transport system and photophosphorylation types

---

Dark reaction

---

C3 and C4 pathways

---

Photorespiration or C2 cycle

---

Factors affecting photosynthesis

---

Test tube and funnel experiment, Ganong’s light screen experiment

---

Mode of nutrition – Autotrophic, Heterotrophic

---

Chemosynthesis

---

Mechanism of Respiration – Glycolysis

---

Mechanism of Respiration – Oxidative decarboxylation , Krebs cycle

---

Mechanism of Respiration – Electron Transport Chain, Energy Yield

---

Ganong’s respiroscope, Pentose phosphate pathway

---

Anaerobic respiration, Respiratory quotient, Compensation point, Kuhne’s fermentation tube experiment

---

Plant growth and Measurement of plant growth

---

Phytohormones Auxins

---

Phytohormones Gibberellins

---

Phytohormones Cytokinin, Ethylene, Abscisic Acid, Growth Inhibitors – Physiological Effects

 

Phytohormones Cytokinin

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Cytokinin is a plant growth substance, which stimulates cell division.
This was isolated by Miller and Skoog in 1954 from Herring fish.
Following the discovery of kinetin many other compounds showing similar activity were discovered. These are collectively called cytokinins. The cytokinin found in the zea mays is called zeatin. Cytokinin is also found in theendosperm of coconut. Cytokinin occurs in various seed plants. They are
found particularly in embryos, developing fruits and roots. Varying
mixtures of auxin and cytokinin influence plant growth and differentiation.

Physiological effects of cytokinin

1. The most important function of cytokinin is the promotion of cell
   division.
2. In association with IAA, cytokinin initiates bud and root formation in callus tissue.
3. External application of cytokinin promotes the growth of lateral buds even if the apical bud is intact.
4. Cytokinin breaks the dormancy of many seeds and also promotes
   germination.
5. Application of cytokinin delays the process of ageing in plants. This is also known as Richmond Lang effect.

Ethylene

Ethylene is a simple gaseous hormone. It is usually present in a
minute quantity. It is synthesised in large amounts by tissues undergoing ageing and acts as a natural plant growth hormone.

Physiological effects of ethylene

Ethylene prevents elongation of stem and root in longitudinal direction. Simultaneously, the tissue enlarges radially resulting in thickening of plant parts.

1. Ethylene promotes positive geotropic growth of roots.
2. A ethylene inhibits the growth of lateral buds in pea seedlings.
3. Ethylene is involved in the ripening of fruits.
4. The ethylene stimulates the formation of abscission zone in leaves, flower sand fruits. This causes leaves, flowers and fruits to shed prematurely.
5. Flowering can be induced by application of ethylene in plants like
   pineapple and mango.
6. Ethylene stimulates rooting of cuttings, initiation of lateral roots and growth of root hair.
7. The ethylene is responsible for breaking the dormancy of buds and seeds.

Abscisic acid

Abscisic acid (ABA) was originally discovered for its role in regulating abscission and bud dormancy. Like other plant hormones, it has multiple functions in the growth of plants.

Physiological effects of abscisic acid

1. Abscisic acid acts as growth inhibitor and induces bud dormancy in a variety of plants.
2. ABA is a powerful growth inhibitor. It causes 50 per cent inhibition
   of growth of oat seedlings.
3. As the name suggests abscisic acid is an hormone that stimulates
   abscission.
4. ABA controls geotropic responses of roots. It stimulates positive
   geotropism in roots.
5. Abscisic acid causes closure of stomata.

Growth inhibitors

Some organic substances produced in the plant inhibit the plant growth.
These substances are called growth inhibitors. They inhibit the elongation in roots, stems and leaves. For example, ethylene is a potent inhibitor of bud growth. ABA inhibits lateral bud growth in tomato.

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Other links 

Plant tissue culture – origin and techniques

---

Plant physiology – photosynthesis and its significance

---

Site of photosynthesis and Mechanism of photosynthesis

---

Electron transport system and photophosphorylation types

---

Dark reaction

---

C3 and C4 pathways

---

Photorespiration or C2 cycle

---

Factors affecting photosynthesis

---

Test tube and funnel experiment, Ganong’s light screen experiment

---

Mode of nutrition – Autotrophic, Heterotrophic

---

Chemosynthesis

---

Mechanism of Respiration – Glycolysis

---

Mechanism of Respiration – Oxidative decarboxylation , Krebs cycle

---

Mechanism of Respiration – Electron Transport Chain, Energy Yield

---

Ganong’s respiroscope, Pentose phosphate pathway

---

Anaerobic respiration, Respiratory quotient, Compensation point, Kuhne’s fermentation tube experiment

---

Plant growth and Measurement of plant growth

---

Phytohormones Auxins

---

Phytohormones Gibberellins

---

Photoperiodism and vernalization, Phytochromes and flowering

 

Phytohormones Gibberellins and Physiological effects of gibberellin

Phytohormones Gibberellins and Physiological effects of gibberellin are illustrated with notes.

Gibberellins

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Gibberellin was first discovered in Japan by Kurusowa.

He observed from his field that some of the rice seedlings had grown much taller than the others.

On further observation, he found that such taller rice plants had shown unusual internodal elongation.

This internodal elongation is known as the ‘bakanae’ or ‘foolish seedling’ disease of rice.

Later, it was discovered that the elongation was due to the action of a substance produced by a fungus, Gibberella fujikuroi.

This substance was successfully isolated from the fungus and it was named as gibberellic acid.

There are over 90 different gibberellins isolated from fungi and from
higher plants.

Gibberellins occur in various plant organs.

They are named as GA1, GA2, GA3, etc. These phytohormones occur in all groups of plants.

Physiological effects of gibberellin

1. Gibberellins produce extraordinary elongation of stem. The elongation of stem is caused by the cell division and cell elongation induced by gibberellic acid.
2. One of the most striking effects of the gibberellins is the reversal of
   dwarfism in many genetically dwarf plants. For e.g. ‘Rosette’ plant
   of sugar beet, when treated with GA undergoes marked longitudinal
   growth of axis attaining the normal size.
3. Rosette plants usually show reduced internodal growth. These plants
   exhibit excessive internodal growth when they are treated with
   gibberellin. This sudden elongation of stem followed by flowering is
   called bolting.
4. Many biennials usually flower during the second year of their growth.
   For flowering to take place, these plants should be exposed to cold
   season. Such plants could be made to flower without exposure to
   cold season in the first year itself, when they are treated with
   gibberellins.
5. Formation of seedless fruits without fertilization can also be induced
   by gibberellin treatment in many plants. eg. Tomatoes, apples,
   cucumbers, etc.,
6. Some of the light sensitive seeds can germinate by the treatment of
   gibberellic acid even in complete darkness. eg. barley,
   m Gibberellin breaks dormancy in potato tubers.

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For more details about Phytohormones Gibberellins and Physiological effects of gibberellin click here

Other links 

Plant tissue culture – origin and techniques

---

Plant physiology – photosynthesis and its significance

---

Site of photosynthesis and Mechanism of photosynthesis

---

Electron transport system and photophosphorylation types

---

Dark reaction

---

C3 and C4 pathways

---

Photorespiration or C2 cycle

---

Factors affecting photosynthesis

---

Test tube and funnel experiment, Ganong’s light screen experiment

---

Mode of nutrition – Autotrophic, Heterotrophic

---

Chemosynthesis

---

Mechanism of Respiration – Glycolysis

---

Mechanism of Respiration – Oxidative decarboxylation , Krebs cycle

---

Mechanism of Respiration – Electron Transport Chain, Energy Yield

---

Ganong’s respiroscope, Pentose phosphate pathway

---

Anaerobic respiration, Respiratory quotient, Compensation point, Kuhne’s fermentation tube experiment

---

Plant growth and Measurement of plant growth

---

Phytohormones Auxins

---

Phytohormones Cytokinin, Ethylene, Abscisic Acid, Growth Inhibitors – Physiological Effects

---

Photoperiodism and vernalization, Phytochromes and flowering

 

Phytohormones Auxins and Physiological effects of auxin

Phytohormones Auxins and Physiological effects of auxin are explained with detail notes.

Phytohormones Auxins

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Auxin was the first plant hormone to be discovered.

They were isolated initially from human urine.

The term auxin is given to generally IAA and other natural and synthetic compounds having similar structure and growth regulating properties.

Generally, auxins are produced in the growing apices of stem and root where from they migrate to the other parts of the plant. Auxins such as IAA and phenyl acetic acid (PAA) are natural auxins.

Synthetic auxins are chemicals synthesised in the laboratory.

They are considered as plant growth regulators. eg. Naphthalene acetic acid, 2,4 – Dichlorophenoxy acetic acid.

Physiological effects of auxin

1. Auxins are well known to promote elongation of stem and coleoptile.It promotes the growth by cell enlargement in stems, particularly by elongation of cells behind the apical meristem.
2. Growth in lateral bud is inhibited when the apical bud of a tall plant
   remains intact. However, the lateral bud grows rapidly on removal
   of apical bud.
3. Suppression of growth in lateral bud by apical bud due to auxin
   produced by apical bud is termed as apical dominance. The reason for this is due to auxin produced in growing tip and it stimulates growth but as it moves downward, suppresses growth in the stems below.
4. Auxin is responsible for initiation and promotion of cell division in
   cambium, which is responsible for the secondary growth. This
   property of induction of cell division has been exploited for tissue
   culture techniques and for the formation of callus.
5. Auxin promotes growth of root only at extremely low concentrations. At higher concentrations, it always inhibits growth of root.
6. When leaves and fruits mature, they shed from the stem. This is
   called abscission. Auxin prevents abscission.
7. Seedless fruits are produced in tomato and apple, by external
   application of auxin on flowers. Such seedless fruits are called
   parthenocarpic fruits.
8. 2,4 – Dichlorophenoxy aceticacid, a synthetic auxin is used to eradicate weeds in the field.

Back to botany topic list

For more details about Phytohormones Auxins and Physiological effects of auxin click here

Other links 

Plant tissue culture – origin and techniques

---

Plant physiology – photosynthesis and its significance

---

Site of photosynthesis and Mechanism of photosynthesis

---

Electron transport system and photophosphorylation types

---

Dark reaction

---

C3 and C4 pathways

---

Photorespiration or C2 cycle

---

Factors affecting photosynthesis

---

Test tube and funnel experiment, Ganong’s light screen experiment

---

Mode of nutrition – Autotrophic, Heterotrophic

---

Chemosynthesis

---

Mechanism of Respiration – Glycolysis

---

Mechanism of Respiration – Oxidative decarboxylation , Krebs cycle

---

Mechanism of Respiration – Electron Transport Chain, Energy Yield

---

Ganong’s respiroscope, Pentose phosphate pathway

---

Anaerobic respiration, Respiratory quotient, Compensation point, Kuhne’s fermentation tube experiment

---

Plant growth and Measurement of plant growth

---

Phytohormones Gibberellins

---

Phytohormones Cytokinin, Ethylene, Abscisic Acid, Growth Inhibitors – Physiological Effects

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Photoperiodism and vernalization, Phytochromes and flowering

 

Plant growth and Measurement of plant growth

Plant growth and Measurement of plant growth are explained.

Plant growth

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Growth is one of the most fundamental and conspicuous characteristics of living organisms.

Growth may be defined as an irreversible increase in mass, weight and size of a living organisms.

In most cases, it results in increase in dry weight and the amount of protoplasm.

Growth in higher plants includes cell division, enlargement and differentiation.

Increase in the number and size of cells by itself cannot account for the development of an organized plant.

For example, when a seed is sown, it does not become a larger seed but it grows as a seedling.

Thus, growth is always accompanied by differentiation.

Differentiation is the transformation of identical cells into different tissues.

Depending upon the various structural, functional and physiological needs of the plant the tissues are of different types.

Growth and differentiation results in development, which leads to gross form of the plant.

Meristematic cells present in the plant body viz., root, shoot apices, and the cambium are responsible for growth in plants.

Phases of growth

The growth in length of the plant is due to the meristematic activity
of the apical meristems that takes place in the root and shoot apices.

Whereas increase in thickness of stem and root is due to the activity of lateral meristem.

You have already learnt in chapter 2 about different types of meristems.

The period of growth is generally divided into three phases viz., formation, elongation and maturation.

In the first phase, new cells are continuously formed by the apical meristem.

In the second phase known as phase of elongation, the newly formed cells enlarge in size.

In the third phase, phase of maturation, cells start maturing to attain permanent size and form.

The rate of plant growth is slow in the initial stages and this phase is called lag phase.

It is followed by a rapid growth phase called log phase.

In the third and final phases, the growth slows down and the organism maintains the size it has already attained.

This phase is known as stationary phase or steady state phase.

The growth in size or increase in number of cells if plotted against time the graph shows ‘S’ shaped curve known as sigmoid growth curve as shown in the figure.

In the annual plants the last phase i.e. steady state phase is followed by senescence i.e. arrest of growth and death.

However, in the case of large trees each growing season exhibits a sigmoidal pattern of growth.

Measurement of growth in plants

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You have already known that the growth in length of the plant is due
to the activity of the apical region of shoot and root.

So in any plant the growth in length can be measured in ordinary measuring scale at an interval of time.

For precise measurement, an instrument called ‘Lever Auxanometer’ is used. It measures the rate of growth of plant in terms of short length.

The auxanometer cons ists of a movable pointer attached to a pulley and a graduated arc fixed to a stand.

A thread passes around the pulley.

One end of the thread is tied to the growing tip of the potted plant.

The other end is tied to a small weight.

As the plant grows in length the pulley rotates and needle attached to the pulley moves down the scale.

From this, growth in length of the plant can be measured at a given interval of time.

The actual growth in the length of a plant is measured as follows.

Plant growth substances

The growth of a plant is regulated through gene action and
environmental conditions.

There are substances, which are produced by plants themselves, which regulate their growth and many physiological and biochemical activities.

These are called plant growth substances.
Regulation of plant growth through chemical mechanisms frequently involves certain molecules known as hormones.

Based on the origin and biological activities plant growth substances are grouped into three – growth regulators, phytohormones and growth inhibitors.

Growth regulator

It is a hormone like synthetic organic compound.

In small amounts, it modifies the growth and development either by promoting or inhibiting the growth. eg. Naphthalene acetic acid (NAA).

Phytohormones

These are organic substances produced by the plant.

They are active in very minute quantities.

They are synthesised in one of the parts of the plant and translocated to another part where they influence specific physiological, biochemical and morphological changes.

The phytohormones are broadly grouped under five major classes namely auxins, gibberellins, cytokinins, ethylene and abscisic acid.

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Other links 

Plant tissue culture – origin and techniques

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Plant physiology – photosynthesis and its significance

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Site of photosynthesis and Mechanism of photosynthesis

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Electron transport system and photophosphorylation types

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Dark reaction

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C3 and C4 pathways

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Photorespiration or C2 cycle

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Factors affecting photosynthesis

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Test tube and funnel experiment, Ganong’s light screen experiment

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Mode of nutrition – Autotrophic, Heterotrophic

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Chemosynthesis

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Mechanism of Respiration – Glycolysis

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Mechanism of Respiration – Oxidative decarboxylation , Krebs cycle

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Mechanism of Respiration – Electron Transport Chain, Energy Yield

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Ganong’s respiroscope, Pentose phosphate pathway

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Anaerobic respiration, Respiratory quotient, Compensation point, Kuhne’s fermentation tube experiment

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Phytohormones Auxins

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Phytohormones Gibberellins

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Phytohormones Cytokinin, Ethylene, Abscisic Acid, Growth Inhibitors – Physiological Effects

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Photoperiodism and vernalization, Phytochromes and flowering