Growth is the essential characteristic of all living organisms.

It can be defined as an irreversible (permanent) increase in mass, weight or volume of cell, organ or organism.

Irreversible increase in size, volume or weight is called apparent growth, as it is external manifestation of growth.

Formation of cellular materials or protoplasm is the real growth.

Growth is a measurable or quantitative phenomenon which can be measured ­in relation to time.

Growth of living being is internal or intrinsic.

Plant growth is diffused like that of animals only during the early embryonic stages.

Later on, plants develop specific areas, called meristems, for growth.

On account of meristems, plant growth is localised.

Characteristic of Plant Growth

1. Growth is localised.

2. Growth continues throughout life, i.e., indeterminate.

3. There is increase in number of parts.

4. It is open ended.

5. Tile young one or seedling can be quite different from adult.

6. The juvenile stage may have different traits.

Phases of Growth

The dividing or meristematic cells pass through following three phases during growth,

(i) Phase of cell division,

(ii) Cell elongation phase,

(iii) Maturation phase.

(i) Phase of cell division. The meristematic cells located at shoot and root apex are thin walled, densely cytoplasmic and show continuous mitotic divisions. It represents initial stage of growth. The rate of growth is naturally slow during this phase.

(ii) Cell elongation phase. The daughter cells, formed by the division of meristematic cells, undergo enlargement. These can be observed proximal to meristematic cells. Initially, the cells enlarge in all dimensions but subsequently enlargement takes place only in a specific direction. Increased vacuolation and new depositions of wall material are other characteristics to this phase. It is the period of maximum and rapid growth. Physiological activities of cells are at their maximum.

(iii) Maturation or stationary phase. As the cells enlarge, they gradually acquire permanent shapes and forms. The final phase of growth is usually called maturation. As maturation proceeds, the cells become fully differentiated. A mature cell may be living or dead, depending upon the requirements of plant.

Growth curve :

The growth of a plant or any of its organs rarely take place at a constant rate.

There are certain regular changes in rate, that can be observed when growth of any parameter (i.e., weight, volume, size, etc.) is plotted against time.

The curve obtained is characteristically S-shaped. It is called sigmoid curve.

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Measurement of growth:

Growth is measured in terms of increase in length, weight; thickness or volume. Following are some of the methods used to measure growth. Various methods and instruments for growth measurement are listed below:

a. Direct method

b. Horizontal microscope

c. Arc auxanometer

d. Preffer's auxanometer.

e. Crescograph (developed by J.C. Bose) Very sensitive growth measuring instrument which can magnify growth by tenthousandtimes.

Differentiation, Dedifferentiation and redifferentiation


(i) Differentiation: It is permanent qualitative change in structure, chemistry and physiology of cell wall and protoplasm of cells, their tissues and organs.

(ii) Dedifferentiation : It is the process of despecialisation of differentiated living cells so that they regain the capacity to divide and form new cells.

(iii) Redifferentiation: Structural, chemical and physiological specialisation of cells derived from dedifferentiated meristematic cells is called redifferentiation.

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Some examples of differenentiation

(i) Enlargement, lignocellulosic secondary wall thickening and loss of protoplasm in case of tracheary elements.

(ii) Loss of end walls in case of vessel elements.

(iii) Loss of nucleus and perforations in sieve tube members.

(iv) Deposition of suberin and tannins in cells.

(v) Differential wall thickening (in guard cells).

(vi) Secretion of mucilage in root cap.



It is the sequence of changes that occur in the structure and functioning of an organism during its life cycle from seed germination to its death.

Broadly, it is sum total of growth and differentiation.

Plants follow different pathways in response to environment or phases of life to form different kind of structures.

This ability is called plasticity, e.g., heterophylly in cotton, coriander and larkspur.

In such plants, the leaves of the juvenile plant are different in shape from those in mature plants.

On the other hand, difference in shapes of leaves produced in air and those produced in water in buttercup also represent the heterophyllous development due to environment.

Plant Growth Regulators


The growth and differentiation of plants is controlled by a special class of chemical compounds called hormones.

These chemicals are synthesized at one place in the plant body and translocated to another where they act in a specific manner.

They are needed in small quantities at very low concentrations.

Plant hormones, also known as growth factors, growth substances, plants growth regulators (PGRs) or phytohormones, can either promote the growth (growth promoters) or may inhibit it (growth inhibitors).

On the basis of their functions in a living plant body, phytohormones can be classified into following groups:

A. Auxins

B. Gibberellins

C. Cytokin ins

D. Ethylene

E. Abscisic acid (ABA)

A. Auxins

Auxins are the best known plant growth regulators.

They have many effects on plant growth.

The earliest experiments on plant hormones were performed by Charles Darwin and his son Francis Darwin.

They worked with the seedlings of canary grass (Phalaris canariensis).

Darwin was amazed to observe that the direction of growth of the coleoptile was influenced only when its tip was exposed to light and not the base where actual growth was taking place.

He further observed that when the coleoptile was illuminated from one side (i.e., unilateral light), it bent towards light, but if the tips were chopped or covered with metal foil, bending response would not occur.

This led Darwin to conclude that certain kind of 'influence' is generated at the tip which is then transmitted to the base (where growth takes place).

However, the influence remained unidentified.

Boysen-Jensen observed that coleoptiles with decapitated tips neither grow, nor bend towards light, (i.e., do not show phototropism).

The normal bending also occurred, if the tip is cut off and replaced with an intervening block of gelatin.

It was thus evident that the influence was due to chemical, diffusing through the block of gelatin.

A. Paal demostrated that coleoptiles would bend even in the dark after certain treatments.

F.W. Went cut off tips of oat coleoptiles and placed them on small agar blocks and let them for a few hours to allow all the 'chemical influence' to diffuse; into the agar.

By this, he isolated auxins in agar block.

He then placed these agar blocks on one side of freshly decapitated coleoptiles.

He observed that the coleoptiles grew and bent away from the side on which the block was placed.

The degree of curvature of the coleoptile was directly proportional to the concentration of the chemical influence in the agar block.

Went named this 'chemical influence' responsible for the phototropic response as auxin (derived from a greek word 'auxein' = to increase or to grow).Hence, Went is credited with the discovery of auxins.

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On the basis of the work done on phototropism so far, following conclusions can be drawn:

(i) Growth regulating auxins are systhesized at the tips (apices) of the plant.

(ii) Auxin synthesis occurs on the illuminated side.

(iii) Auxins are transported from apex to the base or the dark or unilluminated side.


Auxin are synthesized continuously in the shoot and root apices. These are produced in relatively very small amounts in roots. Tryptophan (amino acid) is its precursor.

Auxins from the shoot are transported primarily toward the roots through parenchyma.

This basipetal (from apex to base) movement is called polar transport.

Usually, a low concentration of auxin stimulates the growth, while high concentration would inhibit the growth of the same cell or organ.

Roots, in general, show growth initiation at a very low concentration (0.0001 ppm), while in stem, similar response occurs at a relatively high concentration (10 ppm).

Auxin is active in free state and can be easily extracted. Bound auxin is inactive and meant for storage. e.g., IAA-Aspartic acid, IAA-Inositol, IBA-Alanine.

Chemical nature

Auxins are weak organic acids.

Some auxins occur in plants naturally and are called natural auxins, whereas a large number of synthetic chemicals with similar action are called synthetic auxins.

(a) Natural auxins: The first naturally occuring auxin was isolated by Kogl and Haagensmit (1931) from human urine. It was identified as auxin-a (auxentriolic acid). Later, in 1934 Kogl et. al. obtained another auxin called auxin-b (auxenolinic acid) from corn germ oil (extracted from germinating corn seeds), and heteroauxin from human urine. Heteroauxin, also known as indole-3-acetic acid (IAA), is the best known natural auxin. Besides IAA, Indole Butyric Acid (IBA) is another natural auxin

(b) Synthetic auxins : A number of synthetic compounds showing activities similar to auxins have been produced. Some of these are a-and b -Naphthalene acetic acid (NAA), Phenoxy acetic acid (PAA), 2, 4 -Dichlorophenoxy acetic acid (2, 4-D), 2, 4, 5-Trichlophenoxy acetic acid (2, 4, 5-T), Naphthalene Acetamide (NAAM), ete.­

Functions of auxins

(1) Cell enlargement: Auxins cause cell enlargement by solubilisation of carbohydrates, loosening of cell wall microfibrils, synthesis of new wall materia1 and increase in respiration.

(2) Prevention of lodging: Lower internodes of the stem of cereals are long and weak. As a result the plant bends down or droops (a process known as lodging). Application of NAAM prevents lodging.

(3) Apical dominance: In many plants, apical bud suppresses the growth of lateral buds. This condition is known as apical dominance. A plant with strong apical dominance has little or no branching (e.g., sunflower).

Pruning (removal of shoot tips) of trees, tea plantations and hedge making is actually done to remove apical buds so that lateral buds may be relieved of their suppression. It results in dense growth of hedges.

(4) Responsible for phototropism and geotropism.

(5) Promote root initiation in callus and stem cuttings.

(6) Delay of abscission of leaves by preventing formation of abscission layer. But promotes this in older leaves and fruits.

(7) Induce parthenocarpy (production of seed less fruits e.g., Tomato).

(8) Have feminising effect (increase number of female flowers in plants e.g., Cannabis).

(9) Seasonal activity of cambium is promoted by auxin.

(10) Healing of injury is affected through auxin-induced division in cells around injured area.

(11) Auxin induces negative potential in cell membranes.

(12) Promotes xylem differentiation.

Applications of Synthetic Auxins

1. Rooting in stem cuttings: IBA, IBA-alanine and NAA are used.

2. Parthenocarpy : IAA, IBA.

3. Weedicide : 2,4-D, 2,4,5-T are used for killing broad leaved weeds (generally dicot).

4. Flowering: NAA and 2,4-D for litchi and pineapple.

5. Storage : Methyl ester of NAA for storage of potato.

6. Preharvest fruit drop: 2,4-D for Citrus fruits, NAA for tomato.

7. Vegetable crops: Chlorophenoxypropionic acid is used to improve quality of vegetable crops by inhibiting flower formation. e.g., lettuce.

8. For Dwarf shoots: NAA is used for increasing dwarf shoots and number of fruits in apple.

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Antiauxins : They inhibit aaxin activity.

e.g., TIBA (Triiodobenzoic acid)

PCIB (p-chlorophenoxy isobutyric acid)

Bioassay of Auxin

(i) Avena curvature test

(ii) Split pea test

(iii) Root growth inhibition test

B. Gibberellins

Gibberellins are another important group of growth promoting hormones.

They stimulate growth largely in the shoot system of the plants and have little or no effect on roots.

Japanese rice farmers observed some abnormally tall and thin seedlings in their rice fields which never bore seeds.

This disease was called "bakanae" or "foolish seedling disease".

E. Kurosawa found that the causative agent of the disease was Gibberella fujikuroi, a fungus of the class Ascomycetes.

He produced the same effect on rice plants with an extract of this fungus.

Later, Yabuta and Sumiki isolated the substance responsible for rapid growth from the extract of the fungus and it was named gibberellic acid (GA). Commercial production of GA is still carried out by culturing this fungus in large vats. Fusarium moniliforme is the imperfect stage of this fungus.

Distribution : Gibberellins are found in almost all groups of plants such as algae, fungi, mosses, ferns, gymnosperms and angiosperms. Gibberellins and gibberellin like substances have been observed in most of the higher plants.

Types of gibberellins : Gibberellins are synthesised in the apices of young leaves, embryo, buds and roots and are transported through xylem. There are more than 100 types of gibberellins reported from widely differnt organisms. They are denoted as GA, GA2 and so on. However, GA3 was one of the first gibberellin to be discovered and remains the most intensively studied, common form.

Chemically, II gibberellins are terpenes, a complex group of plant chemicals related to lipids. All are weak acids and have gibbane ring skeleton. Precursor is Acetyl CoA.

Functions of Gibberellins

(1) In rosette forming plants, (e.g. , cabbage, henbane, radish, beet) internode growth is poor but large leaves appear to arise in tufts. The internodes suddenly elongate and the stem becomes normal just before flowering. This is called bolting. This requires long day conditions and gibberellin treatment.

(2) Substitution of cold treatment : Biennial plants flower only when they receive low temperature during winter season. Such plants would, however, flower after gibberellin treatment even if they do not receive suitable low temperature. Thus, biennial plants can be made to flower in a single year by gibberellin treatment.

(3) Parthenocarpy : Gibberellins have been found to be effective in inducing parthenocarpy in tomato, apple, pear,-etc.

(4) Breaking of dormancy: Gibberellins can effectively break the dormancy of potato tubers, winter buds and seeds of many trees. Gibberellins thus also act antagonistically to abscisic acid (ABA).

(5) Increase in fruit size: Gibberellins increases the number and fruit size of grapes {pomalin (GA + BAP) is used for this purpose}.

(6) Production of male flowers : Gibberellins induce production of male flowers on genetically female plants of Cannabis and cucumber. They promote flowering in LDP, even in non-inductive periods.

(7) Internodal elongation : Like auxins, the main effect of gibberellins is on stem elongation. Gibberellins stimulate stem elongation and leaf expansion, but do not affect roots. Thus, Gibberellins restore normal size and growth in genetically dwarf varieties of pear and maize.

(8) Germination of seeds: Promoted (especially in cereals).

(9) Vernalization: Gibberellins can substitute vernalization demands.

Commercial Application:

1. Increasing length of grape stalks.

2. Malt : Increase yield of malt from barley.

3. Overcoming dormancy: In photoblastic seeds of tobacco and lettuce.

4. Delayed ripening : In Citrus.

5. Increase in length of sugarcane, hence yield.

6. Early seed production can be induced in conifers by treating them with GA in juvenile period.

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Antigibberellins : Certain chemicals are antagonistic to gibberellins.

e.g., Phasphan-D,

Amo - 1618

CCC (Chloro Chaline Chloride)

Maleic hydrazide (MH).

Bioassay of Gibberellins

(i) Induction of -amylase in barley endosperm test.

(ii) Dwarf maize test.

(iii) Dwarf pea test.

C. Cytokinins

Skoog et. al. observed that callus formation from internodal segments tobacco can proliferate, only if auxins are also supplied with (a) extract of vascular tissue (b) yeast extract (c) coconut milk or (d) DNA.

Later, Miller et. al. showed that yeast extract contained some growth regulator.

Finally, this growth regulator was isolated from herring sperm DNA and yeast cells and was named kinetin (6-furfurylaminopurine).

The name cytokinin was adopted by Letham (1963).

The first naturally occurring cytokinin to be chemically identified was from young maize (Zea mays) grains.

Hence it was named zeatin.

Similar form was isolated from coconut milk (liquid endosperm of coconut).

Distribution: Cytokinins are most abundant in the tissue where rapid cell divisions occur, like developing fruits, root apices and developing shoot buds are particularly rich in cytokinins.

Functions of Cytokinins

(i) Cell-division: Cytokinins are quite abundant wherever rapid cell division occurs, especially in growing tissues.

(ii) Morphogenesis: Cytokinins promote cell division. In the presence of auxins, cytokinins promote cell division even in non meristematic tissues. In tissue cultures, mitotic divisions are accelerated when both auxin and cytokinin are present. The ratio of high cytokinin to low auxin promote shoot buds in tissue culture.

(iii) Apical dominance: Cytokinins and auxins act antagonistically in the control of apical dominance.

(iv) Delay in senescence: Cytokinins delay the senescence (Richmond Lang effect) of plant organs by controlling protein synthesis and mobilisation of resources. These help to produce chloroplast in leaves. Hence, these are also called antiageing hormones.

(v) Flowering : Cytokinins also induce flowering in certain species of plants such as Lemna and Wolffia and are also responsible for breaking the dormancy of seeds of some plants.

(vi) Phloem transport is promoted.

(vii) Accumulation of salts in the cells is promoted.

(viii) Sex-expression: Promote femaleness.

(ix) Increases resistance to low and high temperature and diseases.

Commercial application:

(a) Tissue culture.

(b) Shelf life of vegetables and cut flowers is increased .

(c) Overcoming senescence.

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(i) Chlorophyll preservation test.

(ii) Cell division test

D. Ethylene

Ethylene is the only gaseous PGR and is effective in conc. 0.01 to 10 ppm.

It is produced by most or all plant organs but maximum production occurs in ripening fruits and senescent leaves.

High concentration of auxin induce the formation of ethylene. Its precursor is methionine.

This hormone could fit in both categories, i.e., inhibitor and promotor, but it is largely a growth inhibitor.

Unlike other plant hormones, ethylene is a gas at normal temperature and pressure.

Thus it travels not only within plant in the intercellular spaces, but also through the air, from site of its production to its target tissue which may be part of the same plant or of another plant.

Like, ethylene given out by apples inhibits the sprouting of potatoes stored in the same godown.

Role of ethylene : It regulates so many physiological processes, hence is one of the most widely used PGR in agriculture.

(i) Inhibition of stem elongation and stimulation of transverse growth : Ethylene causes increase in the girth of the plant, and promotes horizontal growth.

(ii) Fruit ripening:

Ethylene is known chiefly for its effects on fruit ripening. Hence, it is used on a commercial scale to stimulate ripening of bananas, apples, mangoes, citrus fruits, tomatoes and many other fruits. These fruits are picked up before ripening and subsequently ripening can be regulated with ethylene when needed. The ripening of fruits by ethylene is accompanied by increase in their rate of respiration. This is known as respiratory climacteric effect.

Ethylene production by plants is autocatalytic, i.e., a small amount of ethylene will stimulate production of its larger amounts. Thus, a few ripe fruits will initiate ripening of nearby fruits. It is, therefore, a common practice to keep few ripe bananas with unripe bananas to hasten ripening.

(iii) It promotes apogeotropism in roots. Seedlings develops epicotyl hook.

(iv) Senescence : It increases senescence of leaves and flowers.

(v) Abscission : It induces abscission of leaves, flowers and fruits.

(vi) Breaking of dormancy : It breaks the dormancy of storage organs, and initiates germination in peanut seeds

(vii) Root initiation: Low concentration of ethylene induces rooting, growth of lateral roots and root hairs.

(viii) Flowering: Ethylene is used to initiate flowering and synchronising fruit set in pineapples.

(ix) Sex expression: Ethylene, like auxins has a feminising effect on sex expression of genetically male plants of Cannabis and monoecious plants of cucumber.


1. Ethylene in the name of ethephon is used to hasten ripening tomatoes and apples, and to accelerate abscission in flowers and fruits (thinning of cherry, walnut and cotton).

2. Increasing the number of female flowers and fruits in cucumber.

3. Sprouting of storage organs - rhizomes, tubers and other storage organs can be made to sprout by ethylene.

Bioassay of ethylene : Triple response test.

E. Abscisic Acid

In contrast to auxins, gibberellins and cytokinins which stimulate growth, abscisic acid is chiefly a growth inhibitor.

It was discovered almost simultaneously by Addicott (1963) and Wareing (1964).

They named it abscisin II and dormin respectively.

In 1967, it was decided to call it abscisic acid (ABA).

Since then, it has been found in all groups of plants (from mosses to higher plants).

In liverworts and algae, a compound lunularic acid has been found having activities similar to ABA.

Chemically, ABA is a dextro-rotatory cis-sesquiterpene.

ABA is a major inhibitor of growth in plants and is antagonistic to all the three growth promoters, especially gibberellic acid.

Its precursor is violaxanthin (a xanthophyll in chloroplast)

Abscissic acid is produced mainly in mature leaves.

Besides, it is also synthesized in stems, fruits and seeds and then transported to the rest of the plant through vascular tissue. Some of the important roles of abscisic acid in plant growth are as follows.

Bioassay of ABA: Cotton or Bean explant test.

­Functions of abscisic acid

(i) Abscission : It hastens the formation of abscission layer and senescence.

(ii) Transpiration: It helps in closing of stomata by causing potassium ions to leave the guard cells (thus reducing their turgidity) during periods of water shortage or drought and, hence is also known as stress hormone.

(iii) Promotes bud dormancy during winters.

(iv) Seed dormancy : ABA induces seeds dormancy, hence is named dormin, thus it helps the seed to withstand desiccation and other unfavourable factors.

(v) Stoppage of cambial activity : Abscisic acid inhibits cambial activity.

(vi) Flowering: ABA induces flowering in some short day plants like strawberry and black currants.

(vii) It plays an important role in seed development, maturation and dormancy.

(viii) Induces synthesis of carotenoids in green oranges making them yellow.


PHOTOPERIODISM (Term by Garner and Allard)

The response of plants to changes in the relative lengths of day and night is called photoperiodism.


 The relative lengths of dark and light periods in a day vary from place to place and from season to season. The length of light period is called photoperiod. At equators, day length is of 12 hours duration throughout the year. Plants flower only when exposed more or less than a certain light period called critical photoperiod.

Types of plants according to photoperiodic requirements for flowering :

For SDP  while for LDP is critical for flowering

Photoperiodic stimulus is perceived by leaf.

When proper photoperiod is perceived a flowering hormone called florigen is synthesized in leaf and is transported to bud through phloem, where flowering occurs.

Florigen is a hypothetical hormone and is chemically similar to gibberellins.




There are plants, in which flowering is qualitatively or quantitatively dependent on exposure of low temperature.

In annual plants growth usually starts in the spring, flowers are formed in the summer and fruits and seeds are produced in the fall.

Biennials on the other hand, remain vegetative in the first growing season and after prolonged exposure to cold winter temperature, flower in the following season.

Cold treatment during winters is essential for flowering for these plants.

If they do not get it, they either do not flower or their flowering is much reduced.

The effect of cold or chilling treatment that plants get during winters in the field can be given in the laboratory.

As a result of this artificial cold treatment, the biennials and many winter varieties of cereals can be made to flower in the same season.

This low temperature treatment is known as vernalization or yarovization (= making spring like).

The term vernalization was given by Russian scientist Lysenko.

Site of vernalisation : Shoot tip, embryo tip, root apex, as observed by Wellensiek.

Requirements of Vernalisation

1. Low temperature : 0º -5ºC

2. Period of low temperature : Few hours -few days

3. Actively dividing cells (Meristematic cells)

4. Water

5. Aerobic condition

6. Proper nourishment

In the process of low temperature treatment a hypothetical hormone called vernalin (by Melcher's) is produced.

It is now believed that vernalin is a gibberellin like substance, because vernalized plants have higher gibberellin levels than non vernalized plants.

Vernalization is beneficial in reducing the period between germination and flowering.

Thus, more than one crop can be obained during a year.

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