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SyllabusPlant growth and development

PLANT GROWTH AND DEVELOPMENT

1. Growth2. Differentiation, dedifferentiation,

redifferentiation3. Plant Growth regulators4. Gibberelins5. Ethylene6. Photoperiodism

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Plant Growth & Development # 1

PLANT GROWTH & DEVELOPMENT

GROWTHGrowth is a characteristic feature of all living organisms.Growth is an irreversible increase in size of an organ or its part or even of an individual cell.Growth is accompanied by metabolic processes, that occurs at the expense of energy.

PLANT GROWTH GENERALLY IS INDETERMINATEIn plants growth continues throughout their life, so it is called as indeterminate or unlimitedgrowth.Meristem continously divides and add new cells to the plant body, such activity of meristem is calledas open form of growth or indeterminate growth.Growth is diffused in animals but growth in plant is localised. (Presence of meristem at certainlocations in plant body.)Seed germination is the first step of plant growth. Almost all the plants face a period ofsuspended growth.If the suspension of growth is due to exogenously controlled factors (environmental factors) thenit is called quiescence.When the suspension of growth is due to endogenously controlled factors (hormonal, genetic)then it is termed as dormancy.

GROWTH IS MEASURABLEAt cellular level growth can be measured by measuring the increases in the amount toprotoplasm but it is very difficult to measure directly, so growth is measured by a variety ofparameters, they are

(a) Increase in fresh weight(b) Increase in dry weight(c) Increase in surface area/volume(d) Increase in number or size of cells

Growth is measured by Auxanometer.

PHASES OF GROWTH(1) Cell division or cell formation or meristematic phase: Number of cells is increased by

cell division in this phase. The cells of this region have rich protoplasm and conspicuous nuclei, thin and primary cell wall & abundant plasmodesmatal connections.

(2) Cell enlargement or cell elongation phase: Size of cells is increased due to vacuolation & TP (Tugor pressure) and new cell wall depositions in this phase.

(3) Cell maturation or differentiation phase : Cell wall thickening and Protoplasmic modifications.Qualitative changes in cells is important feature of this phase. It leads to formation of mature tissues.

GROWTH RATEIncreased growth per unit time is termed as growth rate.The growth rate shows an increase that may be arithmatic or geometric.


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(a) Arithmatic growth : In arithmatic growth only one daughter cell among the two further di-vides while other differentiates and become mature (stop dividing).

Meristematic cell/Cells capable of division

Differentiated cell/Cells that lose capacity to divide

Ex. Root & Shoot elongation at constant Rate.It is methamatically expressed asLt = L0 + rt where Lt : length at time ‘t’

Time

Hei

ght o

f pla

nt

L0 - length at time ‘zero’r - growth rate/elongation per unit time.

It`s curve is linear.

(b) Geometric Growth : Here both the progeny cells following mitotic divisions retain the abilityto divide and continue to do so.

Ex: Early embryonic development/division in zygote, division in unicellular organism.It is mathematically represented asW1 = W0ert

where W1 - final size (weight, height, number etc.) W0 - initial size at the begining of period. r - growth rate e - base of natural logarithms.

Time

Siz e

/wt.

of o

rgan

Lag phase

Log/Exponential phase

Steady/ stationary phase

In most systems the initial growth is slow (lag phase) and it increases rapudly thereafter at anexponential rate (log or exponential phase), it is also called as “grand phase of growth”.However, with limited nutrient supply, the growth slows down leading to a stationary phase orsteady phase. If we plot the parameter of growth against time, we get typical sigmoid or S-curve.


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A sigmoid curve is a characteristic of living organisms growing in a natural enviroment. It is typical for all cells, tissue and organs of a plant.r = relative growth rate and is also the measure of the ability of the plant to produce new plant material, referred to as efficiency index.

Absolute and Relative growth rates :Absolute growth rate : Total growth which occurs in unit time in plant or plant parts. Relative growth Rate : Total growth which occurs in unit time with respect to initial param-eter in plant or plant parts. Relative growth rate is generally high young developing plant parts.

Ex:

5 cm2 50 cm2

55 cm2

Time period = 7 days

10 cm2

AB

Both posses same absolute growth rate i.e. 5 cm2 in 7 days.But high relative growth rate is in leaf ‘A’ about 100 % while in leaf ‘B’ it is about 10 %.

CONDITIONS FOR GROWTH -Water, oxygen and nutrients are very essential for growth. The plant cells grow in size by cell enlargement which in turn requires water. Turgidity of cells helps in extension growth. Thus, plant growth and further development is intimately linked to the water status of the plant. Water also provides the medium for enzymatic activities needed for growth.Oxygen helps in releasing metabolic energy essential for growth activities. Nutrients (marco and micro essential elements) are required by plants for the synthesis of protoplasm act as source of energy.

DIFFERENTIATION, DEDIFFERENTIATION AND REDIFFEREN-TIATION -

Cells produced after divison (mitosis) in meristem transform or differentite theirselves according to speed function. This act leading to maturation of cells is called as differentiation e.g. formation of primary permanent tissues like parenchyma etc.

The living and differentiated cells sometimes again have to divide according to the requirements of plants. For this purpose these cells regain their dividing capacity, this phenomenon is called as dedifferentiation.

e.g formation interfascicular cambium from the differentiation permanent parenchymatous cells are formation of cork cambium from hypodermis.

Such dedifferenttiated cells divide and produce the cells that once again lose the capacity to divide, but mature to perform specific functions, this is called as redifferentiation. Example of redifferentiated tissues and secondary xylem, secondary phloem, secondary cortex etc.


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Differentiation in plants is open, because cells/ tissue arising out of the same meristem have different structures at maturity and some of them have the ability to return to division stage.

The final structure at maturity of a cell/tissue is also determined by the location of the cell within For example, cells positioned away from root apical meristems differentiate as root-cap, while the pushed to the periphery mature as epidermis.

DEVELOPMENT

DeathCell division

Senescence

Mature cell

Meristematic cell

Maturation

DifferentiationPlasmatic growth

Expansion (Elongation)

Plants follow different pathways in response to environment or phases of life to form differ-ent kinds of structures. The ability is called plasticity, e.g. heterphylly in cotton, conriander and larkspur. In such plants, the leaves of the juvenile plant are different in shape from those in mature plants. (Internal plasticity)

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 (Environmental plas-ticity). This phenomenon of heterophylly is an example of plasticity.

Development is considered as as the sum of growth and differentiation, which is controlledby both extrinsic (environmental) and intrinsic (internal) factors.

(i) Extrinsic factors - light, temperature, water, oxygen, nutrition.(ii) Intrinsic factors - genetic factor (intracellular) and PGR (Intercellular)


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PLANT GROWTH REGULATORS (PGRs)

PGRs are also called plant growth substances or plant hormones or phytochormones.PGRs are of two types -

(1) Plant growth promotors :(i) Auxins (ii) Gibberellin (iii) Cytokinin

(2) Plant growth inhibitors :(iv) Abscisic acid (v) Ethylene

Characteristic ‘or’ chemical nature pf plant growth regulators :The plant growth regulators (PGRs) are small, simple molecules of diverse chemical composition.They could be

(i) Indole compounds (indole-3-acetic acid = IAA) Auxin(ii) Adenine derivatives (N6-fufurlamino purine = kinetin) Cytokinin(iii) Derivatives of carotenoids (abscisic acid = ABA)(iv) Terpenes (gibberellin acid = GA3) Gibberellin(v) Gases (ethylene = C2H4).

AUXIN

DISCOVERY

Term Auxin derived from Greek word ‘auxein’ which means to grow.

First of all Charles Darwin & Francis Darwin (son of C. Darwin) observed the process of phototropism.They observed coleoptile bending in Canary grass (Phalaris), responded to unilateral illumination.After a series of experiments, it was concluded that the tip of coleoptile washe site of transmittable influence (Auxin) that caused the bending of the entire coleoptile. Auxin was isolated by F.W. Went from the tips of coleoptiles of oat seedlings..Biosynthesis of auxin occurs by tryptophan Aminoacid in the presence of Zn++ ion.Now IBA (Indole Butyric acid) have also been isolated from plants (natural auxin) but IAA (Indole Acetic Acid) is mostly widely occuring auxin in plants.

Biosynthesis of auxin occurs at both shoot and root apex and transport of auxin is basipe-tal.

PHYSIOLOGICAL EFFECT OF AUXIN -(1) Apical Dominance (Characteristic function of auxin) :- The phenomenon in which apical bud

dominates the growth of lateral buds is called Apical Dominance.Removal of shoot tips (decapitation) usually results in the growth of lateral buds. It iswidely applied in tea plantations, hedge-making.

(2) Cell Division & Cell Enlargement: Mainly cell elongation. Help in cell division. Auxin alsocontrols the xylem differentiation.

(3) Root initiation :- Rooting on stem cuttings (widely used in plant organs propagation) ispromoted by IBA & NAA.


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(4) Preventtion of Abscission :- IAA, NAA prevent premature abscission of plant organs. Pro-mote the abscission of older mature Leaves & Fruits.

(5) Flower initiation : - Auxin promotes flowering in Pineapple & Litchi plants.

(6) Parthenocarpy :- Seed less fruits can be produced by spray of IAA e.g. Tomato.

(7) Selective weed killer :- 2, 4-D is widely used to kill Dicot weeds, does not affect monocotplants.2, 4-D + 2, 4, 5-T = Agent orange. It is used to prepare weed free lawns.

Synthetic Auxins -

NAA - Napthalene acetic acid.2, 4-D - 2, 4-Dichlorophenoxy acetic acid.2,4,5-T - 2, 4, 5-Trichloro phnoxy acetic acid.

GIBBERELLINS

DISCOVERY -First of all Japanese farmers observed symptoms of the disease in rice seedlings & called it bakanaedisease (Foolish seedling disease)Kurosawa and Sawada confirmed that rice plants become thin, tall & pale due to infection offugus Gibberella fujikuroi (Ascomycetes).

Yabuta and Sumiki were first to extrat a crystalline substance from the Gibberella fungus to whichnamed Gibberellin.

More than 100 type of Gibberellin (GA1, GA2 GA3 ...........GA100) are known. GA3 [C19H26O6] isrepresentation of all Gibberellins.

They are synthesised in buds, shoot, root and germinating seeds.

PHYSIOLOGICAL EFFECTS OF GIBBERELLINS -

(1) Stem/internode elongation (characterstic function of gibberellins) :- Gibberellins induce in-ternal elongation just prior to flowering in Rosette plants (Beet and Cabbage), this phenom-enon is known as Bolting effect.

(2) Elongation of genetically dwarf plants :- When gibberellins are applied to dwarf Maize, Pisumplants then they become tall.

(3) Seed germination :- Gibberellins induces the synthesis of hydrolysing enzymes like a-amylase,Lipas Proteases in seeds and these enzymes help in seed germination by promoting breakdown ofendosperm.

(4) Fruit & Flower enlarger :- Size of grape fruits and length of stalk of bunch is increased byGA Pomalin GA +CK – used as apple enlarger..

(5) In fermentation : - GA increases malt formation. (Malting process) so helpful in brewing(wine) inducance


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(6) Increase height of Sugarcane plant :- GA can increase the yield in sugarcane upto 20tonnes acre.

(7) GA delays the senescence in plant parts so the fruits can be left on the tree longer so as toextend market period ny application of GA.

(8) Spraying juvenile conifers with GAs, hastens the maturity period, thus leading to earlyseed duction.

CYTOKININ (CK)

DISCOVERY

F.Skoog and his coworrker observed in tobacco that callus can proliferate only if, in addition to auxin nutrient medium was supplemented with one of the substance like extract of vascular tissues, extract , coconut milk of DNA.

Skoog and Miller later identified and crystellised an active substance from autoclaved DNA of Herring sperm , which stimulated cell division (mainly Cytokinesis). They named this substance as kinetin does not occur naturally in plants.

The first natural cytokinin was identified & crystalized from immature corn grains (corn ker-nels) coconut milk by Letham & named it as Zeatin.

Cytokinins are derivative of Adenine nitrogen base.

Root tips are major sites of biosynthesis of CK. Also synthesised in developing shoot buds,fruits etc.

PHYSIOLOGICAL EFFECTS OF CYTOKININS -

Cell Division (Characteristic function of cytokinin)

Morphogenesis :- Morphogenetic changes (root and shoot development) are induced by CK inpresence of IAA in tissue culture.

Low CKHigh Auxin

root differentiation High CK

Low Auxin shoot differentiation

Counteraction of apical dominance :- C.K. promotes growth of lateral buds.Delay in senescence (Richmond Lang Effect) :- Senescence is delayed by CK because theyimproves the pholem condition and nutrient mobilisation. Also help to produce chloro-plasts in leaves.Promots lateral shoot growth and adventitious shoot formation.

ABSCISIC ACID (ABA)DISCOVERY -First Growth Inhibitor -was identified and isolated by Bennet Clark (1953) and Ketford from dormant Potato tuber and called Inhibitor-B.Addicott & Okhuma isolated a substance from mature cotton fruits and named it as Abscission - II.Warning & Robinson isolated a growth inhibitor from old Betula leaves & called it Dormin. Later it was proved that inhibitor - B, Abscission-II & Dormin are same and they were


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regarded as Abscisic acid.ABA is mainly synthesised in senescent organs, old leaves.

ABA is also known as stress hormone because it protects plants from adverse conditionslike water stress by inducing closing of stomata. ABA increases tolerance of plants tovarioustype of stresses.Common growth inhibitor in plants is ABA.

PHYSIOLOGICAL EFFECTS OF ABA -

Induces senesence and abscission - ABA cause senscence and abscission of leaves and fruits by increasing the activity of cellulase & pectinase enzymes.

Induce Bud &Seed Dormancy - ABA increases bud & seed dormancy (Inhibits seed ger-mination).

ABA plays a major role in seed maturation enabling seeds to become dormant. Stomatal

closing - ABA close stomata under the water stress conditions. (Anti transpirant).

In most situations ABA act as an antagonist of GAs (Anti GA)

ETHYLENE

DISCOVERY -Ethylene can be included in both groups growth promotor and growth inhibitor but mainly it is a growth inhibitor hormone.Ethylene is a simple gaseous PGR. It is a hydrocarbon reported as a fruit ripening hormone.

H.H. Cousins confirmed the release of volatile substance from ripened oranges that has-tened the ripening of stored unripened bananas. Later this volatile substance was identified as ethylene, a gaseous PGR.

Biosynthesis of Ethylene takes place by methionine amino acid. Ethylene is synthesised in large quantity by ripening fruits and senescent organs.

PHYSIOLOGICAL EFFECTS OF ETHYLENE -

(1) Post harvest ripening of fruits - Citrus, Oranges , Banana, Apple, Tomato etc. Now a days Ethephonyl CEPA (Chloroethyl Phosphonic acid) is used at commerical level as source of ethylene. Ethylene enhancement the respiration rate during ripening of the fruits. this is called respiratory climactic.

(2) Stimulation of senescence & abscission of flower and fruit. Thinning of cotton, cherry, walnut.

(3) Flowering and synchronising fruiting in pineapple. Flowering in mango.

(4) Triple response on stem :- (i) Apical hook formation in dicots seedlings (ii) Swelling of axis(iii) Horizontal growth of seedlings


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(5) Promotes root growth :- Ethylene promotes root growth and also stimulates the formation ofroot hairs, thus helping the plant to increase their absorption surface.

(6) Femaleness (Ferminising effect) in cucumbers. (Promotes female flowers to increase theproduction

(7) Ethylene breaks seed and bud dormancy, initiates germination in peanut seeds, sproutingof potato tubers.

(8) Ethylene promotes rapid internode/petiole elongation in deep water rice plants. It helpsleaves/upper parts of the shoot to remain above water.

PHOTOPERIODISM

Effect or requirement of relative length of day (photoperiod) & night (dark phase) on flowering ofplants called as photoperiodism.

The phenomenon of photoperiodism was first discovered by Garner & Allard on Maryland mam-moth (mutant veriety of Tobacco) and Biloxy soyabean.

Garner & Allard classified the plants in following groups -

(1) SDPs (Short Day Plants) :-

These plants flower on exposure to photoperiod shorter than their Critical day length.

In SDPs the dark period is critical and must be continuous. Thus SDP are also called as LNP(Long Night Plants). If this dark period is interrupted even with a brief exposure of light,the SDP will not flowering.

Examples of SDPs :- Tobacco, soyabean, Viola Xanthium (Cocklebur), Chrysanthemum, Can-nabis Coleus, Chenopodium, Musturd, Dahlia, Sugarcane, Strawberry, Cosmos, Rice, Bryo-phyllum etc.

(2) LDPs (LongDay Plants) :-

These plants flowers when they are exposed to photoperiod longer than their critical dayA brief exposure of light in the dark period stimulates flowering in LDPs.The light period is critical for LDPs.

Gibberellin can induce flowering in LDPs even under non-inductive photoperiod condi-tion.Ex. : Henbane (Hyoscyamus), Spinach, Sugarbeets, Radish, Carrot, Wheat, Larkpur, Bar-ley, Avena Potato.


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(3) DNPs (Day Netrual Plants) :-These plants do not need a specific light period for the flowering.Ex. Maize, Cotton, Tomato, Sunflower, Cucumber

Phytochrmoe :-A light sensitive pigment phytochrome is responsible for flowering induction and present inleaves.Phytochorme exists in two different forms -Pr (Phytochrome Red)Pfr (Phytochrome Far Red)

Both forms of phytochrome are photobiochemically interchangeable into each other.

In LDP flowering inhibitRed light (660 NM)

Far red light (730 NM)PfrPr

DarkIn SDP flowering inhibits

In SDP stimulates In LDP stimulates

Phytochrome - Pfr (P730) is active form which controls many photophysiological processes in plantslike - flowering, seed germination etc.

Stimulation of Critical photoperiod is perceived by leaves.

“Florigen”, Hypothetical hormone (not isolated yet from plants) which migrates from leavesto shoot apical for inducing flowering.

VERNALISATION OR YAROVIZATION

Effect of low temperature on the initiation and development of flowers, was first realised by Klippart (Experiments on winter wheat and Spring wheat)

Mainly embryo tip, shoot apex & leaves perceive stimulus of low temperature.

Concept of hormone ‘Vernalin’ in vernalisation was given by Melcher. This is a hypothetical plant hormone because has not been isolated.


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Vernalisation of seeds or plant propagules in laboratory can be induced at 1oC to 10oC in pres-ence of O2 & H2O.

If vernalized plant propagules are kept at high temperature then effect of vernalisation is reversed,this is called devernalization.

Two types of varieties are found in wheat, barley and rye (cereal) -winter and spring vari-ety. Winter varieties require vernalisation whereas spring varieties do not.

Sugar beet, Cabbage, carrot etc. are biennial plants.

Significance :-

(i) Better & early flowering.(ii) Prevention of precocious reproductive development late in the growing season.(iii) Shorting the life span of the plants.

PLANT SENESCENCE

Period from maturity to death of an organ or plant is know as senescence.

During the senescencem higher rate of catabolism is started, under the control of growth hormoneslike ABA and ethylene. Senescence occurs as a result of ageing and leads to the death of plantparts or whole plant. (Study of senescence and ageing is Phy-Gerontology)

ABSCISSION

Detechment of senecent or mature plant organs like leaves, fruits, flowers due to change in hormonal activity.

There is the formation of separation layer (Abscission layer) at the region of attachment of these parts. Cell wall layers and middle lamella are dissolved by the activity of cellulase and pecitinases enzymes during the abscission.

Hormone ABA is main controller of abscission process.

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