Notes-Part-1-Class-12-Biology-Chapter-7-Plant Growth and Mineral Nutrition-Maharashtra Board

Plant Growth and Mineral Nutrition

Maharashtra Board-Class-12th-Biology-Chapter-7

Notes-Part-1

Topics to be Learn : Part-1

  • Plant growth
  • Phases of growth
  • Conditions for growth
  • Growth rate and types of growth
  • Growth curve
  • Differentiation, De-differentiation. Re-differentiation
  • Development
  • Plasticity
  • Growth Hormones

Topics to be Learn : Part-2  <-(click to view)

  • Photoperiodism
  • Vernalization
  • Mineral nutrition
  • Nitrogen cycle

Plant Growth :

Growth is one of the characteristic features of living organisms.

Two aspects of growth :

(i) Quantitative : Increase in length size, volume, numbers body mass, dry weight, etc.

(ii) Qualitative : It’s about a change in nature of growth.

  • Development- orderly progress
  • Differentiation — leads to higher and more complex state.

Growth : A permanent, irreversible increase in the bulk of an organism, accompanied by the change of form.

Growth in multicellular/vascular plants :

In multicellular (vascular) plants,

  • Growth is indeterminate.
  • Occurs throughout the life indefinitely.
  • It is restricted to some specific region called meristems
  • Meristems :  Meristems are the regions where new cells are constantly and continuously produced. Meristems are of three types based on location viz. Apical, Intercalary and Lateral.
  • In unicellular plants, determinate and uniform growth is observed, while in multicellular plants certain organs like leaf, flowers and fruits show determinate growth.
Know this :

Apical meristem : In vascular plants, growth is restricted to the apices of root and shoot. It is responsible for growth in length/ height and the differentiation or cell types. It contributes to the primary growth.

Intercalary meristem : It is located at the node or at the base of internode of stem. It is primarily responsible for increasing length of internodes and also for formation of leaf primordia and lateral buds.

Lateral meristem : It is located laterally along the axis of dicotyledons and gymnosperms. It is located as strip in the vascular bundles of stem of dicots. It is called vascular cambium. It is responsible for increase in the girth of the stem due to addition of secondary vascular tissues.

Phases of growth :

The cells in the meristem divide, enlarge and get differentiated. Corresponding to these three stages, there are three phases of growth, viz, formation phase, elongation phase and maturation phase.

Three phases of growth :

(i) Formative phase (Phase of cell division) :

  • This is the first phase of growth.
  • In this phase, the meristematic cells undergo mitosis to produce new cells.
  • Cells of meristem are thin walled, non-vacuolated having prominent nucleus and granular cytoplasm.
  • Owing to the formation of new cells, one cell remains meristematic and the other cell undergoes enlargement and differentiation.
  • In this phase, rate of growth occurs at a slower pace (Lag phase).

(ii) Elongation phase (Phase of cell enlargement) :

  • This is the second phase of growth.
  • In this phase, the new cells that are formed, undergo enlargement as a result of which the size and volume of the cells increase.
  • The newly formed cell becomes vacuolated, osmotically active and turgid due to absorption of water.
  • The turgidity results in the enlargement of cell - both lengthwise and breadthwise.
  • The enlargement of cells causes a considerable increase in size and weight of an organ and a plant as a whole.
  • In this phase new wall materials and other materials are synthesized to cope up with the enlargement.
  • The growth rate in this phase occurs at an accelerated pace (exponential or Log phase).

Click here to View Figure-1

(iii) Maturation phase (Phase of cell maturation and differentiation) :

  • Maturation phase is the third and last phase of growth.
  • The enlarged cell now becomes specialized to perform specific function and attains maturity - both morphological and physiological.
  • In this phase, rate of growth slows down and comes to a steady state (Stationary phase).

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Know this :

Root and stem show indefinite or indeterminate growth. However, in organs like leaves, flowers and fruits, growth is determinate. i.e. they grow up to certain genetically destined size. In unicellular plants, growth is uniform and determinate.

Characteristics of growth :

Characteristics of growth :

  • Growth is a permanent increase in size, weight, shape, volume and dry weight of a plant.
  • The change occurring due to growth is permanent and irreversible.
  • Growth is an intrinsic process caused due to internal activities.
  • Growth occurs by cell division and cell elongation followed by cell maturation which lead to the formation of different types of tissues.
  • Growth in plants is mostly localized, i.e. restricted to some regions of plants possessing meristematic tissues or meristems.
  • Growth has a qualitative aspect where development takes place in an orderly manner and differentiation leads to higher and more complex state.

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Conditions for Growth :

Conditions for Growth :

For a proper growth of plant various environmental and physiological conditions are necessary.

  • Carbon/Nitrogen ratio in soil is having effect on growth as both carbon and nitrogen are structural elements in carbohydrates, proteins and other biomolecules.
  • Water is essential component of protoplasm and required for turgidity of cells during cell enlargement phase. It is a medium in which various biochemical reactions occur.
  • Nutrients are necessary for proper growth, Macronutrients and micronutrients have their specific role.
  • Temperature of 25 — 35 °C is optimum for growth.
  • Light is essential for seed germination and photosynthesis.
  • Oxygen is necessary for respiration and supply of energy.
  • Gravitational force decides direction of growth for root system and shoot system.
  • Growth hormones are organic compounds that are involved in various physiological aspects and control of growth.

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Growth Rate and types of growth :

Growth rate : It is the increased growth per unit time. It is also called efficiency index.

Growth in plants can be measured as increase in

  • number-e.g. Cells
  • surface area-e.g. Leaf
  • length-e.g. pollen tube
  • Volume-e.g. fruit
  • Girth-e. g. stem
  • Dry weight

Various methods for measurement of linear growth :

Various methods for measurement of linear growth :

  • Direct method : Measurement with scale
  • Horizontal microscope : Useful for measuring growth in fields.
  • Auxanometer : For linear growth of shoot-2 types — Arc auxanometer and Pfeffer’s auxanometer.
  • Crescograph : Record of primary growth, information of growth per second. It is developed by Sir J. C. Bose.
  • Growth Rate/Efficiency index : Increased growth per unit time. e.g. Increase in area of leaf, size of flower, etc.
  • Absolute growth rate (AGR) : Ratio of change in cell number (dn) over time interval (dt) i.e. AGR = dn/dt i.e. total growth per unit time.
  • Relative growth ratio (RGR) : AGR when divided by total number of cells present i.e. growth of given system i.e. RGR = AGR/n i.e. ratio of growth in given time / initial growth.
  • For describing cell growth in culture AGR and RGR are useful.

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Types of growth :

Two types of growth : (i) arithmetic growth and (ii) geometric growth.

(i) arithmetic growth and (ii) geometric growth :

Arithmetic growth :

  • Rate of the growth is constant hence linear curve.
  • After mitosis one of the daughter cell continues to divide and the other cell takes part in the differentiation and maturation.
  • e.g. elongation of root at a constant rate,
  • Linear curve is obtained when growth rate is plotted against the time.

Arithmetic growth is expressed mathematically by an equation as,

Lt = Lo + rt

Where

  • Lt = Length at time ‘t’
  • Lo = Length at time ‘Zero’
  • r = Growth rate
  • t = Time of growth

Geometric growth :

  • Cell divides mitotically into two.
  • Both the daughter cells continue to divide and redivide repeatedly.
  • Such growth is called geometric growth.
  • Growth rate is slow initially but later on there is a rapid growth at exponential rate.

Geometric growth can be expressed mathematically by an equation as,

W1 = Wo ert

Where,

  • W1= Final size ,
  • Wo = initial size
  • r = growth rate, t = time of growth
  • e = base of natural logarithm

Click here to View Figure-2 

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Growth curve :

Growth curve :

Graphic representation of the total growth against time is known as growth curve.

Click here to View Figure-3 

  • Growth rate is low in lag phase, faster growth rate reaching maximum in exponential or log phase and is gradually slows down in stationary phase.
  • Sigmoid curve is obtained when rate of growth plotted against time for all three phases.
  • Grand period of growth (GPGJ : The total period required for all phases (Lag, log and stationary) to occur is called grand period of growth.

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Differentiation, De-Differentiation, Re-Differentiation :

Differentiation, De-Differentiation, Re-Differentiation :

Differentiation:

  • It is a process of maturation of cells derived from apical meristems.
  • Differentiation is a permanent change in structure and function of cells that leads to its maturation.
  • Cell undergoes major anatomical and physiological change during differentiation process.
  • In hydrophytic plants parenchyma cells develop large schizogenous cavities which help them in aeration, buoyancy and mechanical support.

De-differentiation :

  • It is a process or ability where living differentiated cells regain the capacity to divide thus permanent cells become meristematic. e.g. Cork cambium,
  • Parenchyma cells forming interfascicular cambium for secondary growth.

Re-differentiation :

  • It is a process in which cells produced by de-differentiation lose their capacity of division and become mature.
  • The cells mature to perform specific function.
  • Interfascicular cambium is formed by process of dedifferentiation loses its capacity to divide.
  • Secondary xylem and secondary phloem is formed form this cambium in vascular cylinder.

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Development :

  • Development is progressive changes taking place in shape, form and degree of complexity in an organism.
  • In plants, it includes all the changes taking place in sequence from seed germination to senescence or death of plant.

Click here to View Figure-4 

  • Development is an orderly process.
  • It includes growth, morphogenesis, maturation and senescence.

Plasticity :

  • Plasticity is the capacity of plant being molded or formed.
  • It is ability of plant to develop different kinds of structures in response to environmental factors or stimuli.
  • Different kinds of structures can be developed in plants due to internal stimuli in different phases, i.e. juvenile and adult.
  • Environmental Plasicity is observed in Butter cup (Ranunculus Flabellasis)
  • Heterophylly is shown in plant in different phases or in different environmental conditions.
  • In coriander and cotton plants, two different kinds of leaves are observed in young (juvenile) and mature (adult) plant.
  • In buttercup, two different kinds of leaves are observed in terrestrial (on land) and aquatic habitat.

Click here to View Figure-5 

Growth Hormones :

The term ‘hormone’ was coined first by Starling (1906) in animal physiology.

Growth Regulators or Growth Hormones - These are the internal factors which influence growth i.e. inhibit, promote or modify growth.

  • Growth promoters :  Auxins, gibberellins (GA) and cytokinins (CK).
  • Growth inhibitors : Ethylene and abscissic acid (ABA).
  • Growth regulators. : All phytohormones

Plant hormones are organic substances produced naturally that affect growth or other physiological functions at a site away from their place of production.

To evoke the response hormones are needed in very small amount and they are mainly transported through phloem parenchyma.

Scientist and their work :

  • Charles Darwin : Discovery of auxin with tropism studies of canary grass coleoptile exposure to light.
  • Boysen –Jensen : Observations of bending of coleoptiles with gelatin sheet insertion experiment-effect of auxin.
  • Paal : Observed coleoptile bending due to auxin even in dark.
  • F. W. Went : Successfully isolated natural auxin Avena coleoptile tips in agar blocks – Avena curvature assay.

 Auxins : Term given by F.W. Went

  • First isolated from human urine, while in plants synthesised in apical meristematic region.
  • IAA — i.e. Indole 3 acetic acid - most common natural auxin, synthesised from amino acid Tryptophan.
  • Synthetic auxins — IBA (Indole butyric acid], NAA (Naphthalene acetic acid), 2, 4-D (dichloro Phenoxy acetic acid).

Physiological effects and applications of auxins :

Physiological effects and applications of auxins :

  • Cell elongation and cell enlargement.
  • Apical dominance — Growing apical bud inhibits growth of lateral buds
  • Stimulation of growth of root and stem.
  • Multiplication of cells hence utilized in tissue culture
  • Formation of lateral and adventitious roots 2, 4-D is selective herbicide — kills dicot weeds
  • Induced parthenocarpy— seedless grapes, banana, lemon, orange
  • Promote cell division and early differentiation of vascular tissue xylem and phloem.
  • Induces early rooting in cutting method of artificial vegetative propagation.
  • Foliar spray of synthetic auxins — Flowering induced in litchi and pineapple, prevents early fruit drop of apple, pear and oranges,prevents formation of abscission layer.
  • Increase in rate of respiration.
  • Break seed dormancy and promote seed germination.

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Gibberellins (GA) : Named by Yabuta and Sumuki

  • First isolated from fungus Gibberellafujikuroi by Kurasawa.
  • Rice seedlings show Bakane disease with stem elongation due to this fungus infestation.
  • Yabuta and Sumuki isolated it from fungus culture.
  • Synthesised from mevalonic acid in young leaves, seeds and root, stem tips.
  • GA3 is most common and biologically active — Contains gibbeane ring.

Physiological effects and applications of Gibberellins :

Physiological effects and applications of Gibberellins :

  • Breaking of bud dormancy, seed dormancy.
  • By promoting synthesis of amylase in cereals, their seed germination can be stimulated e.g. Wheat, barley.
  • Increase in length of internodes thereby elongation of stem.
  • Bolting in rosette plants - elongation of internodes before flowering e.g. Cabbage, beet Parthenocarpy in tomato, apple, pear.
  • Stimulates flowering in long day plants.
  • Increase in fruit size and bunch length e.g. grapes.
  • Overcomes effects of vernalization.
  • Inhibition of root growth, delay senescence and abscission.
  • Production of male flowers on female plants.
  • They convert genetically dwarf plants to phenotypically tall plants e.g. maize.

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Cytokinin : Term coined by Letham.

  • Promote cell division — Natural source -Banana flowers, apple and tomato fruits.
  • Discovered by Skoog and Miller in Callus culture of Tobacco — by supplementing media with coconut milk.
  • Present in herring (fish) sperm DNA — Kinetin.
  • Cytokinins are derivatives of adenine, a purine base. Chemically 6-furfuryl amino purine.
  • First natural cytokinin obtained by Letham from maize grain Zeatin.
  • Synthetic hormone — 6 benzyl adenine.
  • Important in plant tissue culture (callus) for morphogenesis.

Physiological effects and applications of cytokinin :

Physiological effects and applications of cytokinin:

  • Promote cell division and cell enlargement
  • Promote shoot formation, buds
  • Cytokinin and auxin ratio controls morphogenesis.
  • Growth of lateral buds, controls apical dominance
  • Delay of ageing and senescence, also abscission
  • Formation of interfascicular cambium
  • Breaks dormancy, promotes germination
  • Reverse apical dominance effect
  • Induce RNA synthesis.

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Ethylene : Denny (1924) reported effect in fruit ripening.

  • Gane (1934) reported natural synthesis of this gaseous hormone in plants.
  • Synthesised in roots, shoot apical meristems and fruits during ripening.
  • It is an unsaturated, colourless, hydrocarbon gas
  • Commercially used source — Ethephon
  • Described as ripening hormone.

Physiological effects and applications of ethylene :

Physiological effects and applications of ethylene :

  • Promotes ripening of fruits
  • Stimulates initiation of lateral roots
  • Breaks dormancy of buds and seeds.
  • Acceleration of abscission activity by forming abscission layer.
  • Inhibits growth of lateral buds, i.e. apical dominance.
  • Retardation of flowering.
  • Enhancement of senescence.
  • Epinasty - Drooping of leaves and flowers e.g. Pineapple.
  • Degreening effect — Stimulate activity of enzyme chlorophyllase causing loss of green colour in fruits of Banana, Citrus.

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Abscissic Acid :

  • Responsible for shedding of cotton balls and was named as abscisin I and II by Carns and Addicott.
  • Isolated from buds of Acer that causes bud dormancy, substance named Dormin by Wareing.
  • These substances were renamed abscissic acid, chemically 15 — C sesquiterpenoid — synthesised from mevalonic acid.
  • Leaves, fruits, roots, seeds synthesise this.

Physiological effects and applications of ABA :

Physiological effects and applications of ABA:

  • Promote abscission of leaves — beneficial for stress — drought
  • Induces dormancy in buds and seeds
  • Accelerates senescence of leaves flowers and fruits.
  • Delay of cell division, cell elongation and suppression of cambial activity— Inhibit mitosis.
  • Causes efflux of K+ ions from guard cells and thus closure of stomata—used as antitranspirant.
  • Stress hormone— Overcome stress by inducing dormancy, inhibiting growth thus face adverse environmental conditions.
  • Inhibit flowering in long day plants and stimulate flowering in short day plants.
  • Inhibits growth stimulated by gibberellin.

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