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

(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).

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.

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.

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.

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 e^rt

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 

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.

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.

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.

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.

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.

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.

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.