Key Notes: Plant Growth & Development - NEET PDF Download

• Development is the sum of two processes: growth and differentiation.
  Plant Growth and Development

• The development of a mature plant from a zygote (fertilised egg) follow a precise and highly ordered succession of events. 
• Some factors govern and control these developmental processes. These factors are both intrinsic (internal) and extrinsic (external) to the plant. 
• Growth is regarded as one of the most fundamental and conspicuous characteristics of a living being. 
• Growth can be defined as an irreversible permanent increase in size of an organ or its parts or even of an individual cell. 
• Generally, growth is accompanied by metabolic processes (both anabolic and catabolic), that occur at the expense of energy. 
• Plant Growth → Generally is Indeterminate.
  ⇛ Plant growth is unique because plants retain the capacity for unlimited growth throughout their life.
  ⇛ This ability of the plants is due to the presence of meristems at certain locations in their body.
• The cells of such meristems have the capacity to divide and self-perpetuate. 
• The product, however, soon loses the capacity to divide and such cells make up the plant body. 
• This form of growth wherein new cells are always being added to the plant body by the activity of the meristem is called the open form of growth. 
• Primary growth of the plants and principally contribute to the elongation of the plants along their axis.
• In dicotyledonous plants and gymnosperms, the lateral meristems, vascular cambium and cork-cambium appear later in life. These are the meristems that cause the increase in the girth of the organs in which they are active. This is known as secondary growth of the plant. 
• Growth is Measurable Growth, at a cellular level, is principally a consequence of increase in the amount of protoplasm. Since increase in protoplasm is difficult to measure directly, one generally measures some quantity which is more or less proportional to it. Growth is, therefore, measured by a variety of parameters some of which are: increase in fresh weight, dry weight, length, area, volume and cell number.
• One single maize root apical meristem can give rise to more than 17,500 new cells per hour Increase in number.
• Cells in a watermelon may increase in size by up to 3,50,000 times- Increase in Size 
• Phases of growth → three phases
  ⇛ Meristematic phase
  ⇛ Elongation phase
  ⇛ Maturation phase 
• The constantly dividing cells, both at the root apex and the shoot apex, represent the Meristematic phase of growth.                                                                                                                                                                                     ⇛ The cells in this region are rich in protoplasm, possess large conspicuous nuclei.
  ⇛Their cell walls are primary in nature, thin and cellulosic with abundant plasmodesmatal connections.
• The cells proximal (just next, away from the tip) to the meristematic zone represent the phase of elongation.
  ⇛ Increased vacuolation, cell enlargement and new cell wall deposition are the characteristics of the cells in this phase.
• Further away from the apex, i.e., more proximal to the phase of elongation, lies the portion of axis which is undergoing the phase of maturation.
  ⇛ The cells of this zone, attain their maximal size in terms of wall thickening and protoplasmic modifications.
• Growth rates → The increased growth per unit time; rate of growth can be expressed mathematically. 
• In Arithmetic Growth                                                                                                                                                                          ⇛ following mitotic cell division, only one daughter cell continues to divide while the other differentiates and matures.
  ⇛ The simplest expression of arithmetic growth is exemplified by a root elongating at a constant rate. 
• In Geometrical Growth                                                                                                                                                                 ⇛ both the progeny cells following mitotic cell division retain the ability to divide and continue to do so. 
• If we plot the parameter of growth against time, we get a typical sigmoid or S-curve.                                ⇛ A sigmoid curve is a characteristic of living organism growing in a natural environment.
  ⇛ It is typical for all cells, tissues and organs of a plant. 
• Quantitative comparisons between the growth of living system can also be made in two ways :
  ⇛ measurement and the comparison of total growth per unit time is called the Absolute growth rate.
  ⇛ The growth of the given system per unit time expressed on a common basis, e.g., per unit initial parameter is called the Relative growth rate. 

Conditions for Growth
⇛ Water, oxygen and nutrients as very essential elements for growth.
         ⇛ Water

• 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
  ⇛ Nutrients (macro and micro essential elements)
• required by plants for the synthesis of protoplasm
• act as source of energy.
  ⇛ Temperature
• every plant organism has an optimum temperature range best suited for its growth.
• Any deviation from this range could be detrimental to its survival.
  ⇛ Environmental signals such as light and gravity also affect certain phases/stages of growth

Differentiation, Dedifferentiation and Redifferentiation 

• The cells derived from root apical and shoot-apical meristems and cambium → differentiate and mature to perform specific functions. This act leading to maturation is termed as differentiation.
• During differentiation, cells undergo few to major structural changes both in their cell walls and protoplasm.
• For example, to form a tracheary element.
  a. the cells would lose their protoplasm.
  b. They also develop a very strong, elastic, lignocellulosic secondary cell walls, to carry water to long distances even under extreme tension.
• The living differentiated cells, that by now have lost the capacity to divide can regain the capacity of division under certain conditions. This phenomenon is termed as Dedifferentiation.
• For example, formation of lateral meristems – interfascicular cambium and cork cambium from fully differentiated parenchyma cells. 
• Such meristems/tissues are able to divide and produce cells that once again lose the capacity to divide but mature to perform specific functions, i.e., get Redifferentiated. 

• Development → includes all changes that an organism goes through during its life cycle from germination of the seed to senescence. 
• Plants follow different pathways in response to environment or phases of life to form different kinds of structures. This ability is called plasticity.
  e.g., heterophylly in cotton, coriander and larkspur.
• Thus, growth, differentiation and development are very closely related events in the life of a plant.
• Development is considered as the sum of growth and differentiation. 
• Development in plants (i.e., both growth and differentiation) is under the control of intrinsic and extrinsic factors.
  ⇛ Intrinsic factors

• intracellular (genetic)
• intercellular factors (chemicals such as plant growth regulators)
  ⇛ Extrinsic factors à light, temperature, water, oxygen, nutrition, etc. 

• Plant growth regulators → small, simple molecules of diverse chemical composition. 

⇛ Indole compounds (indole-3-acetic acid, IAA)
⇛ Adenine derivatives (N6 -furfurylamino purine, kinetin)
⇛ Derivatives of carotenoids (abscisic acid, ABA)
⇛ Terpenes (gibberellic acid, GA3 )
⇛ Gases (ethylene, C₂H4 ). 

• Plant growth regulators / plant hormones / phytohormones.
  ⇛ divided into two groups based on their functions in a living plant body.
  ⇛ GROWTH PROMOTERS → involved in growth promoting activities, such as cell division, cell enlargement, pattern formation, tropic growth, flowering, fruiting and seed formation.

• e.g., auxins, gibberellins and cytokinins.
  ⇛ Growth inhibitors → Play an important role in plant responses to wounds and stresses of biotic and abiotic origin. They are also involved in various growth inhibiting activities such as dormancy and abscission.
• The PGR abscisic acid belongs to this group.
• The gaseous PGR, ethylene, could fit either of the groups, but it is largely an inhibitor of growth activities.  

• The Discovery of Plant Growth Regulators à all five major groups of PGRs have been accidental. 
• Charles Darwin and his son Francis Darwin → Phototropism. 
• Auxin was isolated by F.W. Went from tips of coleoptiles of oat seedlings. 
• The ‘bakane’ (foolish seedling) disease of rice seedlings, → fungal pathogen Gibberella fujikuroi. 
• E. Kurosawa reported the appearance of symptoms of the disease in uninfected rice seedlings when they were treated with sterile filtrates of the fungus. The active substances were later identified as gibberellic acid.
• F. Skoog and his co-workers observed that from the internodal segments of tobacco stems the callus (a mass of undifferentiated cells) proliferated only if, in addition to auxins the nutrients medium was supplemented with one of the following: extracts of vascular tissues, yeast extract, coconut milk or DNA. 
• Skoog and Miller, later identified and crystallised the cytokinesis promoting active substance that they termed kinetin. 
• During mid-1960s, three independent researches reported the purification and chemical characterisation of three different kinds of inhibitors: inhibitor-B, abscission II and dormin. Later all the three were proved to be chemically identical. It was named Abscisic acid (ABA). 
• Cousins confirmed the release of a volatile substance from ripened oranges that hastened the ripening of stored unripened bananas. Later this volatile substance was identified as ethylene, a gaseous pgr. 

Auxins

• Greek ‘auxein’ : to grow
  ⇛ first isolated from human urine.
  ⇛ The term ‘auxin’ is applied to the indole-3-acetic acid (IAA), and to other natural and synthetic compounds having certain growth regulating properties.
• ⇛ Generally produced by the growing apices of the stems and roots, from where they migrate to the regions of their action.
  ⇛ Auxins like IAA and indole butyric acid (IBA) have been isolated from plants.
  ⇛ NAA (naphthalene acetic acid) and 2, 4-D (2, 4-dichlorophenoxyacetic) are synthetic auxins.
  ⇛ All these auxins have been used extensively in agricultural and horticultural practices.

• They help to initiate rooting in stem cuttings, an application widely used for plant propagation. 
• Auxins promote flowering e.g. in pineapples.
• They help to prevent fruit and leaf drop at early stages but promote the abscission of older mature leaves and fruits.
• In most higher plants, the growing apical bud inhibits the growth of the lateral (axillary) buds, a phenomenon called Apical dominance.
• Removal of shoot tips (decapitation) usually results in the growth of lateral buds ü It is widely applied in tea plantations, hedge-making.
• Auxins also induce parthenocarpy, e.g., in tomatoes.
• They are widely used as herbicides. 2, 4-D, widely used to kill dicotyledonous weeds, does not affect mature monocotyledonous plants. It is used to prepare weed-free lawns by gardeners.
• Auxin also controls xylem differentiation and helps in cell division. 

Gibberellins

•  Growth promoter
  ⇛ more than 100 gibberellins reported
  ⇛ widely present (fungi to higher plants).
  ⇛ Denoted as GA1 , GA2 , GA3 and so on.
• Gibberellic acid (GA3 ) was first gibberellins to be discovered and most intensively studied
• All GAs are acidic.
• Wide range of physiological responses in the plants.
• Their ability to cause an increase in length of axis is used to increase the length of grapes stalks.
• Gibberellins, cause fruits like apple to elongate and improve its shape.
• They also delay senescence. Thus, the fruits can be left on the tree longer so as to extend the market period.
• GA3 is used to speed up the malting process in brewing industry.
• Sugarcane stores carbohydrate as sugar in their stems. Spraying sugarcane crop with gibberellins increases the length of the stem, thus increasing the yield by as much as 20 tonnes per acre.
• Spraying juvenile conifers with GAs hastens the maturity period, thus leading to early seed production.
• Gibberellins also promotes Bolting (internode elongation just prior to flowering) in beet, cabbages and many plants with rosette habit.

Cytokinins

• Have specific effects on cytokinesis.
• Discovered as kinetin (a modified form of adenine, a purine) from the autoclaved herring sperm DNA.
• Kinetin does not occur naturally in plants.
• Search for natural substances with cytokinin-like activities led to the isolation of zeatin from corn-kernels and coconut milk.
• Natural cytokinins are synthesised in regions where rapid cell division occurs, for example, root apices, developing shoot buds, young fruits etc.
• It helps to produce new Leaves, chloroplasts in leaves, lateral shoot growth and adventitious shoot formation.
• Cytokinins help overcome the apical dominance.
• They promote nutrient mobilisation which helps in the delay of leaf senescence. 

Ethylene

• Gaseous PGR.
• Synthesised in large amounts by tissues undergoing senescence and ripening fruits.
• Influences of ethylene on plants à horizontal growth of seedlings, swelling of the axis and apical hook formation in dicot seedlings.
• Ethylene promotes senescence and abscission of plant organs especially of leaves and flowers.
• Ethylene is highly effective in fruit ripening.
• It enhances the respiration rate during ripening of the fruits. This rise in rate of respiration is called Respiratory climactic.
• Ethylene breaks seed and bud dormancy, initiates germination in peanut seeds, sprouting of potato tubers.
• Ethylene promotes rapid internode/petiole elongation in deep water rice plants. It helps leaves/ upper parts of the shoot to remain above water.
• Ethylene also promotes root growth and root hair formation, thus helping the plants to increase their absorption surface.
• Ethylene is used to initiate flowering and for synchronising fruit-set in pineapples.
• It also induces flowering in mango.
• Since ethylene regulates so many physiological processes, it is one of the most widely used PGR in agriculture.
• The most widely used compound as source of ethylene is ethephon. 
• Ethephon in an aqueous solution is readily absorbed and transported within the plant and releases ethylene slowly.
• Ethephon hastens fruit ripening in tomatoes and apples and accelerates abscission in flowers and fruits (thinning of cotton, cherry, walnut).
• It promotes female flowers in cucumbers thereby increasing the yield. 

Abscisic Acid

• Play role in regulating abscission and dormancy.
• It acts as a general plant growth inhibitor and an inhibitor of plant metabolism. ⇛ ABA inhibits seed germination.
• ABA stimulates the closure of stomata in the epidermis and increases the tolerance of plants to various kinds of stresses. Therefore, it is also called the Stress hormone.
• Important role in seed development, maturation and dormancy.
• By inducing dormancy, ABA helps seeds to withstand desiccation and other factors unfavourable for growth.
• In most situations, ABA acts as an antagonist to GAs. 
• Any and every phase of growth, differentiation and development of plants, one or the other PGR has some role to play.
  ⇛ complimentary or antagonistic.
  ⇛ individualistic or synergistic.
• Role of PGR is of only one kind of intrinsic control. Along with genomic control and extrinsic factors, they play an important role in plant growth and development. 
• Many of the extrinsic factors such as temperature and light, control plant growth and development via PGR. Some of such events could be:
  ⇛ vernalisation
  ⇛ flowering
  ⇛ dormancy
  ⇛ seed germination
  ⇛ plant movements, etc. 

Old NCERT Syllabus

Photoperiodism

⇛ Some plants require a periodic exposure to light to induce flowering.
⇛ Such plants are able to measure the duration of exposure to light.
⇛ Long day plants require the exposure to light for a period exceeding a well defined critical duration.
⇛ Short day plants must be exposed to light for a period less than this critical duration before the flowering is initiated in them.
⇛ The critical duration is different for different plants.
⇛ There are many plants, however, where there is no such correlation between exposure to light duration and induction of flowering response; such plants are called day-neutral plants
⇛ It is now also known that not only the duration of light period but that the duration of dark period is also of equal importance.
⇛ Flowering in certain plants depends not only on a combination of light and dark exposures but also their relative durations. This response of plants to periods of day/night is termed photoperiodism.
⇛ The site of perception of light/dark duration are the leaves. 

• There is a hormonal substance(s) that is responsible for flowering. This hormonal substance migrates from leaves to shoot apices for inducing flowering only when the plants are exposed to the necessary inductive photoperiod. 

Vernalisation

⇛ There are plants for which flowering is either quantitatively or qualitatively dependent on exposure to low temperature.
⇛ It prevents precocious reproductive development late in the growing season, and enables the plant to have sufficient time to reach maturity. 

⇛ Vernalisation refers specially to the promotion of flowering by a period of low temperature.
⇛ Some important food plants, wheat, barley, rye have two kinds of varieties: winter & spring varieties.
⇛ The ‘spring’ variety are normally planted in the spring and come to flower and produce grain before the end of the growing season.
⇛ Winter varieties, however, if planted in spring would normally fail to flower or produce mature grain within a span of a flowering season. Hence, they are planted in autumn. They germinate, and over winter come out as small seedlings, resume growth in the spring, and are harvested usually around mid-summer.

⇛ Another example of vernalisation is seen in biennial plants.
⇛ Biennials are monocarpic plants that normally flower and die in the second season. Sugarbeet, cabbages, carrots are some of the common biennials.
⇛ Subjecting the growing of a biennial plant to a cold treatment stimulates a subsequent photoperiodic flowering response.