Study Notes on Plant Growth and Development

Study Notes on Plant Growth and Development

  • Growth in plants occur as the stem and root lengthens.
  • Development is the sum of two processes : growth and differentiation.

GROWTH

  • Irreversible permanent increase in size of an organ or its part or even of an individual cell.
  • Growth is accompanied by metabolic processes.
  • This occurs at the expense of energy.
  • For example expansion of leaf is growth.

PLANT GROWTH GENERALLY IS INDETERMINATE.

  • Plant growth is unique
  • They retain the capacity for unlimited growth throughout their life
  • This ability of the plants is due to the presence of meristems at certain location in their body.
  • Cells of meristems have the capacity to divide.
  • Shoot apical meristem and root apical meristem are responsible for the primary growth of the plants.
  • They contribute to the allegation of the plants along their axis.
  • In dicotyledonous plants and gymnosperms the lateral meristems vascular cambium and cork cambium cause the increase in the gift of the organs in which they are active.
  • They appear later in life.
  • This growth is known as secondary growth in plant.

GROWTH IS MEASURABLE.

  • At cellular level growth is the consequence of increase in the amount of protoplasm.
  • Growth can be measured by a variety of parameters.
  • There can be increase in fresh weight, dry weight, length area, volume and cell number.
  • For example 1 single maize root apical meristem can give rise to more than 17500 new cells per hour.
  • In watermelon cells may increase in size by up to 350000 times.

PHASES OF GROWTH

  • There are three phases of growth
  • Meristematic , elongation and maturation
  • The constantly dividing cells both at root apex and shoot apex represent the meristematic phase of growth.
  • The cells in this region are rich in protoplasm and causes large visible nuclei.
  • Their cell walls are primary in nature and cellulosic with abandoned plasmodesmata connections.
  • The cells proximal to meristematic zone represent the phase of elongation.
  • The cell in the phase of elongation undergoes Vacuolation, Cell enlargement ,Cell wall deposition.
  • More proximal to the phase of elongation lies the portion of axis which is undergoing the phase of maturation.
  • The cells in this zone attend their maximum size in terms of wall thickening and protoplasmic modifications.

GROWTH RATES

  • The increased growth per unit time is termed as growth rate.
  • Rate of growth can be expressed mathematically.
  • Growth rate shows an increase that may be arithmetic or geometrical.
  • In arithmetic growth following mitotic cell division only one daughter cell contain used to divide while the other differences and mature.
  • On plotting the length of the organ against time a linear curve is obtained.
  • Mathematically it is expressed as : Lt = Lo + rt
  • In most systems the initial growth is slow (lag phase).
  • It increases rapidly thereafter.
  • This takes place at an exponential rate (log or exponential phase)
  • Here both the progeny cells following mitotic cell division retain the ability to divide.
  • With limited nutrient supply the growth slows down leading to a stationary phase.
  • If the parameter of growth is plot against time we get a typically sigmoid or S curve.
  • A sigmoid curve is a characteristic of living organism growing in a natural environment.
  • The exponential growth can be expressed as:
  • W1 = Wo ert
  • r is the relative growth rate and is also the measure of the ability of the plant to produce new plant material referred to as efficiency index.
  • The final size W1 of depends on the initial size, W
  • Quantitative comparison between the growth of living system can also be made in two ways.
  • First is measurement and the comparison of total growth per unit time is called the absolute growth rate.
  • Second is the growth of the given system per unit time expressed on a common basis.

CONDITIONS FOR GROWTH.

  • The plant cells grow in size by cell enlargement which in turn requires water.
  • Turgidity of the cells helps in extension growth.
  • plant growth and for the development is intimately linked to the water status of the plant.
  • Water also provides the medium for enzyme attic activities needed for growth.
  • Oxygen helps in releasing metabolic energy essential for growth activities.
  • Nutrients are required by plants for the synthesis of protoplasm and act as a source of energy.
  • Every plant organism has an optimum temperature range best suited for its growth.
  • Environmental signals such as light and gravity also affect certain phases / stages of growth.

DIFFERENTIATION , DIFFERENTIATION AND RE DIFFERENTIATION.

  • The cells derived from root apical and shoot apical meristems and cambium differentiate and mature to perform specific functions.
  • This leads to maturation and is termed as differentiation.
  • During this cells undergo few major structural changes both in their cell walls and protoplasm.
  • The living differentiated cells that have lost the capacity to divide can regain the capacity of division under certain conditions.
  • This phenomenon is termed as the dedifferentiation.
  • For example formation of meristems interfascicular cambium and cork cambium from fully differentiated parenchyma cells.
  • Such meristems are able to divide and produce cells that once again lost the capacity to divide but mature to perform specific functions.
  • This is called re differentiation.
  • Differentiation in plants is open because cells / tissues arising out of the same meristem have different structures at maturity.
  • The final structure at maturity of cell / tissue is also determined by the location of the cell within.
  • For example cells position away from root apical meristems differentiate as root cap cells.

DEVELOPMENT

  • It includes all changes that an organism goes through during its life cycle from germination of the seed to senescence.
  • It is also applicable to tissues / organs.
  • Plants follow different pathways In response to environment or phases of life to form different kinds of structures.
  • This ability is called plasticity.
  • For example the leaves of the juvenile plant are different in shape from those in mature plants in some plants like cotton coriander etc.
  • The phenomenon of heterophylly is an example of plasticity.
  • 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 of of plants is under the control of intrinsic and extrinsic factors.
  • Intrinsic factors include both intracellular or intercellular factors while the extrinsic factors include light, temperature, water, oxygen, nutrition etc.

PLANT GROWTH REGULATORS

  • Plant growth regulators are small, simple molecules of diverse chemical composition.
  • They could be indole compounds, adenine derivatives ,derivative of carotenoids, terpenes or
  • Based on functions in a living plant body plant growth regulators can be broadly divided into two groups
  • First group are involved in growth-promoting activities such as cell division, cell enlargement, pattern formation, tropic growth, flowering, fruiting and seed formation.
  • They are also called as plant growth promoters.
  • The second group please an important role in plant responses to wounds and stresses of biotic and abiotic origin.
  • They are involved in various growth inhibitor activities such as dormancy and abscission.

THE DISCOVERY OF PLANT GROWTH REGULATORS

  • There are five major groups of plant growth regulators.
  • Auxin was isolated by F.W. Went from tips of coleoptiles of oats seedlings.
  • Gibberellic acid was identified in plants and fungi.
  • Skoog and Miller identified and crystallised the cytokinesis promoting active substance that they termed kinetin.
  • Chemical characteristics of three different kinds of inhibitors : inhibitor -B, absission II and dormin was reported during mid 1960s.
  • Later they were proved to be chemically identical.
  • It was named abscisic acid.
  • Later ethylene was identified as gaseous plant growth regulators.

PHYSIOLOGICAL EFFECTS OF PLANT GROWTH REGULATORS.

  • Auxins was first isolated from human urine.
  • The term auxin is applied to indole 3 acetic acid and to other natural and synthetic compounds having certain growth regulating properties.
  • They are generally produced by the growing apices of the stems and roots.
  • From there they migrate to the regions of their action.
  • Auxins like IAA and indole butyric acid have been isolated from plants.
  • Synthetic auxins are NAA and 2,4-D.
  • These organs are used in agricultural and horticultural practices.
  • Help to initiate rooting in stem cuttings.
  • Auxins promote flowering.
  • they help to prevent fruit and leaf drop at early stages but promote the abscission of old mature leaves and fruits.
  • The growing apical bud in HIV its the growth of the lateral buds this phenomenon is called apical dominance.
  • Auxins also induce parthenocarpy.
  • They are also used as herbicides.
  • 2,4-D is used to kill dicotyledonous weeds.
  • It does not affect mature monocotyledonous plants.
  • Auxin also controls xylem differentiation and helps in cell division.
  • Gibberellins are another kind of promoter e plant growth regulators.
  • There are more than hundred millions reported .
  • Gibberellic acid was one of the first gibberellins to be discovered.
  • They produce a wide range of physiological responses in the plants.
  • Gibberellins cause fruits like apple to along gate and improve its shape.
  • They also delay senescence.
  • GA3 is used to speed up the malting process ine brewing industry.
  • Kinetin does not occur naturally in plants.
  • Zeatin is the natural substance with cytokinin like activities.
  • It is found in corn kernels and coconut milk.
  • Natural cytokinins are synthesized in regions where rapid cell division occurs.
  • it has to produce new leaves chloroplast in leaves lateral should growth and adventitious growth shoot growth.
  • It helps to overcome the apical dominance.
  • They promote nutrient mobilization which helps in the delay of leaf senescence.
  • Ethylene is the simple gaseous plant growth regulator.
  • It is synthesized in large amounts by tissues undergoing senescence and ripening fruits.
  • Influences of Ethylene on plant includes horizontal growth of seedlings swelling of the axis and apical hook formation in dicot seedlings.
  • It promotes senescence and abscission of plant organs especially of leaves and flowers.
  • It enhances the respiration rate during ripening of the fruits.
  • This is called respiratory climatic.
  • Ethylene breaks seed and bud dormancy.
  • It initiate germination in peanut seeds, sprouting of potato tubers.
  • Ethylene also promotes root growth and hair formation.
  • This helps the plant to to increase their absorption surface.
  • Since ethylene regulate so many physiological processes it is is one of the most widely used plant growth regulators in agriculture.
  • The most widely used compound as source of Ethylene is ethephon.
  • Ethephon in an aqua solution is really absorb and transported within the plant and releases ethylene slowly.
  • Abscisic acid was discovered for its role in regulating abscission and dormancy.
  • It acts as an general plant growth inhibitor an inhibitor of plant metabolism.
  • It also inhibits seed germination.
  • It stimulates the closure of stomata in the epidermis and increases the tolerance of plants to various kinds of stresses.
  • It is called the stress hormone.
  • it helps the seeds to with stand desiccation and other factors unfavourable for growth.
  • It acts as an antagonist to Gas.
  • For any and every phase of growth, differentiation and development of plants one or the other plant growth regulators has some role to play.
  • The roles could be complementary or antagonistic.
  • There are number of events in the life of a plant where more than one plant growth regulator interact to affect that event.

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 required the exposure of light for a period exceeding a very define 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.
  • Day neutral plants do not have any correlation between exposure to light duration and induction of flowering response.
  • Duration of dark period is also important.
  • Flowering in certain plants depends not only on a combination of light and dark exposes but also their relative duration.
  • This response of plants to periods of day / night it is termed as photoperiodism.
  • The site of perception of light / dark duration are the leaves.
  • A hormonal substance migrates from leaves to shoot apices for inducing flowering only when the plants are exposed to the necessary inductive photoperiod.

VERNALISATION

  • It is the phenomenon in which flowering in some plants is either quantitatively or qualitatively dependent on exposure to to low temperature.
  • It helps to to enable the plant to have sufficient time to reach maturity.
  • It mainly refers to the promotion of flowering by a period of low temperature.

REFERENCES

  1. https://ncert.nic.in/textbook/pdf/kebo115.pdf