CHAPTER 15 PLANT GROWTH AND DEVELOPMENT

Chapter 15 Plant Growth And Development-Free PDF

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2 CHAPTER 15 PLANT GROWTH AND DEVELOPMENT https biologyaipmt com. 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, The root apical meristem and the shoot apical meristem are responsible for the 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 and 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,Phases of Growth, The period of growth is generally divided into three phases. namely meristematic elongation and maturation, 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 Fig Detection of zones of. with abundant plasmodesmatal connections elongation by the parallel line. The cells proximal just next away from the tip to the technique Zones A B C D. meristematic zone represent the phase of elongation Increased immediately behind the apex have. elongated most, 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 is termed as growth rate. Thus rate of growth can be expressed mathematically. The growth rate shows an increase that may be arithmetic or geometrical. 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. On plotting the length of the organ against time a linear curve is obtained Mathematically it is. expressed as Lt Lo rt,Lt length at time t,L0 length at time zero. r growth rate elongation per unit time, In geometrical growth the initial growth is slow lag phase and it increases rapidly thereafter at.
an exponential rate log or exponential phase Here both the progeny cells following mitotic cell. division retain the ability to divide and continue to do so However with limited nutrient supply. the growth slows down leading to a stationary phase. 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,The exponential growth can be expressed as. Prepared by,Dr Anurag mittal, 3 CHAPTER 15 PLANT GROWTH AND DEVELOPMENT https biologyaipmt com. Wt final size weight height number etc,Wo initial size at the beginning of the period. r growth rate,t time of growth,e base of natural logarithms. Here 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 Hence the final size of W1 depends on the initial. Fig Diagrammatic representation of a Arithmetic b Geometric growth. and c Stages during embryo development showing geometric and. arithmetic phases,Prepared by,Dr Anurag mittal, 4 CHAPTER 15 PLANT GROWTH AND DEVELOPMENT https biologyaipmt com.
Quantitative comparisons between the,growth of living system can also be made. in two ways,i measurement and the comparison of,total growth per unit time is called the. absolute growth rate,ii 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, Fig Diagrammatic comparison of absolute and relative.
growth rates Both leaves A and B have increased their. area by 5 cm2 in a given time to produce A1 B1 leaves. Conditions for Growth, water oxygen and nutrients are very essential elements 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. oxygen helps in releasing metabolic energy essential for growth activities. Nutrients macro and micro essential elements are required by plants for the synthesis of. protoplasm and act as source of energy, In addition 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 the cells would lose their protoplasm They. also develops a very strong elastic lignocellulosic secondary cell walls to carry water to long. distances even under extreme tension, differentiated cells that 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. meristems interfascicular cambium and cork cambium from fully differentiated parenchyma. While doing so 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. Growth in plants is open i e it can be indeterminate or determinate Now we may say that even. 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 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 cells. while those pushed to the periphery mature as epidermis. DEVELOPMENT, Development is a term that includes all changes that an organism goes through during its life cycle.
from germination of the seed to senescence,Prepared by. Dr Anurag mittal, 5 CHAPTER 15 PLANT GROWTH AND DEVELOPMENT https biologyaipmt com. Fig Sequence of the developmental process in a plant cell. 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. In such plants the leaves of the juvenile plant are different in shape from those in mature plants. 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 This phenomenon of. heterophylly is an example of plasticity,Fig Heterophylly in a larkspur and b buttercup. Thus growth differentiation and development are very closely related events in the life of a plant. Broadly 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 The former includes both intracellular genetic or intercellular factors chemicals. such as plant growth regulators while the latter includes light temperature water oxygen. nutrition etc,Prepared by,Dr Anurag mittal, 6 CHAPTER 15 PLANT GROWTH AND DEVELOPMENT https biologyaipmt com. PLANT GROWTH REGULATORS,Characteristics, The plant growth regulators PGRs are small simple molecules of diverse chemical composition.
They could be,indole compounds indole 3 acetic acid IAA. adenine derivatives N6 furfurylamino purine kinetin. derivatives of carotenoids abscisic acid ABA,terpenes gibberellic acid GA3 or. gases ethylene C2H4, Plant growth regulators are variously described as plant growth substances plant hormones or. phytohormones in literature, The PGRs can be broadly divided into two groups based on their functions in a living plant body. One group of PGRs are involved in growth promoting activities such as cell division cell. enlargement pattern formation tropic growth flowering fruiting and seed formation These are. also called plant growth promoters e g auxins gibberellins and cytokinins. The PGRs of the other group 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.
Interestingly the discovery of each of the five major groups of PGRs have been accidental. Charles Darwin and his son Francis Darwin observed phototropism in Canary grass. F W Went isolated Auxin from tips of coleoptiles of oat seedlings. E Kurosawa reported the appearance of symptoms of the Bakane disease in uninfected rice. seedlings when they were treated with sterile filtrates of the fungus Gibberalla fujikuroi The. active substances were later identified as gibberellic acid. F Skoog and co workers observed callus proliferation in tobacco callus in presence of extracts. of vascular tissues yeast extract coconut milk or DNA in addition to Auxins. Skoog and Miller identified and crystallised the cytokinesis promoting active substance and. termed it kinetin, 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, 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. Fig Experiment used to demonstrate that tip of the. coleoptile is the source of auxin Arrows indicate,direction of light. Prepared by,Dr Anurag mittal, 7 CHAPTER 15 PLANT GROWTH AND DEVELOPMENT https biologyaipmt com. PHYSIOLOGICAL EFFECTS OF PLANT GROWTH REGULATORS, Auxins from Greek auxein to grow was 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. They are 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,All these auxins have been used extensively. in agricultural and horticultural practices, They help to initiate rooting in stem cuttings Fig Apical dominance in plants. an application widely used for plant a A plant with apical bud intact. propagation b A plant with apical bud removed Note the. Auxins promote flowering e g in pineapples growth of lateral buds into branches after. decapitation,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 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, There are more than 100 gibberellins reported from widely different organisms such as fungi. and higher plants They are denoted as GA1 GA2 GA3 and so on. However Gibberellic acid GA3 was one of the first gibberellins to be discovered and remains. the most intensively studied form,All GAs are acidic. They have ability to cause an increase in length of axis It 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. Thus growth differentiation and development are very closely related events in the life of a plant Broadly 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 The former includes both intracellular genetic or intercellular factors chemicals such as plant

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