GROWTH SUBSTANCES AND ITS ROLES IN VEGETABLE CROP GROWTH
GROWTH SUBSTANCES AND ITS ROLES IN VEGETABLE CROP GROWTH
INTRODUCTION <ul><li>Although photosynthesis supplies the carbon and respiration supplies the energy for plant growth, a group of chemicals produced by plants known as plant growth regulators control the growth and development of trees. These chemicals act on plant processes at very low concentrations. Often they are produced at one location and transported to another where they exert their influence; however, they may also act on the same tissue in which they are produced. </li></ul><ul><li>Before discussing the various groups of plant growth regulators the terms Growth and development should be defined. Growth refers to an increase in size by cell division and enlargement . Development is a term used to refer to differentiation of cells into the various tissues and organs (e.g. leaf versus flower). Plant growth regulators influence both growth and development. </li></ul>
PLANT GROWTH SUBTSTANCES <ul><li>The growth of plants is regulated by certain chemical substances, which are synthesized by the plant in very small quantities. These substances are formed in one tissue or organ of the plant and are then transported to other sites where they produce specific effects on growth and development. They are referred to as plant hormones . Plant hormones are organic compounds which are capable of promotion, inhibition or modification of growth. The plant hormones are also known as growth factors, growth hormones, growth substances, growth regulators or phytohormones . </li></ul><ul><li>Phytohormones are grouped into 2 main types - growth promoters have a positive effect on a process and thus promote it, whereas the growth inhibitors have a negative effect and cause inhibition. A particular hormone may promote certain processes, inhibit some others and not affect many others. They act synergistically (co-operative and beneficial) or antagonistically (acting in opposition) with one another. </li></ul>
ROLE IN PLANT GROWTH AND DEVELOPMENT <ul><li>Growth regulators play a very large part in the development of plants. The roles of individual regulators are described below, but please remember that the issue is often cloudy. Apical dominance, for example, may be influenced by the interaction of several growth regulators. </li></ul><ul><li>Growth substances are also called the phytohormones . The phytohormones have been put in five different categories based on their actions. </li></ul><ul><li>They are: ( auxins, gibberellins, cytokinins, ethylene, abscissic acid ). </li></ul><ul><li>The regulators associated with cell enlargement and differentiation are: auxins and gibberellins </li></ul><ul><li>The regulators associated with cell division: cytokinins </li></ul><ul><li>The regulators associated with ageing: ethylene </li></ul><ul><li>The regulators associated with dormancy of buds: abscissic acid </li></ul>
Why do plants have hormones? <ul><li>Here are some plant hormones and what they do. AUXIN stimulates stem elongation, root growth apical dominance, development of fruit ,instrumental in photrophism and gravitropism. CYTOKININS stimulates cell division and growth, seed germination, flowering , and delay senescence. GIBBERELLINS promotes seed and bud germination, stem elongation, leaf growth, stimulate flowering and development of fruit. ABSCISIC ACID Inhibits growth, closes stoma during water stress; counteracts breaking of dormancy. ETHYLENE Promotes fruit ripening; opposes or reduces some auxins, inhibits growth of roots, leaves, flowers, depending on species. </li></ul>
Auxins <ul><li>Auxins (in Greek - auxein means to grow) are one of the most important groups of plant hormones because of their many sided role in plants. These substances were also the first growth factors identified as plant hormones . The principal naturally occurring auxin is indole-3 acetic acid - IAA . </li></ul>
DISCOVERY OF AUXIN <ul><li>History In the last sixty years, a large number of growth regulators have been isolated from plants. The first indication of their existence was given by Darwin (1880) </li></ul>
Darwin's Experiment with the Seeding of Canary Grass <ul><li>Working on canary grass (Phalaris) he found that if a unilateral source of light was given, the coleoptile would bend towards the source of light. He believed that the tip contained a substance which was transmitted to the lower portion where it caused a curve. He also demonstrated that a decapitated coleoptile or a coleoptile which was covered with a tin foil cap failed to respond to unilateral light. But curvature was seen when seedling was buried in fine black sand with tip exposed. </li></ul><ul><li>Boysen Jensen (1910 - 1913) of Denmark found that the phototropic response lost by decapitation of the tip could be recovered if the tip was replaced on the stump. He further demonstrated that if a transverse slit was caused in the coleoptile on the dark side and a piece of mica inserted into the slit, no phototropic response took place. On the other hand there was a phototropic response if the slit and the piece of mica were on the illuminated side. He concluded that a substance migrates down the dark side promoting growth curvature towards light. </li></ul>
<ul><li>Oat Coleoptile Experiment If the agar block was placed laterally on the cut end of the coleoptile, only that side of the coleoptile elongated resulting in a curvature. The side of the coleoptile that received the growth substance elongated faster and caused the curvature towards the opposite side. He called this substance Auxin from the Greek word 'auxein' to grow . </li></ul>
Possible Involvement of Plant Growth Substances in Apical Dominance in Presence of Apical Bud
Auxins are of 2 types <ul><li>Natural Auxins </li></ul><ul><li>These are naturally occurring in plants. The best known and universally present natural auxin is Indole - 3 - acetic acid. Other natural auxins are Indole - 3 - pyuruvic acid, Indole - 3 - ethanol, Indole - 3 - acetaldehyde. </li></ul><ul><li>Synthetic Auxins </li></ul><ul><li>These are the chemicals synthesized by chemists that cause various physiological actions similar to IAA. Some of the synthetic auxins are Indole - 3 - butyric acid (IBA), b - napthalene acetic acid (NAA) and 2,4 - dichlorophenoxy acetic acid (2,4 - D). </li></ul><ul><li>Auxin precursors - They are compounds which can be converted into auxins. </li></ul><ul><li>Antiauxins </li></ul><ul><li>These are chemicals which inhibit the action of auxins. </li></ul><ul><li>Examples: 2, 3, 5 triiodobenzoic acid (TIBA) and napthylthalmic acid (NPA). </li></ul><ul><li>Free auxins - are auxins which can be easily extracted. They are active. </li></ul><ul><li>Bound auxins - are auxins which are difficult to extract. They are inactive </li></ul>
Auxins Characteristics Features: Polar translocation - Apical dominance - Variable Behaviour or root and shoot growth - Root Initiation - Delay in abscision and differentiation of xylem elements. <ul><li>Role of Auxins </li></ul><ul><li>Apical Dominance:Removal of apical bud stimulates lateral buds. Auxins inhibit lateral bud formation since they are synthesised in apex. This phenomena is called apical dominence. Eg: Potato tubers for apical buds forming. </li></ul><ul><li>Cell Division And Elongation: Shoot and Root growth. </li></ul><ul><li>Xylem Differentiation: Auxins helps in establishing contact between vascular tissues of the callus and that of the bud and makes it possible for the bud to grow properly in callus. By adding auxin and sugar continued growth of callus may be obtained and new shoots and even new plant can be produced. </li></ul><ul><li>Nucleic Acid Activities of IAA increases total RNA - synthesises specific enzymes lead to cell enlargement. </li></ul><ul><li>Manifold Activities Play specific role in seed germination, growth, rooting, flowering (Reproductive phase), abscission, parthenorcarpy and tissue culture. </li></ul>
Practical Applications of Auxins <ul><li>Germination: IAA, IBA, NBA, 2,4-D are mostly used in soaking seed for germination- at low concentrations promotes germination but these effects are subjected to variation depending on form and species of plants. </li></ul><ul><li>Root: NAA, 10% induces 100% rooting in mango: Dashri, langra IBA+SUGAR application leads to greater number of roots-structure of roots also changed (Vascular bundles). </li></ul><ul><li>Flowering: Play floragenic role in day neutral plants IAA promotes formation of female flowers. Increased spikelet number, leaf number and weight and number of grains in wheat. NAA & IAA increases boll-set (G.hirsutum) induced more pine-apple. Fruit weight increases. </li></ul><ul><li>Parthenocarpy: IBA, NAA produces seed less/fruits - smaller sized fruits, but more in number, hence yield not affected. </li></ul><ul><li>Fruit setting: By using 2,4,5 T fruit setting and yield of ber/fruit increased. IAA, IBA, and NAA induce high percentage fruit set. </li></ul><ul><li>Prevention of pre-mature drop of fruits: 2,4,D,IAA,IBA, 2,4,5-T, are used to prevent pre-harvest drop of sweet oranges( 100 to 500 ppm) </li></ul><ul><li>Tissue and Organ culture: IAA & Kinetin </li></ul><ul><li>Auxins as inhibitors: High concentration of auxins inhibit the growth and exert toxic effect on plants. In normal case, self produced auxins inhibit the growth and development of lateral buds, and as a result apical buds, remains dormant. </li></ul>
Commercial Applications of Auxins <ul><li>Many synthetic auxins have been developed because they are found useful in many ways. These synthetic auxins are cheaper to develop than the natural auxins. They are also more effective as the plants are not able to degrade them easily. </li></ul><ul><li>Some of the uses of synthetic auxins are given below: </li></ul><ul><li>Fruiting: Naphthalene acetic acid (NAA) and indolebutyric acid (IBA) help in natural or parthenocarpic fruit setting that increases crop yield in tomato, pepper, figs, etc. </li></ul><ul><li>Rooting: It is an important technique to reproduce genetically similar plants, especially ornamental plants. The cuttings are dipped in rooting powders containing NAA or IBA. This promotes root initiation and stimulates their development. </li></ul><ul><li>Weeding: 2,4-dichlorophenoxyacetic acid (2,4-D) and 2,3,6-trichlorobenzoic acid (benzoic acid) are used to kill dicot weeds among the monocot (cereal) crops as the latter are unaffected. </li></ul><ul><li>Storage: 2-methyl-4-chlorophenoxyacetic acid (MCPA) inhibits sprouting of buds in potatoes and hence is used in their storage. </li></ul>
Auxins use in Agriculture and Horticulture <ul><li>Propagation of plants by hormone treatment of cuttings </li></ul><ul><li>Prevention of pre harvest drops of plants. </li></ul><ul><li>Increasing parthenocarpy. </li></ul><ul><li>Increasing fruit set. </li></ul><ul><li>Prevention of sprouting by inhibiting buds. </li></ul><ul><li>Inhibition of prolonged dormancy. </li></ul><ul><li>Control of flowering. </li></ul><ul><li>Defoliation of plants </li></ul><ul><li>Prevention of leaf fall or abscission. </li></ul><ul><li>Thinning of compact fruits. </li></ul><ul><li>Selective weed killer. </li></ul>
Inhibition of Abscission <ul><li>Formation of an abscission layer at the base of petiole or pedicel results in shedding of leaves, flowers or fruits. But auxins inhibit abscission, as they prevent the formation of abscission layer. </li></ul><ul><li>Auxin Spray Prevents Premature Fruit Abscission and Increase in Yield.a) Auxin Sprayed; b) Auxin not Sprayed </li></ul>
Development of Seedless Fruits Parthenocarpy <ul><li>Auxin induces parthenocarpy, i.e., the formation of seedless fruits without the act of fertilisation. </li></ul>
Weedicides Weedicides Many synthetic auxins are used as selective weed killers and herbicides. 2, 4 - D (2, 4 - dichloro phenoxy acetic acid) is used to destroy broad leaved weeds. It does not affect mature monocotyledonous plants.
TRANSPORT OF AUXIN <ul><li>Auxin is transported through the cell membrane of the adjacent plant cells by protein carriers in the plasma membrane. </li></ul><ul><li>These carriers transport the anion of auxin in polar direction, from the top of the cell to the bottom of the cell. </li></ul><ul><li>However the stimulus of light would seem to result in the introduction of these carriers into the side of the cell membranes so that the IAA3 can now be laterally transported. </li></ul>
How Hormones Work - Signal Transduction Pathways
General model <ul><li>Hormone binding to a specific receptor activates chemical and transport steps that generate second messengers, which trigger the cell's various responses to the original signal. </li></ul><ul><li>In the diagram above, the receptor is on the surface of the target cell. In other cases, hormones enter cells and bind to specific receptors inside. Environmental stimuli can also initiate signal pathways. For example, phytochrome conversion is the first step in the transduction pathways that lead to a cell's responses to red light. </li></ul>
Specific example: a hypothetical mechanism for auxin's stimulation of cell elongation.
Specific example: a hypothetical mechanism for auxin's stimulation of cell elongation. <ul><li>1) The hormone binds to an auxin receptor, and (2) this signal is transduced into second messengers within the cell, inducing various responses. (3) Proton pumps are activated, and secretion of acid loosens the wall, enabling the cell to elongate. (4) The Golgi apparatus is stimulated to discharge vesicles containing materials to maintain the thickness of the cell wall. (5) The signal-transduction pathway also activates DNA-binding proteins that induce transcription of specific genes. (6) This leads to the production of proteins required for sustaining growth of the cell. </li></ul>
Polar auxin transport: a chemiosmotic model. <ul><li>In growing shoots, auxin is transported unidirectionally, from the apex down the shoot. Along this pathway, the hormone enters a cell at the apical end, exits at the basal end, diffuses across the wall, and enters the apical end of the next cell. A pH difference between the cell wall (acidic at about pH 5) and the cytoplasm (pH 7) contributes to auxin transport. In the pH 7 environment of the cell, auxin is an anion. (1) When auxin encounters the acidic environment of the wall, the molecule picks up a hydrogen ion to become electrically neutral. (2) As a relatively small, neutral molecule, auxin passes across the plasma membrane. (3) Once inside a cell, the pH 7 environment causes auxin to ionize. This temporarily traps the hormone within the cell, because the plasma membrane is less permeable to ions than to neutral molecules of the same size. (4) ATP-driven proton pumps maintain the pH difference between the inside and outside of the cell. (5) Auxin can only exit the cell at the basal end, where specific carrier proteins are built into the membrane. The proton pumps contribute to this auxin efflux by generating a membrane potential (voltage) across the membrane, which favors transport of anions out of the cell. Now in the acidic environment of the wall again, auxin picks up a hydrogen ion and enters the next cell as an electrically neutral molecule. Polar auxin transport is one specific application of a basic mechanism of energy coupling in cells. This mechanism, chemiosmosis, uses proton pumps to store energy in the form of an H+ gradient and membrane potential, and then taps this energy source to drive cellular work. </li></ul>
Tropisms: Plant Growth Responses and Movement <ul><li>Plant Growth Responses (Cell Division Involved) </li></ul><ul><li>Phototropism - plants grow in response to light (nonreversible growth toward light stimulus. Light causes auxin to move laterally in the apical meristem. An unknown (yellow pigment) receptor absorbs blue light and helps transport auxin to the unlighted side where cells respond by elongating auxin is quickly inactivated by enzymes further down the stem. Gravitropism - nonrevrsible growth toward gravity. Young roots tend to grow down into the soil thanks to the combinations of gravity and inhibitory effects of high auxin concentrations (see auxin) </li></ul><ul><li>Thigmotropism - nonreversible growth toward touch stimulus. Special young slender branches called tendrils respond to touch by growing toward the contacting surface and entwining around it. Both auxin , light, and cell division are probably involved. </li></ul>
How does auxin become concentrated along the lower side of a horizontal root?
How does auxin become concentrated along the lower side of a horizontal root? <ul><li>Auxin it seems is actively pumped from cell to cell until it reaches its lower destination. The auxin pumps are activated by a calcium-protein complex, the protein being a well known activator called calmodulin . Large starch granules, amyloplasts, drift down through the cytoplasm coming to rest and touching the endoplasmic reticulum in the lower half of the cell. ER stores calcium ions which are released once contacted by the amyloplast. The free Ca2+combines with calmodulin which finally activates hypothetical cellular mechanism, falling (amyloplasts) statoliths in the horizontal root cause the release of calcium ions, which activate calmodulin, a well known enzyme activator. The activated enzymes, in turn start calcium and auxin pumps working in the nearby plasma membrane. Both Ca2+ and auxin leave the columella cells and migrate to the lower margin of the root cap, whereupon the auxin begins its journey along the lower side of the root--its accumulation there leading subsequently to the inhibition of cell elongation. </li></ul>
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