Plant Growth Regulators and Auxins
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Questions and Answers

What is the primary function of auxins in plant physiology?

  • Inhibiting root development
  • Stimulating stem elongation (correct)
  • Reducing cell division
  • Promoting lateral bud formation

    • Which of the following is a synthetic auxin?

  • Abscisic acid
  • Indole 3-acetic acid (IAA)
  • Gibberellin
  • Naphthalene acetic acid (correct)

    • What effect does auxin have on the curvature of coleoptiles?

  • It promotes growth only in the root.
  • It inhibits growth in all tissues.
  • It causes uniform growth on both sides.
  • It increases growth on one side resulting in bending. (correct)

    • How does high concentration of auxins affect plant growth?

    <p>They can inhibit growth due to ethylene accumulation. (C)</p> Signup and view all the answers

    What is the first step in the mode of action of auxins?

    <p>Absorption of auxins to binding sites. (B)</p> Signup and view all the answers

    What is apical dominance?

    <p>Inhibition of lateral bud formation by the apical tip. (A)</p> Signup and view all the answers

    Where are the highest concentrations of auxins found in plants?

    <p>In meristematic and actively growing regions. (B)</p> Signup and view all the answers

    What role does auxin binding protein (ABP 1) play in the action of auxins?

    <p>It serves as a receptor that facilitates auxin action. (D)</p> Signup and view all the answers

    Where is abscisic acid synthesized in plants?

    <p>In cells containing chloroplasts or amyloplasts (B)</p> Signup and view all the answers

    Which physiological effect is primarily caused by the presence of abscisic acid?

    <p>Inhibition of seed germination (A)</p> Signup and view all the answers

    What is one of the mechanisms by which abscisic acid inhibits seed germination?

    <p>By inhibiting specific enzymes (A)</p> Signup and view all the answers

    What is the primary role of abscisic acid in plants during periods of stress?

    <p>To induce dormancy (A)</p> Signup and view all the answers

    What pathway partially contributes to the synthesis of abscisic acid?

    <p>Mevalonic pathway (D)</p> Signup and view all the answers

    How does abscisic acid affect the growth of seedlings?

    <p>Inhibits seedling growth (D)</p> Signup and view all the answers

    Which factor does abscisic acid inhibit to affect seed germination?

    <p>Water uptake (D)</p> Signup and view all the answers

    What is the role of carotenoids in the biosynthesis of abscisic acid?

    <p>They break down to produce abscisic acid (D)</p> Signup and view all the answers

    What is the primary function of cytokinins in plant physiology?

    <p>Promoting cell division and organ formation (B)</p> Signup and view all the answers

    What effect does ethylene have on seed germination?

    <p>It breaks dormancy and induces germination. (B)</p> Signup and view all the answers

    Which compound was identified as the first cytokinin?

    <p>Kinetin (D)</p> Signup and view all the answers

    How does ethylene influence the growth of stems?

    <p>It inhibits elongation but enhances radial growth. (A)</p> Signup and view all the answers

    Which effect do cytokinins have on seed germination?

    <p>Promote germination and break dormancy (C)</p> Signup and view all the answers

    At what concentration does maximum growth for ethylene occur?

    <p>0.3 ppm (B)</p> Signup and view all the answers

    Which of the following physiological responses is NOT promoted by ethylene?

    <p>Enhanced rooting in all plant species (B)</p> Signup and view all the answers

    What is the primary mechanism through which cytokinins exert their effects in plants?

    <p>Increased nucleic acid and protein synthesis (B)</p> Signup and view all the answers

    What is the Richmond Lang Effect associated with cytokinins?

    <p>Delaying leaf senescence (B)</p> Signup and view all the answers

    What is the impact of ethylene on flowering in mango and pineapple?

    <p>It promotes flowering in these plants. (C)</p> Signup and view all the answers

    Which plant tissue was used in the bioassays to test the effects of cytokinins?

    <p>Tobacco callus tissue (D)</p> Signup and view all the answers

    What happens during the 'triple response' of etiolated pea plants when exposed to ethylene?

    <p>Stimulation of radial swelling and horizontal growth. (B)</p> Signup and view all the answers

    What role do hydrolytic enzymes play when ethylene is present?

    <p>They are stimulated by ethylene. (A)</p> Signup and view all the answers

    Which cytokinin is most commonly associated with corn?

    <p>Zeatin (D)</p> Signup and view all the answers

    How do cytokinins affect cotyledon and leaf expansion?

    <p>By causing an increase in cell size (B)</p> Signup and view all the answers

    How does ethylene affect the chlorophyll in leaves during senescence?

    <p>It leads to chlorophyll destruction. (C)</p> Signup and view all the answers

    What is one of the primary roles of gibberellins in plants?

    <p>Promoting shoot growth by accelerating cell elongation. (C)</p> Signup and view all the answers

    What process is gibberellin known to induce in certain plants?

    <p>Bolting or premature flowering. (C)</p> Signup and view all the answers

    Which mechanism is NOT associated with the transport of gibberellins?

    <p>Polar transport through phloem. (A)</p> Signup and view all the answers

    Which commercial application involves the use of gibberellins?

    <p>Increasing fruit size in grapes. (D)</p> Signup and view all the answers

    What effect do cytokinins have on plant growth?

    <p>Stimulation of cell division. (C)</p> Signup and view all the answers

    What was discovered by J. Van Overbeek regarding cytokinins?

    <p>Coconut milk can stimulate cell division. (A)</p> Signup and view all the answers

    How do gibberellins affect seed dormancy?

    <p>They break dormancy effectively in potatoes. (D)</p> Signup and view all the answers

    What characteristic of gibberellins allows their application in malt production?

    <p>They increase α-amylase activity in barley seeds. (B)</p> Signup and view all the answers

    What effect does AbA have on seedling growth in Glycine max?

    <p>Inhibits growth by about 50% within 48 hours (D)</p> Signup and view all the answers

    How does AbA influence bud growth in woody plants?

    <p>Induces bud dormancy (C)</p> Signup and view all the answers

    What is the significance of AbA in the control of stomata?

    <p>Causes rapid stomatal closure (B)</p> Signup and view all the answers

    What effect does AbA have on geotropism in plants?

    <p>Stimulates a positive geotropic response despite inhibiting root growth (D)</p> Signup and view all the answers

    Which process has been linked to the senescence factor potentially identified as AbA?

    <p>Inducing leaf yellowing and abscission (D)</p> Signup and view all the answers

    What happens to AbA levels in guard cells during water stress?

    <p>AbA levels increase (C)</p> Signup and view all the answers

    What role does IAA play in relation to AbA in plant tissues?

    <p>Maintains levels of AbA in tissues with apical dominance (A)</p> Signup and view all the answers

    What effect does exogenous application of AbA have on K+ in guard cells?

    <p>Inhibits K+ uptake and proton release (C)</p> Signup and view all the answers

    Signup and view all the answers

    Study Notes

    Plant Growth Regulators

    • Plant growth and development are controlled by extremely low concentrations of chemical substances called plant growth substances, phytohormones, or plant growth regulators.
    • Key regulators include auxins, gibberellins, cytokinins, abscisic acid, and ethylene.

    Auxins - Study Notes

    • Discovered in 1887 by Charles and Francis Darwin.
    • Darwin's experiments involving canary grass (Phalaris canariensis) showed that the tip of the plant was essential for the response to light.
    • Went (1926) isolated auxin from plant tips, identifying it as a chemical called indole-3-acetic acid (IAA).
    • Auxin causes bending of the coleoptile (sheath protecting emerging leaves of a seedling), a response called phototropism.
    • Auxins trigger cell elongation in plants.
    • Application of exogenous auxins can stimulate stem and coleoptile growth, increase root formation, and boost cambial activity.
    • Conversely, high concentrations can inhibit plant growth.
    • Apical dominance (inhibition of lateral bud growth) by auxins is a notable effect.
    • Auxins are involved in phototropism and geotropism (response to gravity) in plants.
    • Auxin transport is polar, moving unidirectionally through the plant.
    • A variety of synthetic auxins exist (e.g., phenoxy acetic acid, naphthalene acetic acid).

    Gibberellins - Study Notes

    • Gibberellins cause a disease called 'bakanae' of rice, resulting in 'foolish seedlings'.
    • They were isolated from the fungus Gibberella fujikuroi in 1939.
    • Gibberellins promote stem elongation, seed germination, and breaking bud dormancy.
    • They also influence flowering and fruit development.
    • Gibberellins aren't transported in a polar fashion, unlike auxins.
    • They can stimulate the hydrolysis of starch to glucose. This can be used as a bioassay.
    • Gibberellins are often used commercially for improving fruit size, color, and quality. Notably, they are crucial in making seedless grapes.

    Cytokinins - Study Notes

    • Haberlandt (1921) first noticed factors stimulating cell division in plant vascular tissues.
    • Coconut milk was discovered to contain a factor stimulating cell division in young plant embryos. Chemical compounds inducing cell division were discovered in tissue culture systems.
    • The active compound discovered in the 1940s is known as kinetin.
    • Cytokinins promote cell division, organ formation, and are used in tissue culture techniques.
    • They are also involved in delaying senescence in leaves and expanding cotyledons in plant embryos.
    • Like gibberellins, they aren't transported in a polar fashion.

    Abscisic Acid - Study Notes

    • It's a potent inhibitor of seed and bud germination.
    • Isolated from cotton fruits (1957).
    • It inhibits seed germination, apical growth, and stomatal opening.
    • Its functions include stimulating abscission (leaf drop), influencing bud dormancy, and affecting stomatal opening.
    • Its biosynthesis occurs in plant tissues undergoing senescence.
    • Has many roles in the overall plant response to stress.

    Ethylene - Study Notes

    • It is a gaseous plant growth regulator.
    • Ethylene is crucial for fruit ripening; its effects include triple response.
    • The triple response, exhibited in stems, features, reduced growth, stem swelling, and horizontal stem growth.
    • Ethylene promotes fruit ripening and also accelerates senescence and abscission.
    • Ethylene has commercial applications, for example in fruit ripening.

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    Plant Growth Hormones PDF

    Description

    Explore the fascinating world of plant growth regulators, focusing on auxins and their critical role in plant development. This quiz covers key concepts such as the discovery, isolation, and functions of auxins in plant physiology. Test your understanding of how these substances influence phototropism and other growth processes.

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