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  1. Unit 1
    Ecology theory
    5 Topics
    1 Quiz
  2. Nutrient recycling
    4 Topics
    1 Quiz
  3. Ecological relationships and population dynamics
    2 Topics
  4. Human impact on the ecosystem
    2 Topics
  5. Study of a grassland ecosystem
    3 Topics
  6. Responses in the Flowering Plant
    4 Topics
    1 Quiz
  7. Unit 3
    Sexual reproduction in the flowering plant
    1 Topic
  8. The human reproductive system
    3 Topics
  9. The menstrual cycle
    2 Topics
  10. Pregnancy
    2 Topics
  11. The circulatory system
    4 Topics
  12. Blood
    5 Topics
  13. The heart
    5 Topics
Lesson 6, Topic 2
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Topic 2: Plant growth regulators

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What are growth regulators ?

Growth regulators

Chemicals the control the growth rate of plants

  • They are produced in the meristrmatic regions and carried to other parts of the plant where they have an effect (they act like a hormone)
  • They are transported mostly in phloem (vascular tissue)

Effect of growth factor concentration and location

  • Their effects depend on their concentration (can have the opposite effect at different concentrations)
  • Their effect also depends on their location in the plant  (can have the opposite effect in the stem and root)

What are growth promoters ?

Growth promoters

Growth promoters increase the rate of growth

  • Examples include auxins  (e.g. indoleacetic acid or IAA) and gibberellins (e.g. gibberellic acid or GA)
  • Growth promoters are made in the meristematic tissue of the shoot tip, young leaves and stems
  • They cause stem and root elongation, fruit formation , phototropism and geotropism

The role of IAA in phototropism

  • IAA is produced in shoot and root tips
  • It travels mainly to the cells away from the light (the shaded side)
  • This higher concentration of IAA on the shaded side causes these cells to elongate faster than those facing the light
  • This uneven elongation causes the plant to bend towards the light

Apical dominance

  • Apical dominance means that auxins produced in the apical meristem pass down the stem and inhibit side branches. This allows the plant to grow tall

What are growth inhibitors?

Growth inhibitors

Growth inhibitors decrease the rate of growth

  • Examples include ethylene and abscisic acid
  • Growth inhibitors are made in stem nodes, ripe fruit and decaying leaves
  • They cause fruit color, fruit flavor, tissues to soften (ripening), the stimulation of more ethylene production
  • If you add a ripe apple to a bunch of unripened bananas, the gaseous ethylene produced by the apple will cause the bananas to also ripen 

Combined effects of promoters and inhibitors

  • Some promoters and inhibitors work together, e.g. gibberellins and auxins promote stem growth 
  • Others have opposite effects, e.g. cytokinins encourage side bud growth whereas auxins inhibit it

Uses of plant growth regulators

  • Growth regulators can be used to help ripen fruit , e.g. ethylene
  • They can also be used as rooting powders (auxin) to increase root formation

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