Higher Biology

1
Higher Biology

• Hormonal Control of Growth in Plants

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Hormonal Control of Growth

• By the end of this lesson you should be able to

• Know the name of 1 plant growth substance.
• Know where IAA is produced.
• Know the effects of IAA or auxins
• on cells
• on shoots and roots
• on bending of stems
• on leaf abscission
• on fruit formation

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Plant Growth Substances

• Also known as hormones.
• They are produced in one part of the plant (in
  tiny quantities) and carried to another part of
  the plant where they have their effect.

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Indole Acetic Acid (IAA)

• IAA is one of a group of plant growth substances
  called auxins.
• IAA is produced in...
• It is produced in
• Root tip
• Shoot tip
• Leaf meristems
• It is transported over
• short distances by diffusion.
• long distances via the phloem (by translocation)

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Indole Acetic Acid (IAA)

• Effects of IAA at the cell level
• Stimulates cell division
• Promotes cell elongation- by increasing the
  plasticity of cell walls, enabling them to
  stretch irreversibly when water enters by osmosis
  during vacuolation.

Secondary growth in lateral meristems (cambium)
Primary growth in apical meristems
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Indole Acetic Acid (IAA)

• Necessary for differentiation to form specialised
  cells.
• Look at the experiment on pages 250-251 of
  Torrance.
• Complete The Effects of different concentrations
  of IAA on root and shoot growth from your
  diagram pack
• Add high concentrations and low concentrations at
  the relevant points.

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Indole Acetic Acid (IAA)

• Effects of IAA at organ level
• On the shoot
• Very high concentrations
• High concentrations
• Low concentrations

Inhibit growth
Stimulate growth
No effect on growth
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Indole Acetic Acid (IAA)

• Effects of IAA at organ level
• On the root
• High concentrations
• Low concentrations

No effect on growth
Stimulate growth
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Bending

• What happens if the IAA is not evenly
  distributed at the root or shoot tip?
• Look at Figure 25.5 in your diagram pack.
• Shade in the areas where IAA is found on the
  shoot.
• What side is the shoot bending towards?
• How is this related to IAA concentration?
• Write a statement linking auxin concentration and
  curvature of the shoot.

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Bending

• Why does the shoot bend?
• The higher concentration of auxin causes a
  greater cell elongation- which means the shoot
  bends.

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Bending in response to light.

• Have you noticed that stems tend to grow towards
  the light?
• This is called phototropism.
• Look at Figure 25.6 in your diagram pack.
• Shade in the areas where IAA is most
  concentrated.
• Complete your Phototropism sheet from your
  diagram pack.

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Bending in response to light.

• Phototropism is the name given to the directional
  growth movement by a plant shoot in response to
  light from one direction.
• phototropism video
• Why does this happen?

The cells on shaded side elongate more, so the
shoot bends
Causing an uneven distribution of auxin
More auxin on the shaded side
The light destroys the auxin
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Apical Dominance

• Read pages 254-255 on Apical Dominance in
  Torrance, and then answer the following
  questions
• What is the position of the apical bud in a
  plant?
• What is the position of the lateral buds in a
  plant?
• What substance does the apical bud produce?
• How does this substance travel to the lateral
  buds?

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Apical Dominance

• What effect does this substance have on the
  lateral buds?
• Describe what would happen to the lateral buds if
  the apical bud was removed.

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Leaf Abscission

• Leaf abscission is the process by which a plant
  sheds its leaves.
• It occurs in deciduous trees in autumn and in
  diseased leaves at any time.
• Prior to leaf fall, the auxin levels drop and a
  thin abscission layer of cells form at the base
  of the leaf stalk.
• The walls of these cells gradually become weaker,
  eventually snap and the leaf falls off. (e.g.
  when the wind blows)

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Leaf Abscission

• High levels of auxin travel from the leaf to
  prevent the abscission layer from forming, during
  the growing season.
• leaf abscission animation

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Fruit Abscission

1. Put the following stages into the correct order
   to describe fruit abscission.

Auxin levels drop
Thin layer of cells form at the base of the
fruit stalk
Walls of the abscission layer become weaker
Auxin levels drop
Walls of the abscission layer become weaker
Fruit falls off
Thin layer of cells form at the base of the
fruit stalk
Fruit falls off
Fruit stalk snaps
Fruit stalk snaps
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Fruit formation

• In a flower what happens to the ovule after
  fertilisation?
• It forms the seed!
• In a flower what happens to the ovary wall after
  fertilisation?
• It forms the fruit!
• Fruit formation is caused by high levels of IAA
  made by the developing embryo.

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Fruit formation

• Unpollinated flowers can be induced to form fruit
  by treating them with IAA.
• Since fertilisation has not happened the fruit is
  formed without a seed!
• This is called parthenocarpy.
• Horticulturalists use this to produce a good crop
  of fruit which all ripen at the same time (to
  make harvesting more economical).

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Fruit formation

• Horticulturalists often spray fruit crops with
  synthetic auxin, towards the end of the growing
  season, to delay the formation of the abscission
  layer at the fruit stalk.
• This prevents the fruit from falling off the tree
  until it is fully ripe.

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Practice Questions

• Torrance
• TYK page 256 Q1, 2, 4 and 5

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Hormonal Control of Growth

• Can you do it?

• Know the name of 1 plant growth substance.
• Know where IAA is produced.
• Know the effects of IAA or auxins
• on cells
• on shoots and roots
• on bending of stems
• on leaf abscission
• on fruit formation