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:

o on cellso on shoots and rootso on bending of stemso on leaf abscissiono 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:1. Stimulates cell division

2. Promotes cell elongation- by increasing the plasticity of cell walls, enabling them to stretch irreversibly when water enters by osmosis during vacuolation.

Primary growthin apical meristems

Secondary growthin lateral meristems(cambium)

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Indole Acetic Acid (IAA)

3. 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:

1. 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:

2. On the root:

High concentrations

Low concentrations Stimulate growth

No effect on growth

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Bending

What happens if the IAA is not evenly distributed at the root or shoot tip?

1. Look at Figure 25.5 in your diagram pack.2. Shade in the areas where IAA is found on

the shoot.3. What side is the shoot bending towards?4. How is this related to IAA concentration?5. 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.1. Look at Figure 25.6 in your diagram

pack.2. Shade in the areas where IAA is most

concentrated.3. 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 videoWhy does this happen?

The light destroys the

auxin

Causing an uneven

distribution of auxin

More auxin on the

shaded side

The cells on shaded side elongate

more, so the shootbends

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Apical Dominance

Read pages 254-255 on “Apical Dominance” in Torrance, and then answer the following questions:

1. What is the position of the apical bud in a plant?

2. What is the position of the lateral buds in a plant?

3. What substance does the apical bud produce?

4. How does this substance travel to the lateral buds?

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Apical Dominance

5. What effect does this substance have on the lateral buds?

6. 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.

Fruit stalk snaps

Walls of the abscission layerbecome weaker

Thin layer of cells form at the base of

the fruit stalk

Auxin levels drop

Fruit falls off

Thin layer of cells form at the base of

the fruit stalk

Auxin levels drop

Walls of the abscission layerbecome weaker

Fruit stalk snapsFruit falls off

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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

Torrance1. 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:

o on cellso on shoots and rootso on bending of stemso on leaf abscissiono on fruit formation