1. Hormonal Control of Growth in Plants
Higher Biology
Hormonal Control of Growth in Plants

2. 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

3. 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.

4. 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)

5. 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

6. Indole Acetic Acid (IAA)
Necessary for differentiation to form specialised cells.
Look at the experiment on pages 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.

7. 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

8. Indole Acetic Acid (IAA)
Effects of IAA at organ level:
On the root:
High concentrations
Low concentrations
No effect on growth
Stimulate growth

9. 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.

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

11. 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.

12. 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

13. Apical Dominance What is the position of the apical bud in a plant?
Read pages 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?

14. 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.

15. 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)

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

17. Fruit Abscission
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

18. 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.

19. 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).

20. 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.

21. Practice Questions
Torrance
TYK page 256 Q1, 2, 4 and 5

22. 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