Download Hormonal Control of Growth in Plants

Higher Biology
Hormonal Control of Growth in
Plants
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 cells
o on shoots and roots
o on bending of stems
o on leaf abscission
o on fruit formation
2
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.
3
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)
4
Indole Acetic Acid (IAA)
Effects of IAA at the cell level:
1. Stimulates cell division
Primary growth
in apical meristems
2.
Secondary growth
in lateral meristems
(cambium)
Promotes cell elongation- by increasing the
plasticity of cell walls, enabling them to
stretch irreversibly when water enters by
osmosis during vacuolation.
5
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.
6
Indole Acetic Acid (IAA)

Effects of IAA at organ level:

On the shoot:
Very high concentrations

High concentrations
Stimulate growth

Low concentrations
No effect on growth
1.
Inhibit growth
7
Indole Acetic Acid (IAA)

Effects of IAA at organ level:
2.
On the root:

High concentrations

Low concentrations
No effect on growth
Stimulate growth
8
Bending
What happens if the IAA is not evenly
distributed at the root or shoot tip?
1.
2.
3.
4.
5.
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.
9
Bending
Why does the shoot bend?

The higher concentration of auxin causes a
greater cell elongation- which means the
shoot bends.
10
Bending in response
to light.
Have you noticed that stems tend to grow
towards the light?

1.
2.
3.
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.
11
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.
The cells on shaded
 phototropism video
side elongate
Why does this happen?
more, so the shoot
Causing an
bends
uneven
distribution
More auxin
The light
of auxin
on the
destroys the
shaded side
auxin

12
Apical Dominance
1.
2.
3.
4.
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?
13
Apical Dominance
5.
6.
What effect does this
substance have on the
lateral buds?
Describe what would
happen to the lateral
buds if the apical bud
was removed.
14
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)
15
Leaf Abscission


High levels of auxin travel from the leaf to
prevent the abscission layer from forming,
during the growing season.
leaf abscission animation
16
Fruit Abscission
Put the following stages into the correct
order to describe fruit abscission.
Auxin levels drop
Thin layer of
1.
Walls of the
abscission layer
become weaker
cells form at
Walls of the
Auxin
levels drop
the base of
abscission
layer
the fruit stalk
become weaker
Fruit falls off
Fruit falls off
Fruit stalk snaps
Thin layer of
cells form at
the
base of
Fruit stalk
snaps
the fruit stalk
17
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.
18
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).
19
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.
20
Practice Questions

1.
Torrance
TYK page 256 Q1, 2, 4 and 5
21
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 cells
o on shoots and roots
o on bending of stems
o on leaf abscission
o on fruit formation
22