1. Plant Responses What do plants respond to? Response to light
Contents
Plant Responses
What do plants respond to?
Response to light
Response to gravity
Response to water
Using plant hormones
Summary quiz

2. Which way up? How do plants always grow the right way up?
What conditions will affect how a plant grows?

3. What do plants respond to?
Plants are very sensitive and their growth is affected by
their environmental conditions.
water
light
gravity
A condition that affects plant growth is called a stimulus.
What are the three types of stimuli that plants respond to?

4. What is a tropism? water light gravity
Plants respond to stimuli by growing to or away from them.
A growth movement in response to a stimulus is a tropism.
water
light
gravity
Movement towards a stimulus is called a positive tropism.
What is a movement away from a stimulus called?
A negative tropism.

5. Different types of tropism
Plant responses to light, water and gravity are given
special names.
water
hydrotropism
light
phototropism
gravity
geotropism
Can you work out what the name of each tropism means?
Which parts of a plant respond to these different stimuli?

6. What do different tropisms mean?

7. Plant Responses What do plants respond to? Response to light
Contents
Plant Responses
What do plants respond to?
Response to light
Response to gravity
Response to water
Using plant hormones
Summary quiz

8. How do plants respond to light?
A plant’s response to light is called phototropism.
Is phototropism a positive or negative tropism?
light
Plants grow towards light, which is a positive tropism.
Why do plants grow towards light?
Plants need light for photosynthesis, so they respond
to light by growing towards it.

9. Phototropism and auxins
Which part of a plant is involved in phototropism?
A growing shoot on a plant responds to light.
light
Auxins are made in the
tip of a growing shoot
and move down the stem.
These plant hormones
speed up growth in
a growing shoot.

10. Phototropism and auxins – light from above
When a shoot gets light from above, the auxins produced at the tip are spread out evenly in the shoot.
How will this affect the growth of the shoot?
light

11. Phototropism and auxins – light from above
Why does a shoot grow straight up when light is from above?
There is an equal amount of auxins in all parts of the shoot, which grow at the same rate, so the shoot grows straight up.
light
light

12. Phototropism and auxins – light from one side
When a shoot gets light from one side, the auxins are not evenly spread out in the shoot.
How will this affect the growth of the shoot?
light

13. Phototropism and auxins – light from one side
Why does the shoot bend towards the light when it gets light from one side?
There are more auxins on the shaded side of the shoot,
so the shaded side grows faster making the bright side bend towards the light.
light
light

14. Phototropism experiment

15. Plant Responses What do plants respond to? Response to light
Contents
Plant Responses
What do plants respond to?
Response to light
Response to gravity
Response to water
Using plant hormones
Summary quiz

16. How do plants respond to gravity?
A plant’s response to gravity is called a geotropism.
Different parts of a plant have different responses to gravity.
shoots grow up
negative geotropism
roots grow down
gravity
positive geotropism
Are these responses positive or negative geotropisms?
Why is important that roots grow down into the soil?

17. Geotropism and auxins
Auxins speed up growth in shoots and slow growth in roots.
If a plant is laid on its side, the auxins produced collect
in the lower side of the root and stem.
How will the auxins affect the growth of the root and stem?

18. Geotropism and auxins
If a plant is laid on its side, why does the root grow down
and the stem grow up?
Auxins slow down growth on the lower side of the root,
so the root curves down.
Auxins speed up growth on the lower side of the stem,
so the stem curves up.

19. Plant Responses What do plants respond to? Response to light
Contents
Plant Responses
What do plants respond to?
Response to light
Response to gravity
Response to water
Using plant hormones
Summary quiz

20. How do plants respond to water?
A plant’s response to water is called hydrotropism.
Roots always grow towards water, a positive tropism.
dry soil
wet soil
Roots will grow sideways or even upwards, towards water.
Do roots have a stronger response to water or gravity?
Roots always have a stronger response to water than gravity to ensure that a plant gets the water it needs.

21. Hydrotropism and auxins – water all around
When roots are well watered from all sides,
the auxins produced in the roots are spread out evenly.
How will this affect the growth of the roots?
wet
soil

22. Hydrotropism and auxins – water all around
Why do roots grow out in all directions when they are well watered from all sides?
There is an equal amount of auxins in all parts of the roots, which grow at the same rate, so the roots grow out in all directions.
wet
soil
wet
soil
wet
soil

23. Hydrotropism and auxins – more water one side
When roots are in soil with more water on one side, the auxins are not spread out evenly in the roots.
How will this affect the growth of the roots?
dry
soil
wet

24. Hydrotropism and auxins – more water one side
Why do roots grow towards the wet side when they are
in soil with more water on one side?
There are more auxins on the wetter side of the roots, which grows slower than the dryer side, so the roots bend towards the water.
dry
soil
wet
dry
soil
wet
soil

25. Plant Responses What do plants respond to? Response to light
Contents
Plant Responses
What do plants respond to?
Response to light
Response to gravity
Response to water
Using plant hormones
Summary quiz

26. What are plant hormones used for?
Plant hormones can be used in gardening and
agriculture to control how plants grow and develop.
Plant hormones are naturally-occurring chemicals
but can also be produced synthetically for the following commercial uses:
growing cuttings
killing weeds
ripening fruit

27. Using plant hormones – growing cuttings
A cutting is a part of plant that
is removed from the main shoot.
What does a cutting no longer have, when it is removed from a plant?
A cutting does not have roots and
so has no supply of water or minerals.
Rooting powder contains plant growth hormones. Dip a cutting into rooting powder and the plant hormones stimulate the cutting to grow new roots.
Why does the gardening business use cuttings and rooting powder to grow new plants?

28. Grow your own plant cutting

29. Using plant hormones – killing weeds
A surprising use of plant hormones is for killing weeds.
Why are weeds a problem in farming and gardening?
Weeds are unwelcome plants that compete with the plants being grown by gardeners and farmers.
Many chemicals that are potential weedkillers are toxic
to animals and humans as well as plants.
Why might a weedkiller with plant hormones that only affects plants be a more favourable option?

30. Using plant hormones – killing weeds
Cereal crops like wheat are grasses with narrow leaves.
Most weeds that grow in fields and gardens have broad leaves.
Selective weedkillers contain plant hormones at concentrations that will only affect plants with broad leaves.
What effect will this type of weedkiller have on a field of wheat overgrown with weeds?
These selective weedkillers upset the normal growth of broad-leaved plants and so kill off any weeds.
The growth of cereal crops and grasses is not affected.

31. Using plant hormones – ripening fruit
Fruit is usually ripe and
ready to eat when it is
bought from a greengrocers
or supermarket.
Fruit farmers are able to control when fruit ripens
using plant hormones.
Why is this important for their business?
Fruit has often travelled many miles before being bought from the shops.
Is it better for fruit to be ripe or unripe during a long journey?

32. Using plant hormones – ripening fruit
Fruit that is picked before
it is ripe is usually hard.
There is less chance of unripe fruit being damaged when transported, which
is important to the farmer
and the buyer.
Plant hormones can be sprayed onto unripe fruit during transportation. These hormones ripen fruit in time for its arrival in the shops.
Why is it a good idea to wash fruit before you eat it?

33. Plant Responses Plant hormones Response to light Response to gravity
Contents
Plant Responses
Plant hormones
Response to light
Response to gravity
Response to water
Using plant hormones
Summary quiz

34. Multiple-choice quiz