1. Growth response of plants
title
Growth response of plants

2. 18.2 How do shoots respond to light?
content
Growth response of plants
18.1 Tropism
18.2 How do shoots respond to light?
18.3 How do roots and shoots respond to gravity?
18.4 How do roots respond to water?
18.5 What substances cause tropisms in plants?
18.6 How do auxins cause phototropism and geotropism?

3. 18.1 Tropism
nervous system
Plants
can still sense external stimuli and respond by movement
endocrine system
stimulus coming from one certain direction
TROPISM
unidirectional stimulus
grow towards/ away from the stimulus
plant will

4. 18.2 How do shoots respond to light?
Phototropism
growth movement of a part of a plant in response to light
growing towards the stimulus
Shoots
positive phototropism
leaves can get more light for photosynthesis
more food is made

5. To investigate phototropism in shoots
Carry out Practical 18.1
To investigate phototropism in shoots

6. 18.3 How do roots and shoots respond to gravity?
Geotropism
growth movement of a part of a plant in response to gravity
growing away from the stimulus
Shoots
Roots
negative geotropism
positive geotropism
leaves will grow away from the soil
leaves can get more light for photosynthesis
roots can get more water and dissolved minerals from the soil
roots can grow deeper for better anchorage

7. To investigate geotropism in roots
Carry out Practical 18.2
To investigate geotropism in roots

8. 18.4 How do roots respond to water?
Hydrotropism
growth movement of a part of a plant in response to water
Roots
positive hydrotropism
roots can get enough water and dissolved minerals for growth

9. 18.5 What substances cause tropisms in plants?
Auxins
plant growth hormones
produced at tips of shoots and roots
diffuse to regions of elongation to promote growth
(but effects vary with concentration & parts of plants concerned)
one common auxin is indole acetic acid (IAA)
cause tropism

10. Region of elongation in plants
more auxins
growth occurs in region of elongation
no increase in length
fewer auxins
plumule
after 1 to 2 days

11. increasing concentration of auxins
-100
-50 50
100
150
200
10-6
10-4
10-2 1
102
104
ppm (parts per million)
GROWTH RESPONSE
most auxin concentrations which increase root growth
are too low to
increase shoot growth
lower auxin concentrations increase the growth of roots
percentage stimulation
percentage inhibition
shoots
roots
increasing concentration of auxins

12. increasing concentration of auxins
higher auxin concentrations increase shoot growth but slow down root growth
-100
-50 50
100
150
200
10-6
10-4
10-2 1
102
104
ppm (parts per million)
GROWTH RESPONSE
percentage stimulation
percentage inhibition
shoots
roots
increasing concentration of auxins

13. increasing concentration of auxins
-100
-50 50
100
150
200
10-6
10-4
10-2 1
102
104
ppm (parts per million)
GROWTH RESPONSE
the concentration which produces the greatest root growth is lower than the one which produces the greatest shoot growth
percentage stimulation
percentage inhibition
shoots
roots
increasing concentration of auxins

14. increasing concentration of auxins
-100
-50 50
100
150
200
10-6
10-4
10-2 1
102
104
ppm (parts per million)
GROWTH RESPONSE
very high auxin concentrations slow down both root and shoot growth
percentage stimulation
percentage inhibition
shoots
roots
increasing concentration of auxins

15. 18.6 How do auxins cause phototropism and geotropism?
Both tropisms are caused by an uneven distribution of auxins.
How auxins cause phototropism of shoots
How auxins cause geotropism of roots

16. Concept diagram Tropism auxins growth response of plants
caused by
auxins
is a
consists of
growth response of plants
work by
produced by
regulating plant growth
tips of shoots and roots
in response to
a stimulus
phototropism
geotropism
hydrotropism
in response to
in response to
in response to
unilateral light
gravity
water

17. Concept diagram unilateral light gravity water
causes
causes
therefore
different concentration of auxins in the shaded and unshaded sides of a plant
different distribution of auxins on the upper and lower sides of a horizontally placed root
therefore
therefore
bending of shoots towards light
bending of roots downwards
bending of roots towards water
hence shoots are
hence roots are
hence roots are
positively phototropic
positively geotropic
positively hydrotropic