18 Growth responses of green plants

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18 Growth responses of green plants

• 18.1 Do plants respond to external stimuli?
• 18.2 What are tropisms?
• 18.3 The growth response of shoots to light
• 18.4 The growth response of roots and shoots to
  gravity
• 18.5 The growth response of roots to water
• 18.6 What are the substances for controlling
  growth
• movements in plants?
• 18.7 More about auxins
• 18.8 The role of auxins in phototropic and
  geotropic responses
• Mind Map

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18.1 Do plants respond to external stimuli?
Do Plants Respond to
External Stimuli?
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18.1 Do plants respond to external stimuli?
?
Will it happen
YES!
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18.2 What are tropisms?
What are Tropism?
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18.2 What are tropisms?
growth
Tropism
plant
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18.2 What are tropisms?
Direction of growth
stimulus
plant
plant
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18.2 What are tropisms?
Phototropism
Geotropism
Hydrotropism
gravity
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18.3 The growth response of shoots to light
The Growth response of
Shoots
to Light
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18.3 The growth response of shoots to light
Phototropism
shoot
root
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18.3 The growth response of shoots to light
Phototropism
shoot
root
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18.3 The growth response of shoots to light
Clinostat
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18.3 The growth response of shoots to light
Clinostat
direction of rotation
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18.3 The growth response of shoots to light
Experiment 18.1 To investigate the phototropic
response of shoots

• Select three pots of young seedlings of similar
  size and label them as pots A, B and C
  respectively.

2. Put pot A in a light-proof box with a small
window on one side on that light reaches the
shoot from one direction only. This is unilateral
light.
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18.3 The growth response of shoots to light
Experiment 18.1 To investigate the phototropic
response of shoots

• Set up pot B in a similar way except that it is
  put on a clinostat. Switch on the clinostat.

• Put pot C in complete darkness. Both pots B and C
  are controls.
• Observe the plants after 1 or 2 days.

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18.3 The growth response of shoots to light
Experiment 18.1 To investigate the phototropic
response of shoots
Question 1 What has happened to the shoots of
plant A? Explain.
Answer Seedlings in pot A are exposed to
unilateral light. The shoots respond by growing
towards the light source.
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18.3 The growth response of shoots to light
Experiment 18.1 To investigate the phototropic
response of shoots
Question 2 What has happened to the shoots of
plant B? Explain.
Answer In pot B, as the effect of unilateral
light is cancelled out by the revolution of the
clinostat, the shoots do not show any curvature
but grow vertically upwards.
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18.3 The growth response of shoots to light
Experiment 18.1 To investigate the phototropic
response of shoots
Question 3 What has happened to the shoots of
plant C? Explain.
Answer In pot C, in complete darkness, the
shoots grow vertically upwards. The plant becomes
taller with slender and longer internodes, and
carries smaller leaves which are yellow in
colour. This condition is called etiolation.
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18.3 The growth response of shoots to light
Experiment 18.1 To investigate the phototropic
response of shoots
Question 4 What is the significance of
phototropism to plants?
Answer The shoot tends to grow towards the light
source. This phototropism ensures that a plant
gets maximum light energy for photosynthesis.
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18.4 The growth response of roots shoots to
gravity
The Growth response of
Roots Shoots
to Gravity
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18.4 The growth response of roots shoots to
gravity
Geotropism
shoot
root
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18.4 The growth response of roots shoots to
gravity
Geotropism
Importance
grow deep into soil to get a firm anchorage
Positive Geotropism
Negative Geotropism
Importance
leaves can receive maximal sunlight for
photosynthesis
roots can absorb maximum amount of water and
mineral salts
shoot
root
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18.4 The growth response of roots shoots to
gravity
Experiment 18.2 To study the geotropic response
of roots and shoots

• Add some moist wool in two Petri dishes.
• 2. Place three bean seedlings on the cotton wool
  in each Petri dish.
• Seedling P placed horizontally
• Seedling Q with radicle pointing upwards
• Seedling R with radicle pointing downwards

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18.4 The growth response of roots shoots to
gravity
Experiment 18.2 To study the geotropic response
of roots and shoots

• Cover the lid and place the whole Petri dish A
  with its edge on a support.
• Attach Petri dish B onto the cork disc of a
  clinostat.

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18.4 The growth response of roots shoots to
gravity
Experiment 18.2 To study the geotropic response
of roots and shoots

• Put both set-ups into a light-proof box.
• Switch on the clinostat in set-up B.
• Observe the seedlings after 1 or 2 days.

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18.4 The growth response of roots shoots to
gravity
Experiment 18.2 To study the geotropic response
of roots and shoots
Question 1 What has happened to the radicles and
plumules in set-up A? Draw a diagram to show the
appearance of the seedlings.
Answer The radicles grow downwards, while the
plumules grow upwards.
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18.4 The growth response of roots shoots to
gravity
Experiment 18.2 To study the geotropic response
of roots and shoots
Question 2 What has happened to the radicles and
plumules in set-up B? Draw a diagram to show the
appearance of the seedlings.
Answer All the radicles and plumules grow
straight because the effect of gravity is
cancelled out by the revolution of the clinostat.
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18.4 The growth response of roots shoots to
gravity
Experiment 18.2 To study the geotropic response
of roots and shoots
Question 3 Why is it necessary to leave the
apparatus in darkness?
Question 4 Do the radicles and plumules show
positive geotropism or negative geotropism?
Answer The apparatus is left in darkness to
avoid the stimulus of light.
Answer In set-up A, the radicles grow downwards
and are positively geotropic, while the plumules
grow upwards and are negatively geotropic.
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18.4 The growth response of roots shoots to
gravity
Experiment 18.2 To study the geotropic response
of roots and shoots
Question 5 What advantage do the seedlings have
by showing such geotropism?
Answer Being positively geotropic, roots grow
downwards deep into the soil to provide a firm
anchorage underground and absorb maximum water
and mineral salts in the soil. Being negatively
geotropic, shoots grow upwards to ensure that a
plant gets enough light energy for photosynthesis.
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18.5 The growth response of roots to water
The Growth response of
Roots
to Water
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18.5 The growth response of roots to water
Hydrotropism
Shoot
Root
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18.5 The growth response of roots to water
Hydrotropism
Importance
Shoot
get maximum water as a raw material for
photosynthesis
Root
get maximum mineral salts to maintain normal
growth
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18.5 The growth response of roots to water
Experiment 18.3 To investigate the relative
effects of gravity and water on the growth
response of green plants

• Prepare a 20cm X 20cm gauze square. Bend it to
  form a shallow tray that is 2cm deep.
• Place an even 1-cm deep layer of damp
  vermiculite.
• Sprinkle about 20 pea seedlings onto the
  vermiculite. Cover the seedlings with damp
  vermiculite.

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18.5 The growth response of roots to water
Experiment 18.3 To investigate the relative
effects of gravity and water on the growth
response of green plants

• Set up the apparatus in darkness as shown in the
  diagram.
• Observe the results after 2 days.

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18.5 The growth response of roots to water
Experiment 18.3 To investigate the relative
effects of gravity and water on the growth
response of green plants
Question Which response is stronger, hydrotropic
or geotropic? Explain.
Answer Some pea seedlings reverse their normal
geotropic response and grow upwards towards the
damp vermiculite. This shows that hydrotropic
response is stronger than geotropic response.
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18.6 What are the substances for controlling
growth movements in plants?
What are the substances for controlling
Growth Movements
in Plants?
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18.6 What are the substances for controlling
growth movements in plants?
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18.6 What are the substances for controlling
growth movements in plants?
Charles Darwin (1880)
Conclusion / Explanation
Experiment
Result
light
The coleoptile grew towards the light.
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18.6 What are the substances for controlling
growth movements in plants?
Charles Darwin (1880)
Conclusion / Explanation
Experiment
Result
The shoot tip was sensitive to light.
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18.6 What are the substances for controlling
growth movements in plants?
Charles Darwin (1880)
Conclusion / Explanation
Experiment
Result
This further confirmed that the tip was sensitive
to light.
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18.6 What are the substances for controlling
growth movements in plants?
Boysen-Jensen (1913)
Conclusion / Explanation
Experiment
Result
A substance produced at the tip diffused through
the gelatin down the shaded side.
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18.6 What are the substances for controlling
growth movements in plants?
Boysen-Jensen (1913)
Conclusion / Explanation
Experiment
Result
mica plate
Substance causing the bending movement moved
down the shaded side of the shoot.
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18.6 What are the substances for controlling
growth movements in plants?
Paal (1919)
Conclusion / Explanation
Experiment
Result
A substance had diffused from the tip
stimulated growth.
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18.6 What are the substances for controlling
growth movements in plants?
Fritz Went (1928)
Conclusion / Explanation
Experiment
Result
The substance passed from the coleoptile tip into
the agar block and then down into the stump.
The stump bent away from
the side with a source of that substance.
agar block
stump
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18.6 What are the substances for controlling
growth movements in plants?
Fritz Went (1928)
Result
Experiment
Result
Went named this substance auxin
agar block
stump
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18.7 More about auxins
More about
auxin
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18.7 More about auxins
organic substances
plant hormone
auxin
can regulate growth other physiological
processes
auxin
auxin
auxin
auxin
act on parts of the plant other than the part
that produces them
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18.7 More about auxins
Site of production
roots tip also produces a small amount of auxins
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18.7 More about auxins
Site of production
auxin exert their effect at
shoot tip produces auxins
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18.7 More about auxins
Effect of auxins on different parts of a plant
Stimulate shoot growth
Higher conc.
Stimulate root growth
No effect on shoot growth
Inhibit root growth
growth stimulation
Lower conc.
shoot
root
growth inhibition
Auxin concentration / parts per million (p.p.m.)
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18.8 The role of auxins in phototropic
geotropic responses
The Role of Auxins in
responses
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18.8 The role of auxins in phototropic
geotropic responses
Phototropic response of shoots
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18.8 The role of auxins in phototropic
geotropic responses
Higher concentration of auxins stimulates shoot
growth
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The shaded side grows faster than the illuminated
side
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18.8 The role of auxins in phototropic
geotropic responses
Geotropic responses of shoots and roots
Auxins diffuse down to the lower side due to
gravity
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18.8 The role of auxins in phototropic
geotropic responses
high concentration of auxins stimulates shoot
growth
high concentration of auxins inhibits root growth
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Mind Map
Tropism
is caused by
growth movement
includes
hydrotropism
geotropism
phototropism

auxins
unilateral stimulus
nature of stimulus
nature of stimulus
nature of stimulus
which are
water
light
pull of gravity

plant hormones
result
produced by
illuminated unilaterally on shoots
on shoots and roots

positive response by roots


shoot tips and root tips
auxins diffuse down to lower side

auxins move to shaded side
importance
stimulate growth at

roots can absorb more water and mineral salts
so the roots
so the shoots
so the shoots

region of elongation in shoots and roots


bending downward
bending upward

bending toward light source

roots can penetrate into soil for anchorage and
to absorb more water and mineral salts

result
result
leaves gain maximum amount of light for
photosynthesis
result
import-ance

positive response by shoots

positive response by roots

negative response by shoots
import-ance
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