form 2-plants tropism

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

18.1 Do plants respond to external stimuli?

Plants

StimulusStimulus

ResponseResponse

YES!YES!

18.1 Do plants respond to external stimuli?

Will it happen?

18.2 What are tropisms?

plant

stimulus

stimulus

stimulus

Response

growth

TropismTropism

directional growth movement made by a plant

directional growth movement made by a plant

in response to a unilateral stimulus

in response to a unilateral stimulus


plant

stimulus

plant

stimulus

positive tropismpositive tropism negative tropismnegative tropism

away from stimuliaway from stimulitowards stimulustowards stimulus

Direction of growthDirection of growth


Phototropism

H2O

H2O H2O

H2OH2O

water

Hydrotropism

gravity

Geotropism

light


18.3 The growth response of shoots to light

shoot

root

light

light

light

Phototropism


shoot

root

light

light

light

Phototropism

Positive phototropism

Positive phototropism

Negative phototropism

Negative phototropism

Leaves can gain maximum amount

of light for photosynthesis

Leaves can gain maximum amount

of light for photosynthesis

Importance:Importance:



ClinostatClinostat

ClinostatClinostat

cancel out the effects of unilateral light & gravity by rotating

the disc

cancel out the effects of unilateral light & gravity by rotating

the disc

direction of rotation

set up as a control for investigation of

growth responses of roots & shoots to

unilateral light & gravity

set up as a control for investigation of

growth responses of roots & shoots to

unilateral light & gravity


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

Experiment 18.1To investigate the phototropic response of shoots

unilateral light

light-proof box

set-up A

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.



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

4. Put pot C in complete darkness. Both pots B and C are controls.

5. Observe the plants after 1 or 2 days. set-up B

unilateral light

clinostat

in complete darkness

set-up C

light-proof box



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.



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.



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.



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.


18.4 The growth response of roots & shoots to gravity

ground

Gravity

Geotropism

shoot root


ground

Geotropism


leaves can receive maximal

sunlight for photosynthesis

leaves can receive maximal

sunlight for photosynthesis

grow deep into soil to get

a firm anchorage

grow deep into soil to get

a firm anchorage

roots can absorb

maximum amount of water and

mineral salts

roots can absorb

maximum amount of water and

mineral salts


Positive Geotropism

Negative Geotropism

shoot root

Gravity


Experiment 18.2To study the geotropic response of roots and shoots

1. 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



3. Cover the lid and place the whole Petri dish A with its edge on a support.

4. Attach Petri dish B onto the cork disc of a clinostat.

Set-up A Set-up B

support

in complete darkness in complete darkness

Petri dishes

plumule radicleP1

P2

Q1 R1

Q2 R2



5. Put both set-ups into a light-proof box.

6. Switch on the clinostat in set-up B.

7. Observe the seedlings after 1 or 2 days.

Set-up A Set-up B

support

in complete darkness in complete darkness

Petri dishes

plumule radicleP1

P2

Q1 R1

Q2 R2



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.

P1

Q1 R1



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.

P2

Q2 R2



Question 3:

Why is it necessary to leave the apparatus in darkness?

Answer:

The apparatus is left in darkness to avoid the stimulus of light.

Question 4:

Do the radicles and plumules show positive geotropism or negative geotropism?

Answer:

In set-up A, the radicles grow downwards and are positively geotropic, while the plumules grow upwards and are negatively geotropic.



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.


18.5 The growth response of roots to water

Shoot

Root

Dry Wet

Hydrotropism


Shoot

Root

Dry Wet

Hydrotropism

get maximum water as a raw material for

photosynthesis

get maximum water as a raw material for

photosynthesis

get maximum mineral salts to maintain normal growth

get maximum mineral salts to maintain normal growth



Positive Hydrotropism

No response

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

1. Prepare a 20cm X 20cm gauze square. Bend it to form a shallow tray that is 2cm deep.

2. Place an even 1-cm deep layer of damp vermiculite.

3. Sprinkle about 20 pea seedlings onto the vermiculite. Cover the seedlings with damp vermiculite.



4. Set up the apparatus in darkness as shown in the diagram.

5. Observe the results after 2 days.

water



water

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.


18.6 What are the substances for controlling growth movements in plants?

coleoptile

first leaf

grass seedused in the experiments for finding out the factors

controlling the growth movements in plants

a protective sheath surrounding the young shoot of the embryo in

plants of the grass family


Charles Darwin (1880)Charles Darwin (1880)

Experiment Result Conclusion / Explanation

light

The coleoptile grew towards the light.

coleoptile


Result

The shoot tip was sensitive to light.

Experiment

decapitated coleoptile

light



Conclusion / Explanation

Experiment Result

This further confirmed that the tip was sensitive to light.


opaque cap

light



light

gelatin allows

substances to diffuse

down

Experiment Result

A substance produced at the tip diffused through the gelatin down the shaded side.

Boysen-Jensen (1913)Boysen-Jensen (1913)



light

Experiment Result

mica plate Substance

causing the bending movement moved down the shaded side of the shoot.

Boysen-Jensen (1913)Boysen-Jensen (1913)

light

light

it does not allow

substances to diffuse

down



Paal (1919)Paal (1919)

Experiment Result

A substance had diffused from the tip & stimulated growth.



Fritz Went (1928)Fritz Went (1928)

Experiment Result

agar block

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.

stump



Went named this substance

auxin

Result

Fritz Went (1928)Fritz Went (1928)

Experiment Result


agar block

stump

18.7 More about auxins

auxin

auxin

auxin

auxin

auxin

plant hormone

organic substances

can regulate growth & other physiological

processes

act on parts of the plant other than the part that produces

them


roots tip also produces a

small amount of auxins

auxins are transported downwards to the root

Site of productionSite of production

shoot tip produces auxins


12345678

mark at 2 mm intervals

shoot tip produces auxins region of

elongation

auxin exert their effect at

12

3

4

5678

Site of productionSite of production


rootshoot

Lower conc.

Lower conc.

Higher conc.

Higher conc.

Stimulate root growth

Stimulate root growth

Inhibit root growth

Inhibit root growth

No effect on shoot growth

No effect on shoot growth

Stimulate shoot

growth

Stimulate shoot

growth

Effect of auxins on different parts of a plantEffect of auxins on different parts of a plant%

gro

wth

s

tim

ula

tio

n%

gro

wth

in

hib

itio

n

Auxin concentration / parts per million (p.p.m.)


18.8 The role of auxins in phototropic & geotropic responses

Phototropic

Geotropic


unilateral light

Unilateral light causes auxins to accumulate on the shaded side of

the shoot

Unilateral light causes auxins to accumulate on the shaded side of

the shoot

1

Phototropic response of shoots

Phototropic response of shoots

auxins


unilateral light

Higher concentration of auxins stimulates shoot growth

Higher concentration of auxins stimulates shoot growth

The shaded side grows

faster than the illuminated

side

The shaded side grows

faster than the illuminated

side

2

3

The shoot grows & bends towards

the light

The shoot grows & bends towards

the light

4


gravity

Auxins diffuse down to the lower side due to gravity

Auxins diffuse down to the lower side due to gravity

Geotropic responses of shoots and roots

Geotropic responses of shoots and roots


the root bends downwards

the root bends downwards

lower side of the shoot grows faster than the upper side

lower side of the shoot grows faster than the upper side

the shoot bends upwards

the shoot bends upwards

high concentration of auxins stimulates shoot growth

high concentration of auxins stimulates shoot growth

lower side of the root grows more slowly

lower side of the root grows more slowly

high concentration of auxins inhibits root growth

high concentration of auxins inhibits root growth

Plant Tropisms

1. Tropism: the way a plant grows in response to stimuli in the environment.

a.Phototropism: growth response to light

-Plants bend towards light

b.Geotrophism: growth response to gravity

-plant roots grow down with gravity, shoots (stems) grow up against gravity and out of the soil.

c.Thigmotropism: growth response to touch

-vines grow up around trees, venus flytrap closes when leaves are touched

Slide # 20

Slide # 21

Geotro

pism

What type of tropism is shown in these pictures?

Thigmotrophism

Thigmotrophism

Geotropism

Phototropism

Phototropism

Mind MapTropism

growth movement

hydrotropismgeotropism

pull of gravity

phototropismunilateral stimulus

planthormones

shoot tips and root tips

light water

auxins

auxins move to shaded side

bending toward

light source

positive response by shoots

positive response by roots

roots can penetrate into

soil for anchorage and to absorb more

water and mineral salts

roots can absorb more water and

mineral salts

auxins diffuse down to lower

side

bending downward

leaves gain maximum

amount of light for

photosynthesis

includes

is caused by

which are

produced by

import-ance

import-ance

resultresult

result

importance

so the shoots

so the roots

on shoots and roots

illuminated unilaterally on shoots

nature of stimulus

nature of stimulus

nature of stimulus

region of elongation in

shoots and roots

stimulate growth at

bending upward

so the shoots

positive response by roots

negative response by shoots

result

form 2-plants tropism

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