Growth Response of Plants Prepared by Ms W.S.Kwan.

Growth Response of Plants

Prepared by Ms W.S.Kwan

Plants have no nervous system.Can plants detect stimulus?Can plants response?

A directional growth movement made by a part

of a stationary plant in response to unilateral

stimulus

Types of TropismTypes of Tropism

hydrotropismgeotropism

stimulus

tropism

watergravity unilateral lightphototropism

Geotropism

• Geotropism is the growth movement (growth response) shown by a plant part in response to gravity

To show To show Geotropism Geotropism in Seedlingsin Seedlings

To show Geotropism in To show Geotropism in SeedlingsSeedlings

Why is it necessary to leave the apparatus in darkness ?Ans: To avoid any stimulus from light.

To show Geotropism in To show Geotropism in SeedlingsSeedlings

What has happened to the radicles and plumules ?Ans: The radicles grow downwards, while the plu

mules grow upwards.

To show Geotropism in To show Geotropism in SeedlingsSeedlings

Do the radicles show positive geotropism (towards gravity) or negative geotropism (away from gravity)?Ans: Positive geotropism.

What advantage do the seedlings have by showing such geotropism ?

Ans: Roots grow downwards deep into the soil to provide a firm anchorage underground and absorb water and mineral salts in the soil.

Importance of GeotropismImportance of Geotropism((Roots grow towards gravityRoots grow towards gravity))

provide firm anchorage for roots

ensure the plant can get adequate supply of water & mineral salts

ClinostatClinostat for control experiments contains a cork disc which can be set to rotate

in a vertical or horizontal plane by a motor Cancel out the effect of unilateral stimulus to make factors uniform (evenly distributed)

To show the effect of To show the effect of Gravity by using a Gravity by using a

ClinostatClinostat

What is the purpose of marking the radicles at equal intervals ?Ans: To find out which region of the radicle grows f

aster.

plastic cover lined with moist blotting paper

X

Y

in complete darkness

Does the direction of growth of the seedlings in clinostat X change ?

Ans: The radicle of the seedlings in clinostat X bends downwards in response to the pull of gravity.

plastic cover lined with moist blotting paper

X

Y

in complete darkness

seedling in X

seedling in Y

Does the direction of growth of the seedlings in clinostat Y change ? Why ?Ans: Since every side of the radicle of the seedling i

n clinostat Y is acted on by gravity, the radicle will grow straight horizontally.

plastic cover lined with moist blotting paper

X

Y

in complete darkness

seedling in X

seedling in Y

What is the purpose of setting up clinostat Y ?Ans: Serve as a control for comparison.

plastic cover lined with moist blotting paper

X

Y

in complete darkness

seedling in X

seedling in Y

Shoots show negatively geotropic

Phototropism

• Phototropism is the growth movement (growth response) shown by a plant part in response to light.

To show To show Phototropism in Phototropism in

ShootsShoots

What is the effect of the clinostat on the shoot of plant B ?Ans: The revolving clinostat cancel out the effect of

unilateral stimulus (light) on the shoot of plant B.

unilateral light

unilateral light

in complete darkness

A B C

light-proof box clinostat

What has happened to the shoots of plants A and B ?Ans: The shoots in pot A respond by growing towar

ds the light source.

The shoot in pot B do not show any curvature but grow vertically upwards.

unilateral light

unilateral light

in complete darkness

A B C

light-proof box clinostat

What has happened to the shoot of plant C ?

Ans: the shoots in pot C grow vertically upwards with slender and longer nodes(elongated), and carries smaller leaves which are yellow in colour (etiolated).

unilateral light

unilateral light

in complete darkness

A B C

light-proof box clinostat

What is the significance of phototropism to plants ?Ans: This tropism ensures that a plant gets enough l

ight energy for photosynthesis.

unilateral light

unilateral light

in complete darkness

A B C

light-proof box clinostat

Importance of Importance of PhototropismPhototropism

((Shoots grow towards light Shoots grow towards light

sourcesource)) enables leaves to be in a position to receive maximum sunlight for photosynthesis

Hydrotropism

• Hydrotropism is the growth movement (growth response) shown by a plant part in response to water.

Importance of Importance of HydrotropismHydrotropism

((Roots grow towards water Roots grow towards water supplysupply))ensures that a plant gets

enough water as a raw material for photosynthesis &

enough mineral salts to maintain normal growth

Types of TropismTypes of Tropism

shoot

root

hydrotropism

no response

positive

geotropism

negative

positive

stimulus

tropism

response

watergravity unilateral lightphototropism

positive

negative

Mechanism

Darwin Experiments

no growth

grows

dark box

few days later

Conclusions :The tip is responsible for the growth

cut & replaced tip

decapitated

Experiments Experiments on Oat Coleoptileson Oat Coleoptiles

unilaterallight

few days later

black covers

Conclusion :

The tips are sensitive to light.(responsible for the detection of light)

Experiments Experiments on Oat Coleoptileson Oat Coleoptiles

Experiments Experiments on Oat Coleoptileson Oat Coleoptiles

few days later

dark box

agar plate

Conclusions :The tip must have produced a diffusible chemical substance which stimulates growth

Experiments Experiments on Oat Coleoptileson Oat Coleoptiles

bending directions

Explanation of results :The side with the tip has a higher concentration of that particular chemical, the growth rate is faster than the other side.

the tip is put aside

bending directions

micaplate

Can the explanation still work ?

AuxinsAuxinscommonest plant hormones

produced by shoot tip regulate growth & other physiological processes in plants

promote growth by inducing cell elongation & cell division.

Effect of auxin on Plant ParEffect of auxin on Plant Partsts

positivegrowth

negativegrowth root

shoot

auxinconcentration

Growth ofplant

high conc. low conc.

auxin concentrationplant growth

shoot

root

Positive (promote)

no effect

Negative (inhibit)

Positive (promote)

highconc.low

conc.

Phototropism in Phototropism in ShootShoot

The shoot tip produces auxins which diffuse downward

The auxins diffuse to the shaded side due to the presence of unilateral light

High auxin concentration stimulates growth in shoot

The shaded side grows faster than the lighted side

The shoot bends towards the light

Geotropism in RootGeotropism in Root

The auxins diffuse to the lower side due to gravity High auxin concentration inhibits growth in root

The lower side of root grows slower than the upper side

The root bends downward

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