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