Biology Form 5 Chapter 3 - Coordination & Response Part 6 - Plant Hormones

• Plants don’t have a brain.

• Some questions to think about.
– How does a tree know when to drop its leaves?
Copyright © 2010 Ryan P. Murphy

• When does a plant know when to grow,
and when to remain dormant?

• How does a tree know when spring has
arrived and it’s time to create buds?

• How can a plant move from one side to
another?
– It doesn’t even have muscles.

• How does a plant know when to create
flowers, and what color they should be?

• How does a plant know when the flower has
been fertilized and it’s time to make seeds?

• How does a banana know when to ripen?

• How does a banana know when to ripen?
“Hey Phyllis” “I’m
getting these strange
marks on my peel.”

• When does a seed know it’s time to
germinate?

• When does a seed know it’s time to
germinate?
– Too early, or too late could be fatal to the young
plant.

• Does anybody know the answer…?

• Answer: Plant Hormones.

 New Area of Focus: Plant Hormones.

• Plant hormones are chemicals that affect
flowering;

flowering; aging;

flowering; aging; root growth;

flowering; aging; root growth; distortion,

killing of leaves,

killing of leaves, stems,

killing of leaves, stems, and other parts;

prevention or promotion of stem
elongation;

elongation; color enhancement of fruit;

prevention of leafing and/or leaf fall;

prevention of leafing and/or leaf fall; and
many other conditions.

 Plant hormones are chemicals that affect
all aspects of the plants life.

Some plant hormones :
 auxin
 gibberellin
 cytokinin
 ethylene
 abscisic acid

• Do plants move?

• Plants do move, although this movement is
very little.

• Video! Phototropism in plants.

 Auxin: Promotes stem elongation and bud
dormancy.
 Phototropism: When plants grow toward a
light source.

• A plant moves by elongating the cells on
the opposite side of light, causing the plant
to bend toward the light.

• Activity! Feeling phototropism in plants.
– Stand up tall, feet slightly apart, and bend
toward the light.
– What part of your body elongates / stretches,
and which shrinks in?

• Note: Plants do not have muscles, so the
elongation and contraction occurs on a
cellular level.
Elongated
Cells
Shortened
Cells

• Auxin also…
– Stimulates cell division.
– Stimulates differentiation of phloem and xylem
– Response of bending in response to gravity
and light.
– Delays growth of lateral buds.
– Can induce fruit setting and growth in some
plants.
– Delays fruit ripening.
– Stimulates growth of flower parts.

• Which plant was given extra gibberellin
hormones?

• Plant B shows extra
stem elongation over
plant A.
• Plant A is the normal
plant / control while
plant B is the variable.
• What is the
responding variable?
Height

 Gibberellins: Make stems longer.

“My mom gave a
lot of gibberellins
as a young
sprout and look
what happened.”

• Gibberellins also…
– Stimulates flowering in
response to long days.
– Breaks seed dormancy
– Induces maleness in
some flowers (sex
expression).
– Can cause fruit
development.
– Can delay dropping of
leaves and citrus fruits.

Fig. 28-19
Effect of Gibberellin

 Cytokinins: Promotes cell division. They
are produced in growing areas like the
tips.
Root Tip

 Abscisic Acid: Opens and closes stomata,
has role in seed dormancy.

• Note: Stomata are small pores (openings)
for gas exchange.

• Abscisic Acid…
– Inhibits shoot growth (inhibits gibberellin).

• Abscisic Acid…
– Inhibits shoot growth.
– Induces seeds to synthesize storage proteins.
– Helps to recognize and fix wounding on an
injured plant.

• What does the one bad apple do…?

• What does the one bad apple do…?
– Why?

• One bad apple spoils the bunch because that
bad apple is releasing ethylene gas which
causes the others to ripen.

• One bad apple spoils the bunch because that
bad apple is releasing ethylene gas which
causes the others to ripen.
– Remove ripe fruit from the bunch to keep fruit
longer.

 Ethelyene: A gas that promotes fruit
ripening.

• Which of these
methods will make
your tomatoes
ripen faster and
why?
• Putting them on
a sunny
windowsill.
• Putting them in
a paper bag.
W
O
R
K
T
O
G
E
T
H
E
R

• Ethylene also…
– Stimulates the release of seed dormancy.
– Stimulates shoot and root growth.
– Stimulates leaf and fruit abscission.
– Creates femaleness in flowers.
– Stimulates flower opening.
– Stimulates flower and leaf dying.

• Class poll, Are plants more complicated,
and perform exciting processes that you
never thought of until studying plant
hormones.
Yes _______ No __________
Learn more about plant hormones (Advanced) at…
http://www.plant-hormones.info/ Easier at…
http://www.ext.colostate.edu/mg/gardennotes/145.html

Plant responses to stimuli are given specific names:
water gravitylight
hydrotropism geotropismphototropism
Are there different types of tropisms?
Touch
•Thigmotropism

Phototropism
is a growth
response to light
Light
Geotropism
is a growth response
to gravity

Which way up?
How do plants always grow the right way up?
Plants respond to gravity to grow in the right direction.

•Roots always grow towards water, which is
positive hydrotropism.
•Roots will grow sideways, or even upwards, towards
water.
•Roots always have a stronger response to water than
gravity to ensure that a plant gets the water it needs.
How do roots respond to water?

How do roots and shoots respond to stimuli?
 Shoots grow towards sunlight.
They are positively phototropic.
Plant stimuli affect certain parts of the plant in different ways.
 Shoots grow away from gravity.
They are negatively geotropic.
 Roots grow away from sunlight.
They are negatively phototropic.
 Roots grow towards gravity.
They are positively geotropic.
What experiments can be used to test these ideas?

A shoot is:
POSITIVELY
PHOTOTROPIC
NEGATIVELY
GEOTROPIC

A root is:
 POSITIVELY GEOTROPIC
 NEGATIVELY PHOTOTROPIC

Advantages of a shoot growing:
 Towards light:
 To trap more light for photosynthesis
 Away from gravity:
 shoot of a germinating seed grows out of
the soil
 more chances for pollination & seed
dispersal

I am growing
towards gravity,
but where is the
water????

Advantages of a root growing towards
gravity:
 To anchor the plant
 To search for water
 To ensure that the root of a germinating
seed always grows downwards, whatever,
the position of the seed in the soil

Study the pictures below and then complete the
table by putting a plus (+) if the shoot or root
grows towards the stimulus and a minus (-) if it
grows away from it.
Stimulus
Light Gravity
Shoot
Root

Study the pictures below and then complete the
table by putting a plus (+) if the shoot or root
grows towards the stimulus and a minus (-) if it
grows away from it.
Stimulus
Light Gravity
Shoot + -
Root - +

Thigmotropism
• Thigmotropism Plant’s growth response to touching a
solid object
• Tendrils and stems of vines (ex. Morning glory) coil when
they touch an object
• Allows vines to climb other plants / objects
• Increases chance of finding light (for photosynthesis)
• Auxin and Ethylene are thought involved.

Chemotropism
• Chemotropism  plant’s growth in response to
chemicals
• Eg. After pollination, pollen tube grows down
through stigma and style and enters ovule
through micropyle

• What type of tropism is shown in these pictures?

The houseplant observation
• For years, people
noticed that
houseplants tended to
lean toward a source of
light.
• Charles Darwin and his
son Francis in 1880,
wondered why. How
does a plant “know”
where to lean?

Darwin’s Oats
• The Darwins
studied the leaning
phenomenon in
oats.
• Oat coleoptiles are
highly light
sensitive, and
growth is fairly
rapid.

A Coleoptile is a :
 hollow, cylindrical sheath that surrounds the
primary leaf of a germinating monocot seed
Coleoptile First leaf

Darwin Experiment 1
Oat shoots tend to bend toward the light. When the tip of the shoot is
covered with a small cap, the shoot does not bend.
Question: Why
doesn’t the shoot
with the cap bend
toward the light?

One hypothesis...
• The Darwins speculated:
• the tip of the plant detects the light and
communicates chemically with the part of the
shoot that bends.
• Question: How could they test these alternative
explanations?
• The cap itself prevents bending.
• Light further down the shoot, rather than on
the tip, causes bending.

Darwin Experiment 2
Some shoots were covered with small caps of glass. Others were covered
with a sleeve that left the tip exposed but covered the lower shoot.
• The cap itself
prevents
bending.
• Light further
down the shoot,
rather than on
the tip, causes
bending.
X
X
Conclusion :
The tip is the place
responsible for bending

Boysen-Jensen
• Peter Boysen-Jensen continued
Darwins’ experiments.

Boysen-Jensen 1
• Boysen-Jensen cut the tips off of
oat coleoptiles
• They did not bend toward the light.
• Question: What does this tell us
about the role of the tip in this
phenomenon?

Boysen-Jensen 2
• Boysen-Jensen cut the tips off coleoptiles
and put the tips back on.
• These coleoptiles bent toward the light.
Conclusion :
The tip is responsible for growth

Boysen-Jensen 3
Boysen-Jensen then put a porous barrier (agar gel) and an
impenetrable barrier (a flake of mica) between the shoot tip and the
rest of the shoot. The shoot with an agar barrier bent toward the
light. The shoot with the mica barrier did not.
Conclusion :
The head must
have produced
a diffusible
chemical
substance
which
stimulates
growth

Boysen-Jensen 4
Boysen-Jensen took a tiny, sharp sliver of mica and pushed it into the
coleoptile so that it cut off communication between the tip and the rest of
the plant on one side only. If the sliver was on the side that was lit, it still
leaned that toward the light, but if it was on the opposite side, the plant
did not lean toward the light.
Explanation of
results :
Higher
concentration of
that chemical that
causes growth on
the side opposite
the light.

F.W. Went identified the factor that was
causing plants to bend toward the light.
Went first cut the tips off of oat coleoptiles and placed them on a block of
agar and allowed juices from the tip to diffuse into the agar.

What is the effect of auxins on plant cells?
Cells absorb water and elongate.

Three regions of a root tip
Cell division region
Cell
differentiation
region
Cell elongation region
Cells become
specialised

Which region does auxin affect?
Cell division region
(root tip that makes auxin)
Cell differentiation region
(cells change their shape;
no effect by auxin)
Cell elongation region
(auxin makes cells absorb water)

Explain why:
markings have
moved away from
each other ONLY in
the lower region.
Cell elongation
region

A shoot growing in even illumination grows
straight up. Why?
auxin
Cells on both sides of the
stem elongate equally.

A shoot growing in one-sided illumination.

A shoot growing in one-sided illumination
grows towards light. How is it possible?
Cells on the two
sides of the stem
GROW UNEQUALLY!!

Describe how a shoot bends towards
one-sided illumination.
Auxins are produced at the ___________ tip and
diffuse down the stem.
More auxin collects on the ____________ side of
the stem.
Auxins cause the cells to absorb water and so
elongate in the cell elongation region.
Thus the ___________ side grows more than the
lighted one.
This results in bending of the stem towards light.
shoot
shaded
shaded

Effect of Auxin on Plant Parts
positive
growth
negative
growth
root
shoot
auxin
concentration
Growth of
plant
low conc. high conc.
auxin concentration
plant growth
shoot
root
No effect positive
positive negative
high
conc.
low
conc.

-the concentration of auxins needed for maximum
growth response in shoots inhibits root growth
-the concentration that stimulates root growth is too
low to stimulate shoot growth

Gravitropism in shoots
• In shoots, auxins
are more
concentrated on the
lower side of the
stem, causing the
cells there to
elongate.
• Why is this
gravitropism and
not phototropism?

Gravitropism in roots
• In roots, however,
auxin concentration
on the lower side of
the root suppresses
cell elongation.
• The upper side of
the root continues
to grow, causing the
roots to bend
downward.

Geotropism in Root
 The root tip produces auxins which diffuse to the
elongation region
 The auxins diffuse to the lower side due to gravity
 High auxin concentration inhibits growth in root
 The lower side grows slower than the upper side
 The root bends downward

•AUXIN is produced in the
• apical meristem at the
tips of the shoots ( also
known as COLEOPTILE)
•From the tip of shoot,
auxin move down to the
region of cell elongation &
stimulates growth of the
cells. The cells become
LONGER.
•IMPORTANT TO KNOW

IMPORTANT INFORMATION TO KNOW
ABOUT AUXIN
• Auxin is sensitive to light.
• It always moves away from light.
• Auxin stimulates the elongation of cells
in the shoots but, inhibits the
elongation of cells in the roots

VIEW THE ACTUAL EXPERIMENT
IN THE NEXT
3 SLIDES

Nastic Movements
 Nastic movements  plant movements
 that occur in response to environmental
stimuli
 but are independent of the direction of
stimuli
 Quick, temporary movements in plants.
 No plant growth, so they can be reversed.
 Regulated by changes in water pressure of

Nastic
movement
in the
sensitive
plant
(Mimosa
pudica)
SEISMONASTI
Stimulus : touch

Thigmonastic Movements
 Thigmonastic ‘thig-mah-NAS-tik’ movements  a
type of nastic movement that occurs in response to
touching or shaking a plant
 Involve rapid plant movements
 Ex. Venus flytrap

PLANT HORMONES
AGRICULTURAL USES

165 of © Boardworks Ltd 2007
 growing cuttings
 killing weeds.
 ripening fruit
Why are these purposes useful for gardeners and farmers?
Plant hormones –
• naturally-occurring chemicals
• can also be produced synthetically
• for use in gardening and agriculture such as:
How can plant hormones be used?

How are cuttings grown?
To stimulate root
growth, cuttings are
dipped into rooting
powder. This
contains plant
growth hormones.
Cuttings are genetically
identical to the parent
plant. This allows
growers to copy
successful plants.

Fruit often has to
travels thousands of
miles from where it is
picked to where it is
sold in the shops.
How are hormones used to ripen fruit?
When the fruit reaches its destination, it can then be sprayed
with artificial plant hormones to encourage ripening.
Fruit is often picked before it is fully ripe. Why might this
help keep the fruit edible?
Unripe fruit is harder and
less likely to bruise than
ripe fruit when transported.
Why might it be a good idea to wash fruit before eating it?

Ethylene and fruit ripening
•With•Without

PARTHENOCARPY
Parthenocarpy is the natural or artificially
induced production of fruit without
fertilization of ovules.
(development of fruit without fertilization)
This method produces seedless fruits
Auxin is sprayed to the flower to stimulate it
to turn into fruit. So no fertilization is
required.

 Sometimes gardeners cut the tops off plants
to make them more bushy

Auxins sometimes stop growth
 auxin passes down the stem and prevents
side branches growing out
 so the plant grows tall and
straight

 If the top is cut off from the plant:
 flow of auxin stops
 side branches will grow out

EXPERIMENTS USING OAT
COLEOPTILES

no growth grows
dark box
few days later
Conclusion :
The tip is responsible for growth
cut & replaced tip
decapitated
Experiments
on Oat Coleoptiles

unilateral
light
few days later
black covers
Conclusion :
the tip is the place responsible for bending
Experiments
on Oat Coleoptiles

Experiments
on Oat Coleoptiles
few days later
dark box
agar plate
Conclusion :
the head must have produced a diffusible
chemical substance which stimulates growth

Experiments
on 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
mica
plate
Can the explanation still work ?
bending directions

Study the diagrams. What would you
expect to happen and why.
The shoot with the covered tip grows (straight
up / towards light) because it (can sense light /
cannot sense light).

The shoot with the exposed tip grows (straight
up / towards light) because it (can sense light /
cannot sense light).

Look at the diagrams below. What does this
experiment tell you about the tip of the shoot
and the part it plays in growth?
Tip produces chemicals for growth.

Look at the diagrams below. How would you
explain the results of this experiment?
Bending resulted due to unequal amount of
chemical . Light affects distribution of chemical.

QUESTION: MAY, 2009
Give a biological explanation for each of the
following statements:
cutting off the tip of a shoot will stop the shoot
from getting taller but it will increase
bushiness; (3)

EXPERIMENTS TO SHOW:
1. Geotropism in a shoot
2. Geotropism in pea radicles
3. Phototropism in oat coleoptiles
4. Phototropism in a shoot
All make use of an apparatus called:
clinostat

Clinostat: a motor rotates slowly
 to make factors uniform (evenly distributed)
 for control experiments
 contains a cork disc which can be set to
rotate in a vertical or horizontal plane by a
motor

AIM: To show geotropism in a plant shoot.
APPARATUS:
CONTROL
Give a precaution for this experiment.
Even illumination.
The apparatus was
left for 3 days.

Why did the shoot attached to the clinostat
grow horizontally ?
As shoot rotated, gravity acted on all sides.
No unilateral stimulus to respond to.

AIM: To show geotropism in pea radicles.
APPARATUS:
 The apparatus was left for 3 days.

QUESTION: MAY, 2005
A biology student wanted to test whether the
position in which seeds are placed in the soil
affects the direction of shoot and root growth.
Design a simple experiment which the student
can use to carry out this investigation.(5)

 Seeds of the same type are placed at
different orientations in a single container.
 A similar container is set up as a replicate.
 Left in darkness for a few days.
 Radicles are observed.

QUESTION: MAY, 2005
When growing plants from cuttings, farmers
usually dip the cutting into Rooting Hormone
powder before planting it in the soil. Briefly
explain how this increases the chances of
successful plant growth. (2)
Roots are stimulated to
form. Thus plant can absorb
more water and ions for
growth.

AIM: To show phototropism in oat coleoptiles.
APPARATUS:

METHOD:
1. Thirty oat grains were soaked in water for 1 day.
2. Ten grains were placed in each of three small pots of
compost and left to germinate.
3. When grown to a height of 1cm, they were treated as
follows:
 One set was covered with a large box, excluding
light;
 Another set was covered with a box with a slit at
the side, allowing one-sided illumination;
 The control was left in the light.
4. The pots were left at room temperature for a few
days.

RESULT:
 In the dark: coleoptiles were yellow, straight and
very long.
 In one-sided illumination: coleoptiles bent towards
the light.
 In the light: coleoptiles were green, straight and
shorter than those in the dark.
CONCLUSION:
 The coleoptiles responded to the stimulus of one-
sided illumination by growing towards it. Coleoptiles
are positively phototropic.
 Light is needed for chlorophyll to form.

AIM: To show phototropism in a shoot.
APPARATUS:
unilateral
light
unilateral
light
light-proof
box
clinostat
A B

unilateral
light
unilateral
light
light-proof
box
clinostat
The shoots in pot A respond by growing towards the
light source. The shoot in pot B do not show any
curvature but grow vertically upwards...
What has happened to the shoots of plants A and B ?
A B

unilateral
light
unilateral
light
light-proof
box
clinostat
A B
What is the effect of the clinostat on the shoot of plant B ?
The revolving clinostat cancel out the effect of
unilateral light on the shoot of plant B.

Question: MAY, 2006
5. The experiment shows the effect of a stimulus
on the growth of a root and a shoot. The seedlings
were kept in the dark throughout the experiment.

a) Continue the diagram to show the direction of growth
of the shoot and root in:
i) the stationary
ii) rotating clinostat. (4)

b) What type of stimulus response is exhibited by
the seedlings? (1)
Positive geotropism.
c) Why were the seedlings kept in the dark? (2)
Any changes observed would be due to gravity.
d) Name the substance produced by the plant which
brings about the growth changes. (1)
Auxin

e) This experiment was
investigated in an orbiting
spacecraft where
weightlessness is observed.
Predict the results obtained
in this situation. (1)
Grow in all directions.

Under which condition (light/dark)
was each seedling of bean grown?
Grown in the
light
Grown in the
dark

What is the advantage of a
longer stem in seedlings
grown in the dark?
light
dark

Four differences between a seedling
growing in the light & another in the dark.
In the light In the dark
1. Short, thick stem long, thin stem
2. Green shoot yellow shoot

Four differences between a seedling
growing in the light & another in the dark.
In the light In the dark
3. Broad leaves small leaves
4. Short internode long internode

QUESTION: MAY, 2009
Give a biological explanation for each of the
following statements:
the internodes of a shoot growing in the dark
are longer than the internodes of a shoot
growing in the light; (3)

Differences in the response shown by:
A shoot in unilateral light A hand touching a hot
object

Differences in the response shown by:
A shoot in unilateral light A hand touching a hot
object
1. Response in positive. 1. Response is negative
2. Slow response. 2. Rapid response
3. Response involves
growth.
3. No growth involved
4. Response does not
involve muscles and
nerves.
4. Response involves
muscles and nerves.

Biology Form 5 Chapter 3 - Coordination & Response Part 6 - Plant Hormones

Biology Form 5 Chapter 3 - Coordination & Response Part 6 - Plant Hormones

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Biology Form 5 Chapter 3 - Coordination & Response Part 6 - Plant Hormones