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3.5.2 Responses in the Flowering Plant
Growth regulation
Tropisms: definition of the following:
Phototropism
Geotropism
Thigmatropism
Hydrotropism
Chemotropism
Examples of phototropism and geotropism
Regulatory system:
Definition of a growth regulator
Transport through the vascular system
Combined effect
Growth promoter and growth inhibitor
Name four methods of anatomical or chemical adaptation that protect plants.
Syllabus P. 37
Contemporary Issue
Use of plant regulators: any 2 examples
Mandatory Activity
Investigate the effect of I.A.A. growth regulator on plant tissue
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Growth regulation
Some external factors that regulate the growth of plants are light intensity, day length, gravity,
temperature.
Among the principal internal factors that regulate these responses is the production of a series of
chemicals called growth regulators in regions of the plant called meristematic regions.
Give some examples of meristematic regions.
Tropisms:
Definition of the following:
Phototropism
Geotropism
Thigmatropism
Hydrotropism
Chemotropism
Emphasise the significance of phototropism and geotropism with
Regulatory system:
Definition of a growth regulator, transport through the vascular system
Different combinations of regulators bring about different effects.
Some regulators promote growth e.g auxins
Others inhibit growth e.g ethane and abscisic acid
Describe any 4 methods by which plants protect themselves from adverse external environments –
anatomical/chemical e.g. heat shock, proteins or stress proteins
T.G. P. 66
Contemporary Issue
Use of plant regulators: any 2 examples
Mandatory Activity
Investigate the effect of I.A.A. growth regulator on plant tissue
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NOTES
A stimulus is anything that causes a reaction in an organism, or in any of its parts.
A response is the activity of the cell or organism as a result of the stimulus.
Growth regulation
The growth of a flowering plant can be controlled by external and internal factors.
Some external factors that regulate the growth of plants are light intensity, day
length, gravity, temperature.
Among the principal internal factors that regulate these responses is the production
of a series of chemicals called growth regulators in regions of the plant called
meristematic regions.
Examples of meristematic regions: apical meristems e.g. root tip and shoot tip
Tropism: A tropism is a change in the growth of a plant in response to a stimulus.
Positive tropisms : Growth towards stimulus
Negative tropisms: Growth away from stimulus

Phototropism: A change in the growth of a plant in response to light

Geotropism: A change in the growth of a plant in response to gravity

Thigmotropism: A change in the growth of a plant in response to touch

Hydrotropism: A change in the growth of a plant in response to water

Chemotropism: A change in the growth of a plant in response to chemicals
Tropisms allow plants to obtain more favourable growing conditions:
Phototropism
 Stem grows towards light
 More favourably positioned to photosynthesise.
Geotropism
 Roots grow towards gravity
 They can penetrate into the soil for better anchorage and absorption
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Types of Tropisms
Thigmotropism
Regulatory system:
Definition of a growth regulator, transport through the vascular system
 Growth Regulators are chemicals which control the growth of a plant.
 Produced in small amount in the meristems of plants
 Transported in the vascular tissue (xylem and phloem)
Different combinations of regulators bring about different effects.
The effects of regulators depend on their concentration i.e. the same regulator can
have different effects at high or low concentrations.
Their effect depends on the location of the plant in which they are acting i.e. the same
concentration of plant regulator can have opposite effects in the stem and root.
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IAA (Indole Acetic Acid)
At low concentrations, IAA causes roots to grow. High concentrations inhibit root
growth.
Low concentrations inhibit shoot growth. At high concentrations, IAA causes shoots
to grow.
Different regulators interact in different ways. Some regulators support each other to
produce a greater effect (Synerginism).
Others interfere with each other and their combination may have no effect
(Antagonism )
e.g. Absissic acid causes dormancy in buds and gibberellins break the dormancy.
Some regulators promote growth e.g auxins are growth promoters
Others inhibit growth e.g ethane and abscisic acid are growth inhibitors
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Note of information
Synthetic auxins (2,4- D) are commonly used as herbicides due to their differential
effects on dicots and monocots.
The herbicide contains a greater concentration of auxin than the amount of auxin
produced naturally naturally produced in the plant.
Dicots are killed by much lower concentrations than monocots.
Synthetic auxins are therefore used to control broad-leaved weeds (mainly dicots) on
lawns and in cereal crops.
The weedkiller causes dicots to elongate so fast that they cannot sustain their own
growth and die.
Describe any 4 methods of anatomical or chemical adaptation by which plants
protect themselves from adverse external environments – anatomical/ structural or
chemical e.g. heat shock proteins or stress proteins
Plants are subject to a range of potentially harmful environmental conditions such as
 Eaten by herbivore
 Infected with disease causing organisms
 Loss of water
 Overheating
To protect themselves, plants have a large number of adaptive features
Anatomical adaptations
 Enclosed by a physical barrier e.g. epidermis or bark
 Cuticle
 Thorns
 Shortage of water causes guard cells to shrivel, closes stomata, reduces loss of
water
OR
Chemical adaptations
 Heat shock proteins surround enzymes, once temperature rises above 400C
these help them maintain their shape.
 Stress proteins are produced in response to micro-organisms. They damage the
micro-organism by attacking their cell walls. They can also stimulate nearby
plant cells to respond to the micro-organism.
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Contemporary Issue
Use of plant regulators: any 2 examples
Plant growth regulators can be produced outside of plants by artificial or synthetic
methods.
1. Rooting powders
Contain a synthetic growth regulator such as NAA (naphtylacetic acid). This
stimulates rapid root formation on stem cuttings.
Horticulturalists use NAA to produce roots on cuttings more quickly than would
normally be the case.
2. Tissue culture
In tissue culturing pieces of plant material are grown to form entire new plants.
If a piece of plant tissue is grown in a high auxin concentration, it will develop into a
mass of similar, undifferentiated cells (called a callus).
By adding different concentrations of auxins, the mass of cells can be stimulated to
form roots, shoots or an entire plant.
Entire plants can
be grown
3. A synthetic auxin is used as a synthetic weedkiller ( 2,4-D)
4. Ethene is sprayed on fruit to promote ripening.
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Mandatory Activity
Investigate the effect of I.A.A. growth regulator on plant tissue.
Materials/Equipment
Radish seeds
IAA solution (0.01% w/v)
Distilled water
2 syringes (10cm3)
8 Graduated droppers
8 Small bottles
Thermometer
Beaker
8 Petri dishes
8 Filter papers
8 Circular acetate grids
Absorbent cotton wool
Disposable gloves
Adhesive tape
Incubator (250C)
Positioning of radish seeds
Procedure
1. Label the petri dishes as follows: 102 ppm, 101 ppm, 1 ppm 10-1 ppm, 10-2
ppm, 10-3 ppm, 10-4 ppm, distilled water (control)
2. Label the bottles in the same way.
3. Using a syringe, add 10 cm3 of the IAA solution to the first bottle (0.01% w/v
or 10-2 ppm).
4. Using the other syringe add 9 cm3 of distilled water to each of the next seven
bottles.
5. Using a dropper, remove 1cm3 of the IAA solution from the first bottle and
add it to the second bottle. Place the cap on the second bottle and mix.
6. Using a different dropper, remove 1 cm3 of solution from the second bottle
and add it to the third bottle. Place the cap on the third bottle and mix.
7. Using a different dropper each time, repeat this serial dilution procedure for
the fourth fifth, sixth and seventh bottles.
8. Discard 1cm3 of solution from the seventh bottle. Each bottle now contains 9
cm3 of solution.
9. Fit a circular acetate grid inside the lid of each dish.
10. Place five radish seeds along a grid line in each dish as shown.
11. Place a filter paper on top of the seeds in each dish.
12. Using the appropriate droppers, add 2cm3 of each solution to its matching
dish. Use the dropper bulb to press gently on the damp filter paper, to reduce
the trapped air.
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13. Spread a piece of cotton wool, about 0.5 cm thick and the approximate area of
the dish, on top of the filter paper in each dish to absorb the excess solution.
14. Add the remaining 8 cm3 of each solution to the cotton wool in the appropriate
dish. Leave for a few minutes, until the cotton well absorbs all the solution.
15. Put the base of each dish in place and secure with a small piece of adhesive
tape on either side.
16. Stand the dishes vertically on their edge, to ensure the roots grow down. Leave
in the incubator for 2-3 days.
17. Measure the length of the roots and shoots of the seedlings in each dish by
using the acetate grids and record.
18. Calculate the total length and the average length of the roots and shoots in
each dish and record.
19. Estimate the percentage stimulation or inhibition of root and shoot growth in
each dish using the following formula:
Percentage stimulation/inhibition
= (Average length – average length of control) x 100
Average length of control
20. A graph should be drawn of percentage stimulation and inhibition of root and
shoot growth against IAA concentration. Put IAA concentration on the
horizontal axis.
21. Replicate the investigation or cross reference your results with other groups.
Prepared by U Moroney
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Results
Concentration Length
of IAA (ppm) of root
(mm)
Length
of root
(mm)
Length
of root
(mm)
Length
of root
(mm)
Length
of roots
(mm)
Seed 1
Seed 2
Seed 3
Seed 4
Seed 5
Total
length
(mm)
Average
length
(mm)
Percentage
stimulation or
inhibition
0
10-4
10-3
10-2
10-1
1
10
102
Concentration Length
of IAA (ppm) of shoot
(mm)
Seed 1
Length
of shoot
(mm)
Length
of shoot
(mm)
Length of Length of
shoot
shoot
(mm)
(mm)
Seed 2
Seed 3
Seed 4
0
10-4
10-3
10-2
10-1
1
10
102
Conclusion/Comment
Prepared by U Moroney
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Seed 5
Total
length
of
shoot
(mm)
Avg
Percentage
length stimulation or
(mm) inhibition
H.3.5.5 Auxins
Study auxin as an example of a plant growth regulator under the headings of
production site(s), function and effects
P40
Study auxin as an example of a plant growth regulator under the headings of
production site(s), function and different effects
T.G. p
NOTES
Auxin (Indole Acetic Acid)
Auxin (IAA) produced in the meristematic tissue in the tips of shoots, roots and buds.
Effect on Roots:
 Low concentrations, IAA causes roots to grow.
 High concentrations inhibit root growth.
Effect on Shoots
 At high concentrations, IAA causes shoots to grow.
 Low concentrations inhibit shoot growth.
Apical dominance
The apical bud of a stem makes IAA in concentrations too high to allow the growth of
side buds further down.
If apical bud is cut off, auxins are removed and so the side buds grow.
Plant growth
regulator
Production site
Function
Auxin
(IAA)
Meristems of
Shoot
Root
Bud
Cell elongation
Effects






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Stem elongation
Root growth
Phototropism
Geotropism
Differentiation
of vascular
tissue
Controls apical
dominance
H.3.5.6 Plant growth regulators and Animal Hormones (Extended study)
Explanation of mechanism of plant response to any one external stimulus.
P40
Explanation of mechanism of plant response to any one external stimulus.
P71
NOTES
Explanation of mechanism of plant response to any one external stimulus.
Coleoptile: protective sheath around the germinating shoots of grasses.
Used because
 response to light easy to observe
 Small
 Easy to grow in large numbers
External Stimulus: Light
Response: Shoot grows towards light: Phototropism. Brought about by auxins



Auxins (IAA) loosen cell walls
This allows them to expand and elongate.
Auxins produced in the growth tips (meristems) of
the stems.
Mechanism of response:
 Plant illuminated from one side
 Auxin moves away (diffuses) from the light
towards the shaded side of the stem
 Greater auxin concentration on the shaded side
 Greater cell elongation on the shaded side
 Uneven elongation
 Stem bends towards light
Summary
Auxins move away from the light.
Concentrate on the shady side.
More growth on this side
Bends towards light
Prepared by U Moroney
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