Response to stimuli
Contents
Introduction
Examples of responses
Responses in the Flowering
Plant
Growth regulation
Tropism
Plant Growth Regulators
Commercial use of
regulators
Plant Protection
Auxin
Other Roles of Auxin
Phototropism and
auxin
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Introduction
Living organisms sense and respond to certain
environmental changes (stimuli), responding in a
variety of ways.
Plants and animals respond in complex ways
e.g. by growth and movement.
Response is a form of defence that allows
certain organisms survive.
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Introduction (cont’d)
Some structures of behaviour include:
a. Chemical or hormonal system
b. Nerve and sense organ system
c. Muscular and skeletal systems
d. An immune system e.g. response to viral
infection.
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Examples of responses
(1/2)
In humans:
1. Eye – moving car – image to brain via nerves
– message to muscles to move – also
message to adrenal glands (adrenaline) –
heart beats faster – more oxygen to cells for
fast reaction.
Above example uses interconnected systems
to respond to a stimulus. What was the
stimulus?
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Examples of responses
(2/2)
2. If a micro-organism (m/o) enters the body the
immune system will respond to it and defend
the body in a number of ways and prevent the
m/o doing any harm.
In plants:
1. Light shining on a shoot from the side causes a
chemical release that stimulates the plant to
grow towards the direction of the light.
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Responses in the Flowering Plant
Growth regulation
Growth of plants is regulated by a number of
external factors, which include
Light intensity, day (or night) length, gravity and
temperature
And the principal internal factors that regulate
these responses is a group of chemicals called
growth regulators – produced in meristematic
regions. Growth regulators = auxins.
Give e.g. of meristematic regions.
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Tropism - definition
tropism: is the growth response of part of a plant
to an external unidirectional stimulus.
Positive tropism - plant grows towards stimulus.
Negative tropism - plant grows away from stimulus.
stimulus: (plural = stimuli): any change in the
environment (internal or external) that causes a
cell or organism to respond.
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Plant Tropisms
Tropism
Growth response of a plant to …
Phototropism
Growth response of a plant to
unidirectional light.
Geotropism
Growth response of a plant to …
Thigmotropism
Growth response of a plant to …
Hydrotropism
Growth response of a plant to …
Chemotropism
Growth response of a plant to …
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Significance of phototropism
Ensures that
a) Plants grow towards the light and
b) Leaves face the sun to maximise
photosynthesis
Demonstrated by placing growing cress seed on a
windowsill
What will happen?
Result– they grow towards the light coming in the
window.
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Significance of geotropism
Ensures that
a) Roots grow downwards and anchor the plant
and
b) Stems grow upwards when seeds germinate.
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Plant Growth Regulators
Definition: plant growth regulators: are chemicals produced in
plants
that affect their rate of growth or development
when they are in very low concentrations.
They are carried in the vascular system of the plant or
diffuse from cell to cell.
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Types of Plant Growth Regulators
Growth promoters
e.g. auxins, giberellins and cytokinins
Growth inhibitors
e.g. ethene (ethylene) and abscisic acid
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Plant growth regulators
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Combinations of regulators
Sometimes two regulators may work together or
against each other
e.g. leaf fall (abscission). Ethene is responsible for
leaf fall – auxin prevents leaf fall.
Young leaves produce auxin – prevents leaf fall.
As leaf ages auxin production stops and ethene
production increased (due to longer nights).
Broad-leaved trees loose their leaves in autumn.
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Commercial use of regulators
Scientists have discovered chemicals that mimic
the effect of certain growth regulators and have
used these to affect plant growth, e.g.
2-4-D a broad-leaved plant (dicot) weedkiller,
it makes the rate of respiration faster than the rate
of photosynthesis.
Spraying a crop with gibberellins will cause fruit
formation without fertilisation and we get the
production of seedless fruit.
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Other examples
Ethene: Bananas are picked when green and
stored in a warehouse. Ethene released into
warehouse and ripens them when they are
needed for sale.
Over-ripe apples release ethene and causes other
apples to ripen – one ‘bad’ apple causes …
Used as rooting powders (demonstrate) and
Used in tissue culturing.
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Plant Protection
(1/2)
Plants have a number of anatomical (structural)
and chemical means for protecting themselves
e.g.
Thorns – on thistles and cacti – prevent plant from
being eaten
Stinging hairs – on nettles - prevent being eaten.
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Plant Protection
(2/2)
Ethene – released from wounds on some plants.
Causes production of new cells – form a callus
that closes the wound. (Ethene is acting as a
growth promoter).
Poisons: some plants produce poisons at the edge
of leaves that kills insects when they chew them.
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Auxin
A sheath called a coleoptile covers the tip of oat
seedlings and other similar plants.
It was discovered by experimentation that the tip of
the coleoptile produces a chemical that diffuses
down the shoot and promotes growth of the
shoot.
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Experiment 1
The coleoptile tip was
removed
There was no growth
until it was replaced
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Experiment 2
The tip was removed
and replaced over
half of the coleoptile.
It grew only on that side
and hence it curved.
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Experiment 3
The coleoptile tip was
removed and placed
on a piece of agar.
The agar alone was
placed on the shoot.
The shoot grew in the
normal way.
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Other Experiments
Using agar blocks with different concentrations of
auxin showed that the higher the concentration
of auxin the greater the growth.
The blocks were placed on one side of the shoot
and caused the shoot to bend.
The higher the concentration of auxin the greater
the bending (growth).
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The effect of auxin conc. on a shoot
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Another effect of Auxin
Auxin also affects the strength of cell walls.
The higher the concentration of auxin the weaker
the cell wall.
This allows plant cells to expand more.
Shoot grows by increasing the normal elongation
process of the cells.
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Other Roles of Auxins
(1/3)
Auxin affects growth of roots. (Produced in shoot tips)
In very weak concentrations it stimulates root growth
very slightly.
At concentrations that cause maximum shoot growth it
inhibits root growth strongly.
Affect of auxins on roots of no consequence in nature.
It is thought that abscisic acid is involved with the
response of the roots to gravity.
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Other Roles of Auxins
(2/3)
Auxins cause apical dominance.
apical dominance: the suppression of the growth
of the lateral buds by the terminal bud, thus
allowing the stem to elongate.
Pruning removes apical dominance and produces
lots of side shoots.
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Other Roles of Auxins
(3/3)
Rooting powder contains auxin.
When it is applied to a plant cutting it encourages
the production of roots at the cut surface.
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Phototropism and auxin
(1/2)
Auxin produced in the shoot tip passes down the
shoot.
This produces an even concentration of auxin
across the shoot resulting in an elongation
(growth) on all cells and the shoot grows
vertically.
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Phototropism and auxin
(2/2)
Light shining on one side of the shoot (unilateral light)
results in an increase in auxin concentration on the
other side (dark side) of the shoot. (auxin moves
away from light).
Cells on dark side have weaker cell walls. This
promotes a greater cell elongation on the dark side
resulting in the shoot growing towards the light.
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The effect of auxins on a shoot
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END
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