Transcript tropisms - year13bio

Biological Orientation
Responses in Plants
Tropisms and Nastic Movements
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Tropisms
• The growth of part of a plant towards or
away from an environmental stimulus.
• If the growth is towards the stimulus, the
tropism is said to be positive, if it is away
from the stimulus it is said to be negative.
• Tropisms are a directional response – the
environmental stimulus comes from one
direction.
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Types of Tropisms
The type of the stimulus
is shown by prefixes:
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Photo
Geo or Gravi
Hydro
Chemo
Thigmo
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Light
Gravity
Water
Chemicals
Touch
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• http://www.youtube.co
m/watch?v=zctM_TW
g5Ik&feature=fvw
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Examples of Tropisms
• The shoot of a plant is positively
phototropic - it grows towards light.
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Examples of Tropisms
• Roots of plants are positively geotropic –
they grow down, and shoots are negatively
geotropic – they grow up
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More Examples of Tropisms
Some roots grow away from copper
pipes – they are negatively
chemotropic
Plants such as runner beans
are positively thigmotropic –
they grow towards a
supporting structure such as
another plant.
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Nastic Responses
• These are responses of plants to stimuli
from the environment that do not come
from a particular direction. These include
responses to changes in temperature
(thermonasty), light intensity
(photonasty),humidity (hydronasty) and
touch (thigmonasty).
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• http://www.youtube.com/watch?v=BLTcVN
yOhUc
• http://www.youtube.com/watch?v=PRo4rg
07_gg&feature=fvw
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Examples of nastic responses
•The leaves of the Venus Fly
Trap and the Mimosa plants
both close up in response to
touch – a thigmonastic
response.
•The flowers and leaves of
many plants close up when
the light intensity decreases
(it gets dark) – a photonastic
response.
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Differences between tropisms
and nastic reponses:
• Tropisms are a directional response, the
stimulus (eg. sunlight through a window)
determines the direction the plant grows.
• Nastic responses are not the result of a
stimulus from a certain direction.
• Nastic responses can be fast, whereas
tropisms are generally slow growth
changes.
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What causes phototropism?
• Experiments first carried out by Charles
Darwin, and often repeated by other
scientists, showed the following results:
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Conclusions from these
experiments
• a chemical is produced in the tip of a plant
in response to light shining from one
direction
• the chemical travels down the plant and
accumulates on the dark side of the shoot
• high concentrations of this chemical cause
the cells on the dark side to elongate, thus
causing the shoot to bend towards the
light
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Further experimental results
Do these experiments confirm the conclusions made
previously?
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But what happens in similar
experiments when plant
coleoptiles are kept in the dark?
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In darkness:
Bending does not occur.
Why?
Normal coleoptile
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• The auxin travels from the tip, even in
darkness. Because both sides of the
coleoptile receive equal auxin
concentrations, they elongate at the same
rate, thus there is no bending.
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Bending occurs towards the left.
Why?
Tip removed and
replaced on side of
coleoptile.
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• The auxin moves from the tip down the
right hand side of the coleoptile. This side
elongates more causing bending to the
left.
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Bending occurs towards the left.
Why?
Glass slip inserted into
left side of coleoptile
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• The auxin cannot travel down the left side
because of the slip of glass, but it can
move down the right. Increased hormone
concentration on the right causes these
cells to elongate, causing bending to the
left.
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Positive thigmotropism
• NCEA 2006
• Discuss the advantages of thigmotropism to a climbing plant.
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Points to include
Define it!
Explain support from other plants
Explain what this support allows the plant to do
Give two advantages due to this response
– Maximising photosynthesis
– Increases pollination and seed dispersal (out-competes other plants)
– BIG PICTURE: HOW DOES THIS MAXIMISE ENERGY EFFICENCY?
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A
M
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Define thigmo tropism
Or
Describes that the plant will use
other plants as support.
Explains benefits of this, eg:
• Such support would help the
plant climb upwards to gain
more light.
• Increasing amount of
photosynthesis.
Clearly discusses the
relationship
between support and gives TWO
advantages to the plant eg:
• Plant can maximise access to
light for photosynthesis,
improving competitive success.
• Plant can put resources into
rapid
upward growth rather than
production of strong internal
support structures outgrowing
competitors.
• Flowers access for pollination.
• Seed dispersal.
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geotropism
• Response of a plant to gravity
• Shoots will grow against
gravity (upwards)
• Roots will grow with gravity
(downwards)
• Auxin is responsible for
geotropism
– inhibits cell elongation in
roots
– stimulates cell elongation in
shoots
– Statoliths (dense starchcontaining plastids) may
accumulate in the direction
of gravity to assist in
geotropism
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Nastic response
• Turgor pressure
• leaf movement is an
osmotic effect, in which
an ion (K+) released into
the leaf tissues makes
the cells on one surface
of the leaves wilt
(because water is leaving
the cells). This makes the
wilting surface slightly
smaller than the unwilted,
opposing leaf surface.
The leaf curls towards the
wilted side.
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