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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:

• Photo • Geo or Gravi• Hydro• Chemo• Thigmo

• Light• Gravity• Water• Chemicals• Touch

• http://www.youtube.com/watch?v=zctM_TWg5Ik&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=BLTcVNyOhUc

• http://www.youtube.com/watch?v=PRo4rg07_gg&feature=fvw

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•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.

Examples of nastic responses

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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:

Normal coleoptile

Bending does not occur.

Why?

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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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Tip removed and replaced on side of coleoptile.

Bending occurs towards the left.

Why?

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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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Glass slip inserted into left side of coleoptile

Bending occurs towards the left.

Why?

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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.

• 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 EDefine 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 starch-

containing 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.