Tropic responses

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TROPIC RESPONSESPlant hormones

Co-ordinated science LO:Define and investigate geotropism and phototropismExplain the chemical control of plant growth by auxins including geotropism and phototropism in terms of auxins regulating differential growth

Biology LO:Define gravitropism Define phototropism Investigate gravitropism and phototropism in shoots and rootExplain phototropism and gravitropism of a shoot as examples of the chemical control of plant growthDescribe the use in weedkillers of the synthetic plant hormone 2,4-DExplain the role of auxin in controlling shoot growth, limited to: auxin made in shoot tip (only) auxin spreads through the plant from the shoot tip auxin is unequally distributed in response to light and gravity auxin stimulates cell elongation

Combined sciences LO:Define and investigate geotropism and phototropismExplain the chemical control of plant growth by auxins including geotropism and phototropism in terms of auxins regulating differential growth

Chemical Coordination in Plants - Tropisms5

Copyright 2015 Henry Exham

Chemical Coordination in Plants - TropismsPlants can respond to their environment6Its not only animals that can respond to their environments.Click on the images to watch the video about each plant

Venus Fly TrapMimosa Pudica


Chemical Coordination in Plants - TropismsTropisms7

These examples are very quick responses which are not typical in plants.A typical response in a plant is a change in growth. For example a plant may detect a certain stimuli like light, and start growing towards it.


Chemical Coordination in Plants - TropismsTropisms8

A growth response of a plant to directional stimuli is called a tropism. This is much slower as it involves cell division to see the results. If it grows towards the stimuli it is a positive tropism. If it grows away it is a negative tropism.


Chemical Coordination in Plants - TropismsPhototropism = a response in which a plant grows towards or away from the direction from which light is coming

9Tropisms are given different names dependent on the stimuli. A tropism where the stimuli is light is called phototropism. The shoot of a plant grows towards the light to maximise photosynthesis. This is called positive phototropism.

Cress seeds grown on a window sill


Chemical Coordination in Plants - TropismsGeotropism10If there is no light under the soil, how does a germinating seed know which way to grow?


Chemical Coordination in Plants - TropismsGeotropism = a response in which a plant grows towards or away from gravity11If there is no light under the soil, how does a germinating seed know which way to grow?

Because the shoot is also negatively geotropic. So will detect gravity and grow in the opposite direction.

The roots are positively geotropic. So will grow down to find mineral ions and water.


Chemical Coordination in Plants - TropismsHydrotropism12The roots of some species show positive hydrotropism.


Chemical Coordination in Plants - TropismsWhat causes these responses?13The plant produces many different plant hormones (plant growth substances).The most important one of these is called auxin.Its produced at the tip of the shoot and diffuses downwards. It causes cell elongation and division.


Chemical Coordination in Plants - TropismsWhat causes phototropism?14Auxin is produced at the tip of the shoot and causes it to grow.


Chemical Coordination in Plants - TropismsWhat causes phototropism?15When light comes from one direction, the auxins get transported to the shaded side.


Chemical Coordination in Plants - TropismsWhat causes phototropism?16The shaded side elongates faster than the illuminated side and the shoot bends towards the light


Weedkillers (herbicide) are synthetic plant hormones, similar to auxins. If these chemicals are sprayed on to plants they can cause rapid, uncontrolled growth and respiration, resulting in the death of the plant.

Some plant species are more sensitive than others to synthetic plant hormones, so weedkillers can be selective.

Many weedkillers kill onlybroad-leavedplants (dicotyledons), leaving grasses (moncotyledons)unharmed.

Effects of weedkillers


Auxin (IAA) is actively transported into cells by a transmembrane transporter and leaves the cells by facilitated diffusion through a different transporter. It turns out that the importer works fine for 2,4-D but that 2,4-D cannot leave the cell through the exporter. It is the resulting accumulation of 2,4-D within the cell that kills it.

Effects of weedkillers


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Tropic responses

Science

Transcript of Tropic responses