5.5.2 Investigating Phototropism and Geotropism Flashcards
What is phototropism in plant shoots?
Plant shoots are positively phototropic, meaning they grow towards light
This ensures they maximise the amount of light they can absorb for photosynthesis
In the late 19th century and early 20th century, various experiments were conducted to investigate the process by which phototropism occurs
Many of the experiments were conducted using coleoptiles (a sheath that surrounds the young growing shoot of grass plants)
What did Darwin’s 1880 experiment show?
Darwin discovered that removing the tip of a coleoptile stopped the phototropic response to a unidirectional light source (light coming from one side) from occurring
To ensure this was not simply due to the wounding caused to the plant, he covered the tip of a coleoptile with an opaque cover or ‘cap’ instead, to block out the light. This also stopped the phototropic response from occurring, showing that the tip of the coleoptile was responsible for detecting light
What did Boysen-Jensen’s experiment in 1913 show?
Boysen-Jensen found that if he replaced the cut tip back on top of the coleoptile and inserted a gelatin block as a barrier in between, the phototropic response was restored
This showed that the stimulus for growth was a chemical (hormone), which was able to travel through the gelatin block
Bosen-Jensen then inserted a mica barrier (mica is impermeable to chemicals) halfway through the coleoptile just below the tip, first on the lit side and then on the shaded side
When the mica barrier was inserted into the lit side, the phototropic response occurred
When the mica barrier was inserted into the shaded side, the phototropic response did not occur
This confirmed that the stimulus for growth was a chemical (hormone) and showed that it was produced at the tip, before travelling down the coleoptile on the side opposite to the stimulus (i.e. the shaded side)
It also showed that the stimulus acted by causing growth on the shaded side (rather than inhibiting growth on the lit side)
What did Paál’s experiment in 1919 show?
Paál cut off the tip of a coleoptile and then replaced it off-centre in the dark
The side of the coleoptile that the tip was placed on grew more than the other side, causing the coleoptile to curve (similar to a phototropic response)
This showed that, in the light, the phototropic response was caused by a hormone diffusing through the plant tissue and stimulating the growth of the tissue
What did Went’s experiment in 1926 show?
Went placed the cut tip of a coleoptile on a gelatin block, allowing the hormones from the tip to diffuse into the block
The block was then placed on the coleoptile, off-centre and in the dark
As in Paál’s experiment, the side of the coleoptile that the block was placed on grew more than the other side, causing the coleoptile to curve
The greater the concentration of hormone present in the block, the more the coleoptile curved
How can you control growth by elongation?
Indole-3-acetic acid (IAA), which is an auxin, is a specific growth factor found in plants
IAA is synthesised in the growing tips of roots and shoots (i.e. in the meristems, where cells are dividing)
IAA coordinates phototropisms in plants by controlling growth by elongation
IAA molecules are synthesised in the meristem and pass down the stem to stimulate elongation growth
The IAA molecules activate proteins in the cell wall known as expansins, which loosen the bonds between cellulose microfibrils, making cell walls more flexible
The cell can then elongate
What is the phototrophic mechanism?
Phototropism affects shoots and the top of stems
The concentration of IAA determines the rate of cell elongation within the region of elongation
If the concentration of IAA is not uniform on either side of a root or shoot then uneven growth can occur
When the shoots grow towards the light it is known as positive phototropism
It is described as positive because growth occurs towards the stimulus
In shoots, higher concentrations of IAA results in a greater rate of cell elongation
Experiments have shown that IAA moves from the illuminated side of a shoot to the shaded side
The higher concentration of IAA on the shaded side of the shoot causes a faster rate of cell elongation
This causes the shoot to bend towards the light
What is geotropism in roots and shoots?
Gravity affects both plant shoots and roots, but in different ways
When shoots grow away from gravity it is known as negative geotropism
Gravity modifies the distribution of IAA so that it accumulates on the lower side of the shoot
As seen in the phototropic response, IAA increases the rate of growth in shoots, causing the shoot to grow upwards
When roots grow towards gravity it is known as positive geotropism
In roots, higher concentrations of IAA results in a lower rate of cell elongation
The IAA that accumulates at the lower side of the root inhibits cell elongation
As a result, the lower side grows at a slower rate than the upper side of the root
This causes the root to bend downwards
How can you investigate the effect of IAA on root growth in a practical?
Use the scalpel to cut a 1cm section from the root tip of each seedling
Mark the root tips at 2mm marks
Divide the root tips into three groups and place them in test tubes of water
The water helps to keep the plant tissue alive
Group A receives treatment 1
Remove the ends of the root tips using the scalpel
Transfer root cuttings with the end removed to an agar block
A uniform light source is present
Group B receives treatment 2
Transfer intact root tips to an agar block
A light-proof container is placed over the seedlings to prevent light from entering
Group C receives treatment 3
Transfer intact root tips to an agar block
Apply a directional light source to one side of the root tips
Leave all the roots in their treatment conditions for 3 hours
Use the 2mm marker lines to determine if growth has occurred
Note if the growth has been even on both sides
What would the results from this experiment show?
In group A (tips removed) the roots grow evenly on both sides
IAA is synthesised in the root tips so removing them means that no IAA is produced
There is no inhibition of cell elongation
In group B (no light) the roots grow slightly less than group A but evenly on both sides
There is an equal concentration of IAA on both sides of the root tip
The inhibition of cell elongation is equal on both sides of the root tip
The roots do not grow as long as those in group A due to the presence of IAA
In group C (directional light) the cells on the illuminated side of the root grow longer than those on the shaded side
There is a greater concentration of IAA on the shaded side
This results in greater inhibition of cell elongation on the shaded side
So the illuminated side grows at a faster rate
The roots bend away from the light – negative phototropism
What would be the limitations of this experiment?
Although the same species of plant is being used the plants will still have differences
Certain genotypes may be more prone to bending or have slightly different sensitivities to IAA
The 2mm marks used for measuring growth may get smudged or move
If the root is mishandled the marks can be altered, which will affect the results
The evenness of growth can be hard to determine using the naked eye
Only general comments can be made about whether there has been even growth on both sides of the roots
