5.5.2 Investigating Phototropism & Geotropism

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)

• Darwin's Experiment (1880)
  • 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

• Boysen-Jensen's Experiment (1913)
  • 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)

• Paál's Experiment (1919)
  • 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

• Went's Experiment (1926)
  • 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

Four historical phototropism experiments were conducted to investigate the process by which phototropism occurs

Controlling 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

The phototropic 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

Higher concentrations of IAA on the shaded side increases the rate of cell elongation so that the shaded side grows faster than the illuminated side

Geotropism in plant shoots and roots

• 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

Practical: investigating the effect of IAA on root growth

• Experiments can be carried out to investigate the effect of IAA on root growth in seedlings

Apparatus

• Seedlings (of the same age and plant species)
• Cutting tile
• Scalpel
• Light source
• Lightproof container
• Blocks of agar (all the same volume)
• Marker/pen
• Test tubes
• Water

Method

• 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

Results and analysis

• 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

Limitations

• 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

The different treatments produce different levels of growth in the root tips. The IAA molecules inhibit cell elongation in roots