Tropisms (Cambridge O Level Biology)

Tropisms

• Plants can respond to changes in the environment (stimuli) for survival, e.g. light, water, gravity
• Their responses are usually much slower than animals
• They grow either towards a stimulus (known as a positive response) or away from a stimulus (known as a negative response)
• The responses are known as tropisms

• It is very important to a plant that its roots and shoots grow in the right directions 
• Shoots must grow upwards, away from gravity and towards light, so that leaves are able to absorb sunlight
  • This means that shoots have a positive phototropic response and a negative gravitropic response
• Roots need to grow downwards into the soil, away from light and towards gravity,  in order to anchor the plant and absorb water and minerals from the soil particles.
  • This means that roots have a negative phototropic response and a positive gravitropic response

Plant Gravitropism Diagram

Placing a plant on its side shows the gravitropic responses (also known as geotropic responses)

• Plants respond to stimuli by producing a growth hormone called auxin which controls the direction of growth of roots or stems
• Therefore we say plants control their growth chemically
• Auxin is mostly made in the tips of the growing stems and roots and can diffuse to other parts of the stems or roots; spreading from a high concentration in the shoot tips down the shoot to an area of lower concentration. 
• • This is an important point. Only the region behind the tip of a shoot is able to contribute to growth by cell division and cell elongation. This part of a shoot is called the meristem. Auxin stimulates the cells behind the tip to elongate (get larger); the more auxin there is, the faster they will elongate and grow  
• If light shines all around the tip, auxin is distributed evenly throughout and the cells in the meristem grow at the same rate - this is what normally happens with plants growing outside
• When light shines on the shoot predominantly from one side though, the auxin produced in the tip concentrates on the shaded side, making the cells on that side elongate and grow faster than the cells on the sunny side
• This unequal growth on either side of the shoot causes the shoot to bend and grow in the direction of the light

Auxin and Phototropism Responses

Positive phototropism in plant shoots

• Auxin plays a role in a plants response to gravity, affecting plant shoots and roots in different ways
• When shoots grow away from gravity it is known as negative geotropism
  • Gravity modifies the distribution of auxin so that it accumulates on the lower side of the shoot
  • As seen in the phototropic response, auxin 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 auxin results in a lower rate of cell elongation
  • The auxin 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

Investigating Phototropisms

• Three identical plants are set up as shown below (A, B and C)

Investigating the phototropic response

• The seedlings in A grow towards the light source
• In B the effect of the light only coming from one direction has been cancelled out by using a clinostat
  • A clinostat is a device that revolves slowly and repeatedly, so the shoots are evenly exposed to light
• This means all sides of the seedlings get equal exposure to light so they do not curve towards the light source but grow straight up
• In C the seedlings grow straight up looking for light and the plant becomes tall and slender with yellowing leaves due to the lack of light

Investigating Gravitropism

Investigating the gravitropic response (set-up)

• Add some damp cotton wool to two Petri dishes
• Place 3 bean seedlings in the cotton wool in each petri dish
  • A - radicle facing downwards
  • B - horizontally
  • C - radicle (root grows from here) facing upwards

• Cover each dish with a lid 
• Attach one petri dish to a support so that it’s on its side
• Attach the second petri dish to a clinostat  (as shown in the diagrams above). 
• Place both in a light-proof box (so that the seedlings are in complete darkness), leave for two days and then observe the growth of the seedlings

Investigating the gravitropic response (results)

• In the first petri dish, all radicles (roots) have grown downwards (positive gravitropic response) regardless of which way they were initially facing (horizontal, up or down) and all plumules (shoots) have grown upwards (negative gravitropic response)
• In the second petri dish, all radicles and all plumules have all grown neither up nor down but straight outwards in whichever direction they were placed as the effect of gravity has been cancelled out by the revolving of the clinostat - they have shown no gravitropic response at all
• The experiment needs to be done in a lightproof box to cancel out the effect of light on the growth of the seedlings