topic 15

• Gravitropism


  A response in which parts of a plant grow towards or away from gravity

  
• Phototropism


  A response in which parts of a plant grow towards or away from light

  
• Role of auxin in controlling shoot growth

  1. Auxin is made in the shoot tip

  2. Auxin spreads through the plant from the shoot tip

  3. Auxin is unequally distributed in response to light and gravity

  4. Auxin stimulates cell elongation
• Plants can respond to changes in the environment (stimuli) for survival e.g. light, water and gravity
• Plants grow either towards a stimulus (positive response) or away from a stimulus (negative response)

  
• These responses are known as tropisms
• Shoots must grow upwards, away from gravity and towards light, so that leaves are able to absorb sunlight

  
• 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

  
• Roots have a negative phototropic response and a positive gravitropic response

  
• Investigating gravitropism


  1. Add damp cotton wool to two petri dishes

  2. Place 3 bean seedlings in the cotton wool in each petri dish

  3. Attach one petri dish to a support so that it's on its side

  4. Attach the second petri dish to a clinostat

  5. Place both in a light-proof box, leave for two days and then observe the growth of the seedlings

  
• In set-up A, 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 set-up B, 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
• Investigating phototropism


  1. Select 3 potted seedlings of similar size and water them

  2. Place one of them under a cardboard box with a window cut in one side so that light reaches the shoot from only one direction (set-up A)

  3. Place one of them in an identical situation but on a clinostat (set-up B)

  4. Place another under a cardboard box without a window so it is in complete darkness (set-up C)

  
• 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. 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