Tropisms

• Plants are usually held firmly in position by their roots and are, therefore, unable to move from one place to another
• Plant growth factors

  • They affect growth of the whole or part of a plant
  • They are not secreted by glands and do not travel in a transport system
  • They can sometimes affect the tissues that release them
  • They are produced in the growing regions of the plant and can stimulate or inhibit growth
• IAA (indoleacetic acid)

  • Controls directional growth seen in tropisms in flowering plants
  • Produced in the tips of shoots and roots
  • Moved to other parts of the plant by diffusion or active transport
  • Uneven distribution of IAA causes tropisms in the shoot and root
• Tropism


  A plant's growth response to an external, directional stimulus, taking the form of bending/turning in a certain direction
• Observations of plant growth

  • Plant shoots grow towards light
  • Plant roots grow away from light
  • Young roots of seedlings grow towards gravity
  • Young stems of seedlings grow away from gravity
  • Plant roots grow towards water
• Plants rely on growth to respond to different directional stimuli to maintain their roots and shoots in a favourable environment
• If plant growth is towards the stimulus, it is a positive response/tropism. If it is away from the stimulus, it is a negative response/tropism
• Plants produce 'chemical messengers' called plant growth factors to respond to stimuli
• Auxins were the first class of plant growth factors discovered
• Root response to unilateral light

  Auxin moves away from the light source and accumulates on the dark side, inhibiting growth on the shaded side causing the root to bend away from the light, resulting in a negative phototropic response
• Shoots exhibit positive phototropism and negative geotropism
• Roots are said to be positively gravitropic while shoots are said to be negatively gravitropic
• More IAA at the bottom cells causes elongation loss, while less IAA at the top cells causes more elongation, resulting in the root curving downwards
• Woodlice often turn in the opposite direction after being forced to turn in a particular direction, increasing their chances of finding more favorable conditions
• Directional response is called taxis, while non-directional response is called kinesis
• Gravity redistributes IAA so that it collects on the underside of the root, inhibiting cell elongation
• Roots exhibit positive geotropism and negative phototropism
• Role of IAA in Controlling Gravitropism

  Gravitropism is the growth of plant roots and shoots in response to gravity. When a seedling is placed horizontally, the root grows downwards and the shoot grows upwards due to the redistribution of auxin. IAA moves down to the lower side, causing different effects in the shoot and root
• Choice chamber can be used to investigate how animals respond to different environmental conditions like light intensity and humidity
• Experimental data interpretation: RP10 - Taxes and kineses