Responses in plants

• flowering plants, like animals, increase their chances of survival by responding to changes in their environment
• • sense the direction of light and grow towards it to maximise light absorption for photosynthesis
  • they can sense gravity, so their roots and shoots grow in the right direction
  • climbing plants have a sense of touch, so they can find things to climb and reach the sunlight
• A tropism is the response of a plant to a directional stimulus (a stimulus coming from a particular direction)

  plants respond to stimuli by regulating their growth
• a positive tropism is growth towards the stimulus
• a negative tropism is growth away from the stimulus
• Phototropism - the growth of a plant in response to light

  • shoots are positively phototropic and grow towards light
  • roots are negatively phototropic and grow away from light
• Gravitropism: is the growth of a plant in response to gravity.

  • Shoots are negatively gravitropic and grow upwards
  • roots are positively gravitropic and grow downwards
• Auxins: plants respond to directional stimuli using specific growth factors - these are hormone like chemicals that speed up or slow down plant growth
• gravitropism = geotropism
• growth factors called auxins are produced in the tips of shoots and diffuse backwards to stimulate the cell just behind the tips to elongate them - this is where the cell walls become loose and stretchy, so the cells get longer
• if the tip of a shoot is removed, no auxin will be available and the shoot stops growing
• auxins stimulate growth in shoots but high concentrations inhibit growth in roots
• there are other classes of growth factors that affect growth in different ways e.g. a growth factor called gibberellin stimulates flowering and seed germination
• Indoleacetic acid (IAA): an important auxin that is produced in the tips of shoots and roots in flowering plants
• IAA is moved around the plant to control tropisms:

  • it moves by diffusion and active transport over short distances
  • via the phloem over long distances
  • results in different parts of the plant having different concentrations of IAA
  • the uneven distribution of IAA means there is uneven growth of the plant
• phloem is a tissue that transports sugars around a plant
• phototropism:

  • IAA moves to the more shaded parts of the shoots and roots, so there is uneven growth
  • IAA concentration increases on the shaded side - cells elongate and the shoot bends towards the light
  • IAA concentration increases on the shaded side - growth is inhibited so the root bends away from the light
• Gravitropism: IAA moves to the underside of shoots and roots, so there is uneven growth:

  • IAA concentration increases on the lower side - cells elongate and shoot grows upwards
  • IAA concentration increases on the lower side - growth is inhibited so the root grows downwards
• root growth is inhibited by high concentrations of IAA - the opposite is true in shoots - high concentrations of IAA promote shoot growth
• e.g. question:

  an experiment was carried out to investigate the role of IAA in shoot growth. 8 shoots, equal in height and mass, had their tips removed. Sponges soaked in glucose and either IAA or water were then placed where the tip should be. 4 shoots where then placed in the dark (experiment A) and the other 4 were exposed to a light source, directed at them from the right (experiment B). After 2 days the amount of growth in mm and direction of growth was recorded.
• growth factors are produced in the shoot tips - by removing the tips, any IAA already present is removed
• the results show how the movement of IAA controls phototropism in plant shoots:

  • in experiment A shoot A, the IAA diffused straight down from the sponge into the left hand side of the shoot - stimulated the cells in this side to elongate, so the shoot grew towards the right
  • in shoot B the opposite occurred, making the shoot grow towards the left
  • shoot C equal amounts of IAA diffused down both sides - making all cells elongate at the same rate
• • in experiment B, the shoots were exposed to a light source
  • the IAA diffused into the shoot and accumulated on the shaded side (left hand side) regardless of where the sponge was placed
  • shoots A,B,C all grew towards the right bc most IAA accumulated on the left, stimulating cell elongation there
• when asked about experimental design - think about controls:

  • in this experiment the negative control treatment was the sponge soaked in water (and glucose) which was included to show that it was the IAA causing the observed effects and nothing else
  • all sponges were soaked in glucose to provide energy for growth of shoots
  • photosynth cannot take place in the dark so the growth of seedlings in experiment A may have been limited if they weren't provided with glucose (external energy source)