Tropisms

• what is a tropism?

  plant directional growth in response to environmental cues/stimulus
• describe examples of tropism

  • Positive or negative
  • Permanent change
  • Phototropism, geotropism, chemotropism, thigmotropism and hydrotropism
• What is thigmotropism?

  Growth in response to touch
• What is hydrotropism?

  • Growth response of plant towards water
  • Helps them gain the raw materials needed for photosynthesis
• What’s chemotropism?

  • Growth response of plants to chemicals. Pollen tubes grow down style as they are attracted by chemicals in the ovary (fertilisation)
• What’s geoteopism?

  • Growth response towards gravity (roots)
  • Plants can be anchored into the soil and take up water for photosynthesis and nitrates (minerals for amino acids)
• What parts of the plant respond to positive and negative phototropism?

  • Shoot- positive phototropism
  • Root- negative phototropism
• Describe the process of positive phototropism

  • All shoots bend towards lights (positive phototropism)
  • Shaded side elongates faster than lit side
  • Blue light receptors (phototrophins) in plant shoots get phosphorylated (shaded side doesn’t)
  • Photophosphorylation of phototropins causes sideways movement of auxin to the shaded side
  • Auxin stimulates cells to grow longer
• Describe the experiment for investigating phototropism in plants

  1. Germinate and grow seedlings in different conditions- one in dark, one in all round light. measure record and observe, time lapse photography useful too
  2. Germinate and grow seedlings in unilateral light- different colour filters to test which wavelengths trigger phototrophic resposnse
  3. Repeat classic experiments with coleoptiles (the plants used)
• What is etiolation?

  • Process in flowering plants grown in incomplete presence of light/the dark
  • Characterised by long weak stems, smaller leaves due to longer internodes and a pale yellow colour (chlorosis) as the plant is deprived of light so little chlorophyll develops in the leaves
• How do you investigate for geotropism?

  • Clinostat (rotating drum)
  • Plants grown on slowly rotating clinostat, at 90° intervals
  • Usually in all round light or darkness (control variable to make sure that the only thing causing the tropism in the plant is gravity)
• What is a tropism?

  Plant directional growth in response to stimuli or environmental cues
• Where does positive phototropism occur?

  In shoots
• Where does negative phototropism occur?

  In roots
• Where does positive geotropism occur?

  In roots
• Does negative geotropism exist in roots?

  no
• What parts of the plant respond to positive and negative geotropism?

  Shoot- negative geotropism

  Root- positive geotropism
• What is a plant that folds in response to touch?

  Mimosa pudica
• why is most of the research on tropisms conducted on germinating seeds/young seedlings eg- coleoptiles?

  • easy to manipulate
  • grow and respond rapidly to stimuli
  • so any changes show up quickly
  • changes affect the whole organism rather than a small part as with mature plants as the plant is smaller, making tropisms easier to observe and measure
• why do shoots grow towards a light source when it is unilateral?

  to expose the shoot to as much even, all round light as possible
• true or false? plants in the dark will grow straight upwards like plants in all round light, but will also grow faster straight upwards than plants in all round light

  TRUE
• what was an original theory about what light did to auxin?

  • light destroys auxin
  • disproved by experiments showing that levels of auxin in shoots are the same regardless of whether being kept in the dark or under unilateral illumination
• why do plants grow faster in the dark than in all round light?

  • plants grow rapidly upwards in dark to reach light to photosynthesise (eg when a shoot breaks through the soil)
  • seedlings that break through soil first don't have to compete with other plants to photosynthesise
  • gibberellin is responsible for extreme internodal elongation when a plant is grown in the dark
  • once plant is exposed to light, slowing of upward growth is valuable as plants need resources for growing of leaves, strengthening stems, etc, so [gibberellin] reduces
  • rapid upwards growth of plants in the dark = etiolation
• describe experimental evidence for the growth of a shoot in response to light

  • shoot tip removed
  • lightproof cover
  • impermeable barrier of mica
  • gelatin block which allows chemicals to pass through but not electrical messages

  
• describe experimental evidence for what happens to a shoot grown in unilateral light

  effects occurring when the shoot is split

  
• describe effects shown in roots and shoots when plants are grown on a clinostat

  • shoots grow straight out as effects of gravity are evenly distributed on shoot
  • root growth changes direction towards gravity when they are moved