Plants and growth incl tropisms

• Plant growth factors


  Chemicals that regulate plant growth response to directional stimuli
• Plant growth factors


  • Produced in plant growing regions (apical meristems)
  • Diffuse from cell to cell/ phloem mass transport
• Positive phototropism in shoots


  1. Indoleacetic acid (IAA) diffuses to shaded side of shoot tip

  2. IAA diffuses down shaded side, causing active transport of H+ ions into cell wall

  3. Disruption to H-bonds between cellulose molecules & action of expansins make cell more permeable to water (acid growth hypothesis)

  4. Cells on shaded side elongate faster due to higher turgor pressure

  5. Shoot bends towards light
• Positive gravitropism in roots


  1. Gravity causes IAA to accumulate on lower side of the root

  2. IAA inhibits elongation of root cells

  3. Cells on the upper side of the root elongate faster, so the root tip bends downwards