Plant Growth Factors

• A tropism is a plant growth in response to a directional stimuli
• Plant shoots demonstrate positive phototropism and negative geotropism, which increases the rate of photosynthesis
• Plant roots demonstrate positive geotropism, negative phototropism and positive hydrotropism. This allows for more stability, mineral ion absorption and water
• IAA is an auxin that causes the plant cells to elongate
• IAA is produced in very small quantities in the meristem of shoots and roots
• IAA in the shoot

  1. IAA produced initially diffuses down both sides of the shoot
  2. Light causes IAA to accumulate on the shaded side, increasing its concentration
  3. IAA attaches to receptors on the membrane of cells
  4. This releases hydrogen ions, lowering the pH and breaking bonds between microfibrils so it's more easily stretched
  5. Shaded side grows faster
• IAA in the root

  1. IAA initially diffuses down all sides of the roots as it is produced
  2. Gravity causes IAA to move to the lower region in the root tip
  3. Concentration is greater in the lower side
  4. IAA attaches to receptors on the membrane of cells and the lower side grows slower
  5. Top side grows faster so the root arrows downwards towards the centre of the Earth
• IAA inhibits cell elongation in the roots so grows downwards
• IAA stimulates cell elongation and so grows upwards