Plant Hormones

• Auxin is a plant growth hormone
• Auxin is a plant hormone that controls growth near the tips of shoots and roots
• Auxin controls the growth of a plant in response to light (phototropism) and gravity (gravitropism or geotropism)
• Auxin is produced in the tips and moves backwards to stimulate the cell elongation process which occurs in the cells just behind the tips
• If the tip of a shoot is removed, no auxin is available and the shoot may stop growing
• Extra auxin promotes growth in the shoot but inhibits growth in the root - producing the desired result
• Shoots grow towards light

  • When a shoot tip is exposed to light, more auxin accumulates on the side that's in the shade than the side that's in the light
  • This makes the cells grow (elongate) faster on the shaded side, so the shoot bends towards the light

  
• Shoots grow away from gravity and roots grow towards gravity

  • When a shoot is growing sideways, gravity produces an unequal distribution of auxin in the tip, with more auxin on the lower side
  • This causes the lower side to grow faster, bending the shoot upwards
  • A root growing sideways will also have more auxin on its lower side
  • But in a root the extra auxin inhibits growth. This means the cells on top elongate faster, and the root bends downwards