Plant hormones

• Plants have to be able to respond to stimuli in their environment in order to survive, but they do it much more slowly than animals
• Plants can sense light and grow their shoots towards it to maximize photosynthesis
• Plants can sense gravity and grow their roots downwards to fix themselves into the soil and find more water and minerals
• Some plants can sense touch and climb around other objects
• Plants rely entirely on hormones to respond to their environment, as they don't have a nervous system like animals
• Auxins


  Plant hormones that control growth at the ends of shoots and roots
• How auxins work


  1. Produced in the tips of shoots and roots

  2. Dissolve in the cell solution and diffuse backwards along the shoot or root
• Auxins in shoots


  • Stimulate growth, causing shoots to grow
• Auxins in roots


  • Inhibit growth
• Phototropism


  Response to light
• Geotropism/Gravitropism


  Response to gravity
• Auxins always accumulate on the shaded side rather than the sunny side, and the lower side rather than the upper side
• Shoots


  Positively phototropic (grow towards light), negatively geotropic (grow away from ground)
• Roots


  Negatively phototropic (grow away from light), positively gravitropic (grow down towards ground)
• If roots are exposed to light, auxins accumulate on the lower shaded side, causing the upper side to grow faster and the root to curve downwards
• If roots are horizontal, auxins accumulate on the lower side, inhibiting growth and causing the upper side to grow faster, making the root curve downwards