5.17 - Plant Hormones (Auxins)

• Plants rely entirely on hormones - act locally
• Auxins are produced in the shoots and roots of a plant
• Shoots of a plant are positively phototropic (grow towards light) and are negatively geotropic (grow away from the ground)
• Auxins control growth in a plants shoots and roots
• When auxins are produced, they then dissolve in the solution of the cells, and diffuse backwards along the shoot or root.

  Shoots = Auxins stimulate growth

  Roots = Auxins inhibit growth
• Phototropism 


  Plants response to light
• Geotropism


  Plant response to gravity
• Once auxins are produced int the shoots or roots of a plant, they will always accumulate on the shaded side of a shoot or the lower side of a root.
• If light is on the left hand side of a plant, the auxin will accumulate on the right hand side which is shaded.

  As auxin stimulates growth, the right side of the plants cells will grow faster and form a curve which shows the plant growing more obviously towards the left hand side, towards the light.

  The same occurs for gravity, the auxin will accumulate on the lower side of the shoot, and will cause the plant to bend upward.
• Roots are negatively phototropic (grow away from light) and positively geotropic (grow towards the ground).
• If light is present, roots will accumulate auxins on the lower shaded side of the root, which inhibits the growth, causing the upper side to grow faster and bend around to grow downwards.

  The same happens with gravity. The auxins will accumulate on the lower side, inhibiting the growth of the root, and the upper side will grow at a quicker rate, causing the root to bend around and grow downwards.
• Which stimuli can plants detect?

  Light

  Gravity

  Touch
• Do plants have a nervous system?


  No