Plant Hormones

• auxins are plant hormones which control growth at the tips of shoots and roots. they move through the plant in solution (dissolved in water)
• auxin is produced in the tips and diffuses backwards to stimulate the cell elongation process which occurs in the cells just behind the tips
• auxin promotes growth in the shoot, but actually inhibits growth in the root
• auxins are involved in the growth responses of plants to light (phototrophism) and gravity (gravitrophism)
• • when a shoot tip is exposed to light, it accumulates more auxin on the side that's in the shade than the side that's in the light
  • this makes the cell grow (elongate) faster on the shaded side, so the shoot bends towards the light
• by bending towards the light, the shoot will be able to absorb more light for photosynthesis, which enables the plant to grow. shoots growing completely in the dark will be tall and spindly - the auxin in the tips makes them elongate quickly on all sides. a taller shoot has a better chance at finding light.
• • 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
• shoots are negatively gravitropic (grow away from gravity)
• roots are positively gravitropic (grow towards gravity)
• • a root growing sideways will 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 roots bend downwards
• roots are negatively phototropic (grow away from light)
• • if a root starts being exposed to some light, more auxin accumulates on the more shaded side
  • the auxin inhibits cell elongation on the shaded side, so the root bends downwards, back into the ground