Geotropism & Phototropism (Edexcel IGCSE Biology: Double Science)

• A response to light is a phototropism and a response to gravity is a geotropism (or gravitropism)
• As shoots grow upwards, away from gravity and towards light (so that leaves are able to absorb sunlight), shoots show a positive phototropic response and a negative geotropic response
• As roots grow downwards into the soil, away from light and towards gravity (in order to anchor the plant and absorb water and minerals from the soil), roots show a negative phototropic response and a positive geotropic response

Geotropism and Phototropism Table

Auxins

• Plants produce plant growth regulators (similar to hormones in animals) called auxins to coordinate and control directional growth responses such as phototropisms and geotropisms
• Auxins are produced in the tips of the shoots and the roots; they diffuse to the cells below the tips and have the following effects:
  • In the shoots, auxins promote cell elongation (growth); more auxin = more cell elongation = more growth
  • In the roots, auxins inhibit cell elongation (growth); more auxin = less cell elongation = less growth

• The distribution of auxin in the shoots is affected by light and gravity, whereas the distribution in the roots is primarily affected by gravity alone
  • If a shoot or root is placed on its side, auxins will accumulate along the lower side as a result of gravity; so the uppermost side has a lower auxin concentration
  • In the shoots, the lower side grows faster than upper side (more auxin = more cell elongation), so the shoot grows upwards
  • In the roots, the lower side grows slower than the upper side (as auxin inhibits cell elongation and growth in roots), so the root grows downwards

• Unequal distributions of auxin cause unequal growth rates in plant roots and shoots

The difference in the geotropic response of roots and shoots results from their different sensitivities to auxin