• Plants need to be able to grow in response to certain?stimuli
  • For example, plants need to be able to grow?in response to light, to ensure their leaves can absorb light for?photosynthesis
  • They also need to be able to grow?in response to gravity, to ensure that shoots grow upwards and roots grow downwards

   

• The?directional growth responses?made by plants in response to light and gravity are known as?tropisms
  • A response to light is a?phototropism?and a response to gravity is a?geotropism?(or gravitropism)

   

• If the growth is?towards?the stimulus, the tropism is?positive?and if the growth is?away from?the stimulus, the tropism is?negative
  • 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

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