3.2.7 Plant Hormones - Tropisms

Tropisms

• 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

Table of Tropisms

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); less auxin = less cell elongation = less growth

Auxin and gravitropisms

• 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 geotropism response in roots and shoots

Auxin and phototropisms

• If light shines on a growing shoot from all directions, auxin is distributed evenly throughout the shoot and the cells in the shoot grow at the same rate - this is what normally happens with plants growing outside
• When light shines on the shoot predominantly from one side, the auxin produced in the shoot tip concentrates on the shaded side of the shoot, making the cells on that side elongate and grow faster than the cells on the sunny side
• This unequal growth on either side of the shoot causes the shoot to bend and grow in the direction of the light

Positive phototropism in plant shoots as a result of auxin

Other plant hormones

• In the same way that auxins control growth, other plant hormones are required to control other stages of the plant lifecycle, such as:
  • Germination (controlled by the hormone gibberellin)
  • Fruit ripening (controlled by the hormone ethene)
  • Flower opening (controlled by the hormone gibberellin)
  • Shedding of leaves (controlled by the hormone ethene)

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