Plant Hormones

Stem > always grows towards light

Roots > always grow away from light

Auxins (plant hormones controlling its growth) are produced in the growing tips of shoots and roots

  • Auxins respond to light, gravity and moisture
  • Auxins cause some cells to elongate or grow at a faster rate

Tropism = plant’s response to light, gravity or moisture

  • Positive tropism = response towards stimulus
  • Negative tropism = response away from stimulus

Geotropism = plant’s response to gravity

  • Positive geotropism = roots grow towards direction of gravity
  • Negative geotropism = shoots grow away from direction of gravity

This video shows how Geotropism works

Hydrotropism = plant’s response to water

  • Positive hydrotropism = shoots always follow direction of water

Phototropism = plant’s response to light

This video shows how Phototrophism works

Light destroys auxins. Auxins on sunlit side of plant are destroyed, growth slows. Growth of plant on shaded side continues > plant growth leans towards light

Commercial Ues of Plant Hormones

1. Growing cuttings: cuttings are dipped into powder containing growth hormones. Plants are cloned quickly and cheaply

2. Producing fruit without seeds: Growth hormones sprayed on unpollinated flowers > grow fruit without fertilisation

3. Ripening fruit: Plant hormone ethene ripens fruit. Sprayed onto unripened fruit during transportation to supermarkets

4. Increasing size of fruit: e.g. grapes

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