Auxins & Apical Dominance (OCR A Level Biology): Revision Note

Auxins & Apical Dominance

• The auxins that are produced at the growing tip at the apex (i.e. the top) of a plant stem cause the stem to grow upwards and also stop lateral (side) buds from growing

  • This is known as apical dominance

• Most of the time, it is best for plants to grow upwards towards the light, as this ensures the plant has access to more energy for photosynthesis

• Most of the time, sideways growth is not so useful to plants and so the apical dominance caused by auxins ensures that growth is preferentially upwards

• If the growing tip at the apex of a plant is removed (e.g. due to grazing from herbivores), lateral buds grow from the top of the plant, as the source of auxins has been removed and there is no longer apical dominance

• With time, however, the lateral shoots that grow from these lateral buds do curl up towards the light, meaning the plant continues to grow in an upwards direction

• This response can be easily shown experimentally, as seen in the images below

  • The apical bud of the first test plant is removed (decapitated)

  • This allows the lateral buds to grow

  • A second (genetically identical) test plant is decapitated but this time the cut tip is immediately replaced with an agar block containing auxin

  • This restores the inhibition of lateral bud growth and no lateral buds grow

A simple experiment showing the role of auxins in apical dominance