5.5.4 Auxins and Apical Dominance

Question 1

What do auxins cause?

Answer 1

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

Question 2

Why is growing upwards better?

Answer 2

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

Question 3

What happens if the growing tip of the apex plant is cut off?

Answer 3

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