Auxin and phototropism

Greg Foot explains how plants respond to light and gravity, and the role of auxins in controlling and coordinating plant growth

In order to survive, plants require light and water for photosynthesis. They have developed responses called tropisms to help ensure they grow towards adequate sources of light and water.

There are two main types of tropisms:

  • positive tropisms - the plant grows towards the stimulus
  • negative tropisms - the plant grows away from the stimulus

Phototropism is the growth of plant shoots towards the light.

Responses to stimuli of different parts of the plant

  • In the plant stem, responses to light are known as a positive phototropism, which means the stem grows towards the light.
  • In the plant root, responses to light are known as a negative phototropism, which means the root grows away from the light.
Image of a plant growing towards the light
Positive phototropism in plant stems

Auxins

Auxins are a family of plant hormones. An example of a specific auxin is IAA, which is known as Indole acetic acid.

They are mostly made in the tips of the growing stems and roots, which are known as apical meristems, and can diffuse to other parts of the stems or roots.

Unequal distribution of auxin can cause unequal growth rates in plants and shoots. Auxins change the rates of elongation in plant cells and control growth of stems and roots.

Stems and roots respond differently to high concentrations of auxins:

  • cells in stems grow more
  • cells in roots grow less

Phototropisms

In a stem, the shaded side contains more auxin and grows longer, which causes the stem to grow towards the light. The plant does NOT bend towards the light. It grows because the auxin causes the cells to elongate on the shaded side, so this side grows more. This unequal growth of the two sides, results in the growth of the stem towards the light.

If lit from above, the plant will grow upwards.

A visual to show four stages of positive geotropism.Three sets of plants. A: furthest from light, pointing up, have grown about 2cm. B: grow straight, have grown twice as high as A. C: have grown highest, but are bending towards light.Typical results by oat seedlings grown in a box with a light source
Seedling ASeedling BSeedling C
TreatmentThe tips have been removedNo light reaches the tipsMore light reaches one side of the tips
Effect on auxin concentrationNo auxin is producedEqual concentration of auxin on both sidesGreater concentration of auxin on shaded side
ResultThe stems do not grow longerThe stems grow evenly and longer on both sidesThe cells on the darker side of the stems grow longer

Auxins have the opposite effect on root cells. In a root, the shaded side contains more auxin and grows less - causing the root to bend away from the light.

Auxin and gravitropism

Gravitropisms

Phototropism is a response to the stimulus of light, whereas gravitropism (also called geotropism) is a response to the stimulus of gravity.

Plants responses to gravity:

  • when the stem grows against the force of gravity, this is known as a negative gravitropism
  • when a root grows in the direction of the force of gravity, this is known as a positive gravitropism

In a root placed horizontally, the bottom side contains more auxin and grows less - causing the root to grow in the direction of the force of gravity.

The opposite happens in a stem. When a stem is placed horizontally, the bottom side contains more auxin and grows more - causing the stem to grow upwards against the force of gravity.

A visual to show geotropism, a response to the stimulus of gravity. The image should show how a plant responds to gravity.Stems grow upwards (negative gravitropism)A visual to show how auxin and gravity cause the plant to grow in different directions when it is placed horizontally.Roots grow downwards (positive gravitropism)