Auxins 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 a response to the stimulus of 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

Auxins

Auxins are a family of plant hormones. 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.

Auxins control the growth of plants by promoting cell division and causing elongation in plant cells (the cells get longer).

Stems and roots respond differently to high concentrations of auxins:

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

Phototropism

This is caused by an unequal distribution of auxin.

Diagram showing plants pointing towards the sunTypical results shown by oat seedlings grown in a box with a light source

In a stem, the shaded side contains more auxin and grows longer, which causes the stem to grow towards the light. It is vital to note that the plant does NOT bend towards the light.

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 the 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.