Auxin and Tropic Responses

What is auxin?

Growth hormone. Stimulates the growth of cells. Crucial for plants response to stimuli.

Positive Phototropism (most plants show this)

• Plant is exposed to light on one side

• Auxin moves towards shaded side of the root; stimulates more cells to grow here

• Shoot bends towards the light

• Plant receives more light; photosynthesis can occur at a faster rate

Negative geotropism (most shoots show this)

• Auxin moves to the lower side of the stem / shoot

• Cells of the shoot grow more on the side with more auxin; stimulates more cells to grow there

• Shoot bends and grows away from ground

• Beneficial: light levels are likely to be higher further away from the ground

Positive geotropism (most roots show this)

• Auxin moves to the lower side of the stem / shoot

• Cells of the root grow more on the side with less auxin; stimulates more cells to grow here

• Makes the root bend and grow downwards

• Beneficial: More likely to be increased levels of water and nutrients lower down, and provides stability for plant