Biology

4.9 - Plant Hormones & Tropisms

7 min

Plant responses

Auxins

Tropism rules and shoots

Tropisms in roots

note 1 of 2

Phototropism and gravitropism

Plants respond to their surroundings by growing in particular directions.
A tropism is a growth response in which part of a plant grows towards or away from a stimulus.
Phototropism - The response to light.
Gravitropism (also called geotropism) - The response to gravity.

A response is positive if the plant grows towards the stimulus, and negative if it grows away from it.

How shoots and roots respond

Plant part	Response to light	Response to gravity
Shoot	Grows towards light - positively phototropic	Grows away from gravity (upwards) - negatively gravitropic
Root	Grows away from light - negatively phototropic	Grows towards gravity (downwards) - positively gravitropic

Why these responses help the plant survive

Shoots bend towards light - This lets the shoot and its leaves absorb more light for photosynthesis, helping the plant make food and grow.
Roots grow downwards - Growing down anchors the plant and helps the root reach water and dissolved minerals.
Roots grow away from light - This keeps them in the soil.

A seedling in soil lit from a sun labelled light, with an upward arrow labelled shoot (grows toward light), a downward arrow labelled root (grows downward), and a downward arrow below the root labelled gravity.

note 2 of 2

The role of auxin

Auxin is a plant growth hormone.
It is made in the tips of shoots and roots.
It moves backwards from the tip, dissolved in solution, to the region just behind the tip, where it makes cells elongate (grow longer).
Uneven elongation of cells on one side is what bends a shoot or root.
Auxin has opposite effects in the two organs:
1. It promotes growth in shoots.
2. It inhibits growth in roots.
So the same uneven distribution of auxin bends a shoot and a root in opposite directions.
If the shoot tip is removed, no auxin is made and the shoot may stop growing.

Responses in shoots

Light from one side - Auxin builds up on the shaded side. Those cells elongate faster than the lit side, so the shoot bends towards the light (phototropism).
Lying horizontally - Gravity makes more auxin gather on the lower side. The lower cells elongate faster, so the shoot bends upwards, away from gravity.

Responses in roots

Lying horizontally - Gravity makes more auxin gather on the lower side, but extra auxin inhibits root growth. The lower cells grow more slowly, the upper cells elongate faster, so the root bends downwards, towards gravity.
Exposed to light - More auxin collects on the shaded side, where it inhibits elongation, so the root bends down and away from the light.

Two curved tips side by side: a green shoot bending towards light, auxin dots on its shaded side stimulating elongation, and a brown root bending away from light towards gravity, auxin there inhibiting elongation.

key terms

TermDefinition

TropismA growth response in which a plant grows towards or away from a stimulus.

PhototropismA plant's growth response to light.

GravitropismA plant's growth response to gravity.

AuxinA plant hormone made in the tips of shoots and roots that controls cell elongation.

end of notes

Key points to remember

Phototropism and gravitropism

Plants respond to their surroundings by growing in particular directions.

A tropism is a growth response in which part of a plant grows towards or away from a stimulus.

Phototropism - The response to light.

Gravitropism (also called geotropism) - The response to gravity.

A response is positive if the plant grows towards the stimulus, and negative if it grows away from it.

How shoots and roots respond

Plant part	Response to light	Response to gravity
Shoot	Grows towards light - positively phototropic	Grows away from gravity (upwards) - negatively gravitropic
Root	Grows away from light - negatively phototropic	Grows towards gravity (downwards) - positively gravitropic

Why these responses help the plant survive

Shoots bend towards light - This lets the shoot and its leaves absorb more light for photosynthesis, helping the plant make food and grow.

Roots grow downwards - Growing down anchors the plant and helps the root reach water and dissolved minerals.

Roots grow away from light - This keeps them in the soil.

The role of auxin

Auxin is a plant growth hormone.

It is made in the tips of shoots and roots.

It moves backwards from the tip, dissolved in solution, to the region just behind the tip, where it makes cells elongate (grow longer).

Uneven elongation of cells on one side is what bends a shoot or root.

Auxin has opposite effects in the two organs:

It promotes growth in shoots.
It inhibits growth in roots.

So the same uneven distribution of auxin bends a shoot and a root in opposite directions.

If the shoot tip is removed, no auxin is made and the shoot may stop growing.

Responses in shoots

Light from one side - Auxin builds up on the shaded side. Those cells elongate faster than the lit side, so the shoot bends towards the light (phototropism).

Lying horizontally - Gravity makes more auxin gather on the lower side. The lower cells elongate faster, so the shoot bends upwards, away from gravity.

Responses in roots

Lying horizontally - Gravity makes more auxin gather on the lower side, but extra auxin inhibits root growth. The lower cells grow more slowly, the upper cells elongate faster, so the root bends downwards, towards gravity.

Exposed to light - More auxin collects on the shaded side, where it inhibits elongation, so the root bends down and away from the light.