4.12 Plant Hormones - Auxins

Question 1

What are auxins? (4)

Answer 1

• Auxins are plant hormones which control and coordinate growth at the tips of shoots and roots. They move through the plant in solution (dissolved in water).
• Auxin is produced in the tips and diffuses backwards to stimulate the cells to elongate (grow), which occurs in the cells just behind the tips.
• Auxin promotes growth in the shoot, but actually inhibits growth in the root.
• Auxins are involved in the growth responses of plants to light (phototropism) and gravity (gravitropism).

Question 2

Describe how shoots are positively phototrophic

Answer 2

• When a shoot tip is exposed to light, it accumulates more auxin on the side that’s in the shade than the side that’s in the light.
• This makes the cells grow (elongate) faster on the shaded side, so the shoot bends towards the light.

Question 3

Describe how shoots are negatively gravitropic

Answer 3

• When a shoot is growing sideways, gravity produces an unequal distribution of auxin in the tip, with more auxin on the lower side.
• This causes the lower side to grow faster, bending the shoot upwards.

Question 4

Describe how roots are positively gravitopic

Answer 4

• A root growing sideways will also have more auxin on its lower side.
• But in a root the extra auxin inhibits growth. This means the cells on top elongate faster, and the root bends downwards.

Question 5

Describe how roots are negatively phototropic

Answer 5

• If a root starts being exposed to some light, more auxin accumulates on the more shaded side.
• The auxin inhibits cell elongation on the shaded side, so the root bends downwards, back into the ground.

Question 6

What is the point of phototropism and gravitropism?

Answer 6

Phototropism and gravitropism increase a plant’s chance of survival. For example, by growing towards the light, a plant shoot is able to absorb more light for photosynthesis, and by growing downwards, a plant root is more likely to anchor itself into the soil and find water and minerals.

Question 7

Describe how to investigate the role of auxins in photosynthesis (4)

Answer 7

For these experiments you need some shoots with coleoptiles, e.g. oats, grasses.
1) Plant three lots of three shoots in damp soil. Leave one lot of shoots as they are. Put foil caps on the tips of another lot of shoots. Wrap foil around the bases of the final lot of shoots.
2) Put each group of shoots in a cardboard box with a hole in one side. Put a light source next to the hole.
3) Leave the shoots for a few days . You should notice that: the shoots without any foil have grown towards the light, the shoots with foil caps have grown straight upwards and the shoots with foil around their bases have grown towards the light.
4) This experiment shows that growth towards the light is only prevented when the shoot tips are covered, so the auxins must be active in the tip.

Question 8

Which 3 stimuli can plants detect?

Answer 8

• Touch
• Gravity
• Light

Question 9

What are the two terms that refer to the growth of parts of plants in response to the force of gravity?

Answer 9

• Gravitropism
• Geotropism

Question 10

Auxins always accumulate on the:

Answer 10

Lower, shaded side of shoots/roots.

Question 11

Tips for remembering which side of the shoot or root auxins accumulate on:

Answer 11

• Light breaks down auxins, so there will be more auxin on the shaded side.
• Gravity pulls auxins downwards, so there will be more auxin on the lower side.