Plant hormones

Messages without nerves

Growth movements are slow, but they still need a message. The part that senses the light is not always the part that bends. So the plant needs a chemical message. This is done by plant hormones, also called phytohormones.

A plant hormone is a chemical that is:

• made in one part of the plant, in a very small amount;
• carried to other parts, where it diffuses into the cells;
• able to control the growth of those cells.

Auxin and the bend towards light

The best known plant hormone is auxin. It is made at the tip of the shoot.

When light falls on the shoot from one side:

1. The tip senses the light.
2. Auxin moves away from the light, towards the shaded side of the shoot.
3. On the shaded side, auxin makes the cells grow longer.
4. The shaded side grows more than the lit side, so the shoot bends towards the light.

When light falls from above, the auxin spreads evenly, both sides grow alike, and the shoot grows straight up.

Scientists found this by simple experiments on young grass shoots. Try them here.

You should find three results.

• With its tip cut off, the shoot does not bend. The tip is needed.
• With a foil cap over the tip, the shoot grows straight up even in light from one side. The tip is the part that senses light.
• With the tip open and light from one side, the shoot bends towards the light.

Other plant hormones

Three of them promote growth. Abscisic acid is different: it inhibits growth. It helps a plant to survive hard times, such as a long dry spell, by stopping growth. It also causes the wilting of leaves.

Farmers and gardeners use these hormones. Gibberellins can make stems longer, and cytokinins can keep cut leaves green for longer.

Try it in Python

The program shares the auxin from the tip between the two sides of the shoot, and works out how much each side grows.

Practise with new numbers

Common mistakes

• "Auxin is made all along the stem." It is made at the tip of the shoot, then moves down.
• "Auxin moves towards the light." It moves to the shaded side.
• "Plant hormones are carried in blood." Plants have no blood. Hormones diffuse from cell to cell and move in the plant's own tissues.
• "All plant hormones make plants grow." Abscisic acid slows growth down.
• Thinking that the tip bends. The tip senses the light, and the region just below it bends.

Remember these

• A plant hormone is a chemical made in one part and carried to other parts, where it controls growth.
• Auxin is made at the shoot tip. In light from one side, it moves to the shaded side and makes those cells grow longer, so the shoot bends towards the light.
• Gibberellins help stems grow. Cytokinins promote cell division.
• Abscisic acid inhibits growth and causes leaves to wilt.

Check yourself

Try each one on your own first. If you are stuck, open a hint. Then check with Show the answer.

1. Where is auxin made, and where does it go when light comes from one side?

2. A shoot has its tip removed and is lit from one side. Will it bend? Explain.

3. Which plant hormone is the odd one out, and why: auxin, gibberellin, cytokinin, abscisic acid?

4. A gardener wants a plant with a long stem. Which hormone could help?

5. Why do the leaves of a plant wilt in a long dry spell, and how does this help it?