Plant hormones and tropisms – GCSE Combined Science Biology

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Plants cannot walk around, but they can move by growing. A shoot grows towards a window. A root grows down into the soil. Plant hormones control this.

A hormone is a chemical messenger made in one part of a living thing. It travels to another part and changes how it works. Farmers and gardeners also use plant hormones to grow better crops.

By the end you will be able to explain how plants bend towards light and gravity. You will name three plant hormones and say what each one does. You will also describe a simple tropism investigation.

Plant responses

A tropism is a growth response to a stimulus, which is a change in the surroundings. Plants respond by growing in a particular direction.

Phototropism is growth in response to light. Gravitropism is growth in response to gravity. It is also called geotropism. Growth towards a stimulus is positive. Growth away from it is negative.

Shoots grow towards light and away from gravity. Roots grow towards gravity and usually away from light.

Auxin and light

Auxin is a plant hormone made in the tips of shoots. It moves through the plant and controls cell growth. In a shoot it makes cells elongate, which means grow longer.

When light shines from one side, auxin moves to the shaded side. The cells there elongate more. The shaded side grows faster, so the shoot bends towards the light. Think of a car with one wheel turning faster than the other. It curves.

This helps the plant to get more light for photosynthesis.

Auxin and gravity

Put a seedling on its side. Auxin collects on the lower side of both the shoot and the root. But the two parts respond in opposite ways.

In the shoot, auxin makes the lower cells elongate more. The lower side grows more, so the shoot bends upwards.

In the root, a high concentration of auxin stops cells elongating. The lower side grows less, so the root bends downwards. The same hormone can have opposite effects, because shoots and roots respond differently.

Gibberellins and ethene

Gibberellins are plant hormones. They end seed dormancy, which is a resting stage when a seed does not grow. They start germination, when the seed begins to grow into a plant.

Gibberellins also promote flowering and can increase the size of fruit.

Ethene is a gas. It controls cell division and the ripening of fruit. Ripe fruit is softer and sweeter. Plants make these hormones naturally, and growers can use them too.

Using hormones in farming

Auxin is used in rooting powder. A gardener dips a cutting in the powder, and it quickly grows roots. Auxin is also used in weedkillers. Broad-leaved weeds take it up and grow too fast, so they die. Narrow-leaved crops such as wheat are not harmed.

Ethene controls the ripening of fruit. Fruit can be picked unripe, which makes it easier to transport. It is then treated with ethene when it reaches the shop. Gibberellins can end seed dormancy and increase fruit size.

Investigating tropisms

To investigate phototropism, grow seedlings in boxes. Give one box light from one side only. Give another light from all sides, and keep another in the dark.

To investigate gravitropism, lay seedlings on their side on damp cotton wool. Watch the shoots and roots over a few days.

Keep the temperature, water and type of seedling the same. Use several seedlings and measure the angle of bending. Angles give you numbers to compare.

Recap

Phototropism is a response to light. Gravitropism is a response to gravity.

Auxin collects on the shaded or lower side. It makes shoot cells elongate and stops root cells elongating.

Gibberellins end dormancy and promote flowering. Ethene ripens fruit.

Uses are rooting powder, weedkillers, seedless fruit and ripening control.