Plant Hormones

TropismsUses of plant hormones

How do plants always grow the right way up?

What conditions will affect how a plant grows?

Plants are very sensitive and their growth is affected by their environmental conditions.

A condition that affects plant growth is called a stimulus.

What are the three types of stimuli that plants respond to?

light

water

gravity

What is a tropism?Plants respond to stimuli by growing to or away from them.

Movement towards a stimulus is called a positive tropism.

What is a movement away from a stimulus called?

A growth movement in response to a stimulus is a tropism.

light

water

gravity

A negative tropism.

Different types of tropismPlant responses to light, water and gravity are givenspecial names.

Can you work out what the name of each tropism means?

water

gravity

light

hydrotropism

geotropism

phototropism

Which parts of a plant respond to these different stimuli?

What do different tropisms mean?

How do plants respond to light?

A plant’s response to light is called phototropism.

Is phototropism a positive or negative tropism?

light

Plants grow towards light, which is a positive tropism.

Why do plants grow towards light?

Plants need light for photosynthesis, so they respond to light by growing towards it.

Phototropism and auxins

Which part of a plant is involved in phototropism?

Auxins are made in the tip of a growing shoot and move down the stem.

These plant hormones speed up growth in a growing shoot.

light

A growing shoot on a plant responds to light.

Phototropism and auxins – light from above

When a shoot gets light from above, the auxins produced at the tip are spread out evenly in the shoot.

How will this affect the growth of the shoot?

light

Phototropism and auxins – light from above

Why does a shoot grow straight up when light is from above?

There is an equal amount of auxins in all parts of the shoot, which grow at the same rate, so the shoot grows straight up.

light light

Phototropism and auxins – light from one side

When a shoot gets light from one side, the auxins are not evenly spread out in the shoot.

How will this affect the growth of the shoot?

light

Phototropism and auxins – light from one side

Why does the shoot bend towards the light when it gets light from one side?

There are more auxins on the shaded side of the shoot, so the shaded side grows faster making the bright side bend towards the light.

light light

Phototropism experiment

A plant’s response to gravity is called a geotropism.

Different parts of a plant have different responses to gravity.

Are these responses positive or negative geotropisms?

Why is important that roots grow down into the soil?

gravity

shoots grow up

roots grow down

negative geotropism

positive geotropism

Geotropism and auxins

Auxins speed up growth in shoots and slow growth in roots.

If a plant is laid on its side, the auxins produced collect in the lower side of the root and stem.

How will the auxins affect the growth of the root and stem?

Geotropism and auxins

If a plant is laid on its side, why does the root grow down and the stem grow up?

Auxins slow down growth on the lower side of the root, so the root curves down.

Auxins speed up growth on the lower side of the stem, so the stem curves up.

What are plant hormones used for?

Plant hormones can be used in gardening and agriculture to control how plants grow and develop.

Plant hormones are naturally-occurring chemicals but can also be produced synthetically for the following commercial uses:

growing cuttings

killing weeds

ripening fruit

A cutting is a part of plant that is removed from the main shoot.What does a cutting no longer have, when it is removed from a plant?

A cutting does not have roots and so has no supply of water or minerals.

Rooting powder contains plant growth hormones. Dip a cutting into rooting powder and the plant hormones stimulate the cutting to grow new roots.

Why does the gardening business use cuttings and rooting powder to grow new plants?

Grow your own plant cutting

Using plant hormones – killing weeds

A surprising use of plant hormones is for killing weeds.

Why are weeds a problem in farming and gardening?

Weeds are unwelcome plants that compete with the plants being grown by gardeners and farmers.

Many chemicals that are potential weedkillers are toxic to animals and humans as well as plants.

Why might a weedkiller with plant hormones that only affects plants be a more favourable option?

Cereal crops like wheat are grasses with narrow leaves.

Most weeds that grow in fields and gardens have broad leaves.

Selective weedkillers contain plant hormones at concentrations that will only affect plants with broad leaves.

What effect will this type of weedkiller have on a field of wheat overgrown with weeds?

These selective weedkillers upset the normal growth of broad-leaved plants and so kill off any weeds.

The growth of cereal crops and grasses is not affected.

Fruit is usually ripe and ready to eat when it is bought from a greengrocers or supermarket.

Fruit farmers are able to control when fruit ripens using plant hormones.

Why is this important for their business?

Fruit has often travelled many miles before being bought from the shops.

Is it better for fruit to be ripe or unripe during a long journey?

Fruit that is picked before it is ripe is usually hard.

There is less chance of unripe fruit being damaged when transported, which is important to the farmer and the buyer.

Plant hormones can be sprayed onto unripe fruit during transportation. These hormones ripen fruit in time for its arrival in the shops.

Why is it a good idea to wash fruit before you eat it?