1 of 34© Boardworks Ltd 2011 Plant Hormones. 2 of 34© Boardworks Ltd 2011

1 of 34 © Boardworks Ltd 2011

Plant Hormones



What do plants need to grow?

This plant hasn’t been looked after very well. What do plants need to grow healthily? sunlight

water

mineral nutrients

If a plant cannot get these from its environment, can it move to somewhere else?

Plants are rooted to one place. However, they can control their direction of growth.

an air supply


What controls plant growth?

Plants grow in response to their environment.

The shoots of these tulips grow upwards and the roots grow downwards.

To what environmental factors do plants respond to make sure they grow properly?

If a plant was unable to do this, it might not be able to get enough water or sunlight from its environment.


What do plants respond to?

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

Any condition that affects plant growth is called a stimulus.

light water gravity

Which stimulus will have the strongest effect on the plant?

What are three types of stimuli that plants respond to?


Which stimulus?



A tropism is a growth movement in response to a stimulus.Plants grow towards or away from stimuli.

A growth movement towards a stimulus is called a positive tropism.

A growth movement away from a stimulus is called a negative tropism.

What is a tropism?

Will all the parts of a plant respond to a stimulus in the same way?

What is growth away from a stimulus called?


Plant responses to stimuli are given specific names:

What do the names of each tropism mean?

water gravitylight

hydrotropism geotropismphototropism

Which parts of a plant respond to these different stimuli?

Are there different types of tropisms?


Defining tropisms


How do tropisms affect growth?

This plant does not have a very straight stem. What might have caused this?

The shoots of plants grow towards sunlight.

The plant was placed on a window sill and received light from one direction only.

What is the name of this type of plant response?

The response to sunlight is called phototropism.


Phototropism experiment


Do plants have hormones too?

The growth of shoots and roots is controlled by a group of hormones called auxins. These chemicals are produced in the tips of the shoots and roots.

Like humans, plants use hormones, (chemical messengers)to control their development.

Hormones produced by plants control growth, flowering and fruit ripening.

What environmental stimuli might cause a plant to produce auxins?


Hormones and phototropism


Discovery of auxins


Which way up?

How do plants always grow the right way up?

Plants respond to gravity to grow in the right direction.


Hormones and geotropism


Roots always grow towards water, which is a positive tropism.

Roots will grow sideways, or even upwards, towards water.Roots always have a stronger response to water than gravity to ensure that a plant gets the water it needs.

How do roots respond to water?


How do roots and shoots respond to stimuli?

Shoots grow towards sunlight. They are positively phototropic.

Plant stimuli affect certain parts of the plant in different ways.

Shoots grow away from gravity. They are negatively geotropic.

Roots grow away from sunlight. They are negatively phototropic.

Roots grow towards gravity. They are positively geotropic.

What experiments can be used to test these ideas?


Tropisms and auxins – true or false?



growing cuttings

killing weeds.

controlling dormancy

Why are these purposes useful for gardeners and farmers?

Plant hormones are naturally-occurring chemicals, but they can also be produced synthetically for uses in gardening and agriculture, such as:

How can plant hormones be used?

making seedless fruit.

ripening fruit


A cutting is a part of plant that is removed from the main shoot. Do cuttings have everything they need to grow?

How are cuttings grown?

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

To stimulate root growth, cuttings are dipped into rooting powder. This contains plant growth hormones.

Cuttings are genetically identical to the parent plant. This allows growers to copy successful plants.


Growing plant cuttings


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

How do plant hormones kill weeds?

Plant hormones can be used instead to make weedkillers that only affect certain plants.

However, many chemicals that are potential weedkillers are toxic to animals and humans, as well as plants.

Most weeds have broad leaves. Cereals, which have narrow leaves, can be protected from weeds by using weedkillers that only effect the growth of broad-leaved plants.


Fruit often has to travels thousands of miles from where it is picked to where it is sold in the shops.

How are hormones used to ripen fruit?

When the fruit reaches its destination, it can then be sprayed with artificial plant hormones to encourage ripening.

Fruit is often picked before it is fully ripe. Why might this help keep the fruit edible?

Unripe fruit is harder and less likely to bruise than ripe fruit when transported.

Why might it be a good idea to wash fruit before eating it?


Which hormone makes fruit ripen?

Using artificial ethylene to ripen fruit is not just a modern practice. Ancient Egyptians exposed figs to natural gas, which contains ethylene, to encourage ripening.

Ethylene is a plant hormone that controls fruit ripening. It can be sprayed on unripe fruit to speed up ripening.

Why might it be important to keep fruit away from coal-burning engines during transportation?

Some fruit, such as bananas, actually produce a large amount of ethylene, which can cause other nearby fruits to ripen.


Controlling dormancy

Seeds can lie dormant for many years. Dormant seeds don’t germinate, even in warm moist conditions.

Dormancy is important as it ensures seeds don’t all germinate at the same time. This means that if there is a short period of poor growing conditions, such as a drought, some of the dormant seeds will survive.

Dormancy is controlled by plant hormones. Farmers can add artificial hormones to seeds to prevent them from germinating.

The record for the longest dormancy is held by a 1,300-year-old lotus seed which finally sprouted in 1995.


Seedless fruit

Plants usually reproduce using seeds found inside fruit, however people prefer to eat fruit without seeds.

For some plants, farmers can produce seedless fruit by:

Most plants only grow fruits if they have developing seeds. Plants that grow seedless fruit are often sprayed with plant hormones like auxin and giberellin to stimulate fruit growth, even though there are no seeds.

preventing pollination

selective breeding to create plants that have too many chromosomes, and so can’t produce seeds.


Using hormones



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1 of 34© Boardworks Ltd 2011 Plant Hormones. 2 of 34© Boardworks Ltd 2011