06 Plant Hormones

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- GCSE Bitesize - Plant hormones

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Science

Plant hormones

A 't ropism' is a growth in response to a stimulus. Plants

grow towards sources of water and light , which they need

to survive and grow.

Auxin is a plant hormone produced in the stem tips and

roots, which controls the direction of growth. Plant

hormones are used in weedkillers, rooting powder and to

control fruit ripening.

Tropisms

The direction of plant growth

Plants need light and water forphotosynthesis

[photosynthesis: The chemical change that occurs in the

eaves of green plants. It uses light energy to convert carbon

dioxide and water into glucose. Oxygen is produced as a by-

product of photosynthesis. ]. They have developed responses

called tropisms to help make sure they grow towards sources of

ight and water.

There are different types of

tropisms:

positive tropism towards

the stimulus

negative t ropism away from

the stimulus

phototropism growth inresponse to the direction of

light

geotropism growth in response to the direction of

gravity

Responses of different parts of the plant

Positive phototropism in plantstems


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response part

of

plant

direction of

growth

advantage

positive

phototropism

stem

tip

growth

towards

light

to get maximum light

for photosynthesis

negative

phototropism

root

tip

growth

away from

light

less chance of drying

out

positive

geotropism

root

tip

towards

gravity

more chance of

finding moisture

negativegeotropism

stemtip

away fromgravity

more chance offinding light

Plant hormone experiments

Controlling the direction of growth

Auxin is a plant hormone responsible for controlling the

direction of growth of root tips and stem tips in response to

different st imuli including light and gravity.

Auxin is made at the tips of stems and roots. It's moved in

solution to older parts of the stem and root where it changes the

elasticity of the cells. More elastic cells absorb more water and

grow longer, causing bending in the stem or root. It's thought

that light and gravity can interfere with the transport of auxin

causing it to be unevenly distributed.

You may have noticed that a houseplant grows towards thewindow and turns its leaves towards the light. It does this

because light coming from the window side of the plant destroys

the auxin in that side of the stem. So growth on that side slows

down.

On the shaded side of the plant there is more auxin. So growth

on this side speeds up. The result is that the shoots and leaves


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are turned towards the light forphotosynthesis.

Auxin is produced in the tip of growing shoots.

f the tips are removed from growing shoots they cannot produce

auxin, so phototropism cannot occur.

f the tips are covered, light cannot break down the auxin, so

phototropism cannot then occur either.

Using plant hormones Higher tier

Weedkillers

Selective weedkil lers kill some plants but not others. This can

be useful for getting rid of dandelions in a lawn without killing the

grass, or getting rid of thistles in a field without killing the wheat

plants. The selective weedkiller contains growth hormone thatcauses the weeds to grow too quickly. The weedkiller is

absorbed in larger quantities by the weeds than the beneficial

plants.

Rooting powder

Rooting powder makes stem cuttings quickly develop roots.

Rooting powder contains growth hormones.

Controlling fruit ripening

Some hormones slow the ripening of fruits and others speed it

up. These hormones and their inhibitors are useful for delaying

ripening during transport or when fruit is displayed in shops.

Dormancy

Dormancy stops seeds germinating until conditions are ideal for

growth. Hormones can be used to remove the dormancy of aseed so it can germinate at all times of year. Buds and flowers

can also be naturally dormant. Hormones can also be used to

make plants grow bushier, make them flower or control the

growth of hedge plants.

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06 Plant Hormones