Plant Auxins

Noadswood Science, 2011

Plant Auxins

To understand how plant auxins function

Wednesday, April 19, 2023

Which Way

How do plants always ‘know’ which way is up when they grow?!

Which Way

Plants are very sensitive and their growth is affected by their environmental conditions (a stimulus)

light

water

gravity

Tropism

Plants respond to stimuli by growing to or away from them – a growth movement in response to a stimulus is a tropism (towards stimulus = positive tropism, away from stimulus = negative tropism)

Light

Phototropism

Water

Hydrotropism

Gravity

Geotropism

Tropisms

Phototropism (Light)

A plant’s response to light is called phototropism – plants grow towards light, which is a positive tropism (plants need light for photosynthesis, so they respond to light by growing towards it)

light 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

Phototropism (Light)

light light

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…

Phototropism (Light)

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

Setup the phototropism experiment as shown in the diagram using seedlings and a light source on one side of the box…

Geotropism (Gravity)

A plant’s response to gravity is called a geotropism – different parts of a plant have different responses to gravity…

Gravity

Shoots grow up

Roots grow down

Negative geotropism

Positive geotropism

Geotropism (Gravity)

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

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.

Hydrotropism (Water)

A plant’s response to water is called hydrotropism – roots always grow towards water, a positive tropism

Wet soilDry soil

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

Equal Hydration

When roots are well watered from all sides there is an equal amount of auxins in all parts of the roots, which grow at the same rate, so the roots grow out in all directions…

Wetsoil

Wetsoil

Wetsoil

Wetsoil

Unequal Hydration

When roots are in soil with more water on one side, the auxins are not spread out evenly in the roots

Drysoil

Wetsoil

Drysoil

Wetsoil

There are more auxins on the wetter side of the roots, which grows slower than the dryer side, so the roots bend towards the water

Agriculture

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

Ripening fruit

Killing weeds

Cuttings

A cutting is a part of plant that is removed from the main shoot

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

Rooting Powder

Weed Killers

Cereal crops like wheat are grasses with narrow leaves – most weeds that grow in fields and gardens have broad leaves

Selective weed killers contain plant hormones at concentrations that will only affect plants with broad leaves

These selective weed killers upset the normal growth of broad-leaved plants and so kill off any weeds – the growth of cereal crops and grasses is not affected

Ripening Fruit

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

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 (but you should wash your fruit before you eat it)!