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Plant Auxins
Noadswood Science, 2011
Monday, May 22, 2017
Plant Auxins
 To understand how plant auxins function
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)
water
light
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)
Water
Hydrotropism
Light
Phototropism
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
 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
Phototropism (Light)
light
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…
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…
Shoots grow up
Negative geotropism
Roots grow down
Gravity
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
 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
Dry soil
Wet soil
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…
Wet
soil
Wet
soil
Wet
soil
Wet
soil
Unequal Hydration
 When roots are in soil with more water on one
side, the auxins are not spread out evenly in the
roots
Dry
soil
Wet
soil
Dry
soil
 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
Wet
soil
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)!