Plant hormones
A tropism is a growth response and plant hormones
coordinate
a growth
Auxin is a plant hormone which controls the growth of
roots and shoots. They move to cells by diffusion in
response to different stimulus e.g.
Gravity – gravitropism/geotropism
Water – hydrotropism
Light – phototropism
Touch – thigmotropism Explain these in terms of auxin distribution.
Auxin controls the response to shoots and roots to gravity
but in different ways. In shoots auxin promotes growth – Uses of plant hormones
causes cell elongation - but in roots auxin inhibits growth. Growth hormones are used in agriculture and
A positive tropism is when the hormone moves the plant horticulture
towards the stimulus whereas a negative tropism is when Auxin can be used as rooting powders that are
the hormone moves the plant away from the stimulus e.g. placed on the end of a cutting to stimulate the
Shoots grow towards light – positive phototropism, growth of new roots and help the cutting grow into
but grow up against gravity – negative gravitropism a new plant.
Roots grow towards water – positive hydrotropism High doses of plant hormones can be used as
but away from light – negative phototropism weed killers/herbicides as it makes the plant go
How the distribution of auxin varies in a shoot into rapid, uncontrolled growth which kills them.
Herbicides only kill weeds because they tend to
have broad leaves so they absorb more of the
hormone than narrow leaved plants.
They are also used for promoting growth in tissue
culture cloning.
Other plant hormones
Gibberellins
Used in the brewing industry to end seed
dormancy and speed up germination of barley
seeds. They also promote flowing in out of season
pot plants and increase the size of fruit
Ethene
It controls cell division and is used in the food
industry in controlling the ripening of fruit during
transport and storage
A tropism is a growth response and plant hormones
coordinate
a growth
Auxin is a plant hormone which controls the growth of
roots and shoots. They move to cells by diffusion in
response to different stimulus e.g.
Gravity – gravitropism/geotropism
Water – hydrotropism
Light – phototropism
Touch – thigmotropism Explain these in terms of auxin distribution.
Auxin controls the response to shoots and roots to gravity
but in different ways. In shoots auxin promotes growth – Uses of plant hormones
causes cell elongation - but in roots auxin inhibits growth. Growth hormones are used in agriculture and
A positive tropism is when the hormone moves the plant horticulture
towards the stimulus whereas a negative tropism is when Auxin can be used as rooting powders that are
the hormone moves the plant away from the stimulus e.g. placed on the end of a cutting to stimulate the
Shoots grow towards light – positive phototropism, growth of new roots and help the cutting grow into
but grow up against gravity – negative gravitropism a new plant.
Roots grow towards water – positive hydrotropism High doses of plant hormones can be used as
but away from light – negative phototropism weed killers/herbicides as it makes the plant go
How the distribution of auxin varies in a shoot into rapid, uncontrolled growth which kills them.
Herbicides only kill weeds because they tend to
have broad leaves so they absorb more of the
hormone than narrow leaved plants.
They are also used for promoting growth in tissue
culture cloning.
Other plant hormones
Gibberellins
Used in the brewing industry to end seed
dormancy and speed up germination of barley
seeds. They also promote flowing in out of season
pot plants and increase the size of fruit
Ethene
It controls cell division and is used in the food
industry in controlling the ripening of fruit during
transport and storage
