Hormone control in plants
Doc Brown's Biology Revision Notes
Suitable for GCSE/IGCSE/O level Biology/Science courses or equivalent
This page will answer many questions e.g.
How do hormones control plant functions?
What are auxins?
What is phototropism?
What is gravitropism or geotropism?
What controls the ripening of plants?
Control in plants
Know that plants
also produce hormones and respond to external stimuli.
a) Know and understand that plants are sensitive to,
and respond to, light, moisture and gravity:
their shoots grow towards light and against the
force of gravity,
their roots grow towards moisture and in the
direction of the force of gravity.
b) Know that plants produce hormones to coordinate and control
Auxin is a plant hormone
that controls the growth of the tips of shoots and roots
Auxin acts by enabling the plant
to respond to, and ...
... the tips of shoots to grow
towards light, the effect is called phototropism,
... the tips of shoots to grow
upwards (against gravity) and the tips of roots downwards (with gravity),
the effects are called gravitropism,
... the tips of roots to seek
moisture in the soil.
Know that auxins controls phototropism and
Auxin is produced in the tips
and moves back by diffusion to stimulate cell growth - cell enlargement-elongation.
If the tip of shoot is cut off,
the shoot may stop growing because the auxin hormone is no longer available.
Auxin can promote growth in
shoots but a high concentration of auxin can inhibit growth in the root to
ensure it grows in the right direction.
You should understand the role
of auxins in phototropism and gravitropism.
c) The responses of plant roots and shoots to light,
gravity and moisture are the result of unequal
distribution of hormones like auxin, causing unequal
growth rates and changes in growth direction.
d) Plant growth hormones are used in agriculture and
horticulture as weed killers and as rooting hormones and is a very important use
of plant hormones like auxin.
Some plant growth hormones can
be used as selective weed killers to disrupt the growth of weeds but leave
the crops unaffected.
Desired crops of grasses and
cereals are narrow leafed plants but many weeds have broad leaves.
Selective plant growth hormone
based weed-killers have been developed that affect the growth development of
broad-leafed weeds and eventually kill them, BUT, do not affect the grasses
and cereals with narrow leaves.
Some plant cuttings won't always
readily grow in soil or compost, but by adding a rooting powder to the
compost containing a plant hormone like auxin, the growth of roots and
subsequent shoots are greatly encouraged.
Plant hormones can be used to
control the ripening of fruit or produce seedless fruit.
The ripening can be controlled
while the fruit are still on the tree/bush or during transport to the
You can therefore pick fruit
before it is ripe and still quite firm - which means its less easily damaged
You can then choose the time
when the ripening hormone is added so the fruit is as fresh as it can be for
you the consumer via the wholesaler, market vendor, small shop or giant
Most fruit plants with seeds in
the core, require pollination by insects, otherwise the fruits and seeds
will not grow.
By applying growth hormones to
the unpollinated flowers of some fruit plants, the fruits grow BUT not the
seeds! Sometimes the plant hormones are applied after pollination, but still
prevent the seeds developing.
A very handy way of producing
common varieties of seedless fruits like watermelons, grapes, bananas and
many seedless citrus fruits, such as oranges, lemons and limes.
Gibberellin is another plant
growth hormone used to stimulate seed germination, stem growth (taller) and
Sometimes combinations of
hormones eg auxins plus gibberellins can be used in conjunction with each
other to have greatly enhanced effect eg producing very tall plants.
You are expected to use
appropriate skills, knowledge
and understanding to:
Any practical work and
investigations you did should also be revised (which should also
be revised, helps in understanding 'how science works' and context
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