(a) Introduction to
meristems and hormone
control in plants
Plants grow and develop in three ways:
(i) Cell division -
mainly in the tips of roots and shoots in regions called meristems.
(ii) The growth in height of plants
is due to plant cells expanding and elongating.
(iii) Plants usually grow
continuously e.g. putting out new roots, branches and leaves - so
plant cells can differentiate throughout their lifetime.
Know that plants
also produce hormones and respond to external stimuli.
These effects are called tropisms
and they are caused by an unequal distribution of plant hormones.
A definition of a tropism is to say
it is a plant growth movement in a particular direction due to a
directional stimulus.
The plant growth hormone auxins are unequally
distributed in response to light, gravity and moisture (water).
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.
Know that plants produce hormones to coordinate and control
the
growth at the tips of shoots and roots.
These hormones, called auxins, are
soluble in water and so can diffuse around to where they are needed.
Auxins are produced in the tips of
shoots and roots of plants.
Plants need to be able to detect and
respond to stimuli from their environment in order to survive.
The plant hormone auxin acts by enabling the plant
to respond to external stimuli e.g.
the tips of shoots to grow
towards light, the effect is called phototropism,
the tips of shoots to grow
upwards against gravity (gravitropism or geotropism),
the tips of roots downwards with gravity
(gravitropism or geotropism),
the tips of roots grow to seek
moisture in the soil (hydrotropism),
and note that auxin promotes
growth in the shoots, but inhibits growth in the root.
Know that hormones called auxins controls phototropism and
gravitropism (geotropism).
Auxin is produced in the tips
of shoots and roots, being soluble, it moves back by diffusion to stimulate cell growth
throughout the plant -
a process of cell enlargement and cell elongation.
This process occurs in the cells immediately
behind the tips of shoots and roots AND the change in growth direction
is due to an unequal distribution of auxin.
If the tip of shoot is cut off,
the shoot may stop growing because the auxin hormone is no longer available.
NOTE - auxin can promote growth in
shoots but a high concentration of auxins can inhibit growth in the root to
ensure it grows in the right direction - so things get a bit complicated.
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 - examples described in detail
below.
Meristems
In plants, the only cells that divide
by mitosis are found in plant tissues called meristems.
Meristem tissue is found in the tips
of roots and shoots - the parts of plants that are growing
(diagram on right).
Meristems make unspecialised cells
that can divide and form any type of cell the plant needs.
These unspecialised or
undifferentiated plant cells
effectively acts the same as animal stem cells.
However, unlike human stem cells,
they can differentiate to form any type of cell for the lifetime of the
plant.
e.g. these unspecialised cells can form
the specialised tissue cells of the xylem and phloem.
In the tips the meristems cells
undergo cell division by
mitosis and the daughter cells
grow longer and longer to increase the length of the shoot or root and
ultimately the full sized mature plant.
Plant hormones like auxins
promote cell division in the meristems and the increase in size
of the daughter cells.
As the cells get larger, so
does the vacuole (not shown), filled with water to give physical
support to the plant as a whole.
The cells at the end of the
tips remain meristematic - that is they stay undifferentiated to
provide any type of cell needed for new growth.
When cells from meristems are
extracted you can them grow them in tissue culture.
This is a very useful procedure
because tissue culture enables you to produce clones of plants
quickly, that are genetically identical and can be grown
economically by horticulturalists.
Section on plants in
Cloning -
tissue culture of plants
TOP OF PAGE for
SUB-INDEXES
(b) How the plant growth hormone
auxins works!
Phototropism - shoots are positively phototropic -
they grow towards light
PHOTOTROPISM of shoots
Shoot tips growing towards light -
positive phototropism (positively phototropic - shoots grow towards
the light)
Light is needed for photosynthesis,
so plant need to know in which direction to point their shoots to ensure
its food supply.
When light shines on a shoot,
more auxin concentrates on the side that is in the shade (less light
intensity side) giving an
unequal distribution of auxin.
This stimulates growth to elongate the cells
more on the shaded side so the shoot
bend upwards towards the light.
In bending towards the light the
shoot can absorb more light for photosynthesis and hence plant growth.
Shoots that grown in poor light (very
low light levels) grow tall and very thin (spindly) because the auxin in
the tip makes the cells grow elongated quickly, but on all sides. A
taller shoot has a better chance of finding light.
A simple experiment to demonstrate
phototropism
You can do a simple experiment with
cress seeds on moist filter paper in a petri dish.
You cover the petri
dish with a box with black inside walls but with a hole cut in one side.
You
then leave the box where it is exposed to bright light e.g. near a
laboratory window with the hole pointing outwards.
leave it for a week
and then carefully remove the box and you will find the cress shoots
were growing towards the hole in the side of box - that is growing
towards the light.
Phototropism and a fallen tree!
The tree had blown over in a
gale, but sufficient roots survived, so that the once horizontal
branches, are now growing vertically 'tree-like' towards the light
in what is quite a densely wooded area.
Gravitropism - shoots are negatively gravitropic - they grow up against
the down force of gravity
GRAVITROPISM of shoots
Shoots growing up against
gravity - negative gravitropism/geotropism (negatively
gravitropic/geotropic)
Plants must sense gravity in order
that shoots grow upwards in the right direction to access the maximum
light intensity (and roots grow downwards for water and mineral ions).
If a shoot starts to grow
sideways - at an angle or horizontally, gravity causes more auxin to concentrate on the lower
side producing an unequal distribution of the auxin hormone.
Therefore the lower side cells are stimulated to grow faster causing
the shoot to grow and bend upwards.
By growing upwards the shoots can
better access the light for photosynthesis.
Shoots will still grow upwards eve in
the dark - absence of light makes to difference in principle to negative
gravitropism.
Gravitropism - roots are positively gravitropic - they
grow down in the
direction of the force of gravity
GRAVITROPISM of
roots
Roots growing down with gravity
- positive gravitropism/geotropism (positively gravitropic/geotropic)
Plants must sense gravity in order
that roots grow downwards in the right direction to absorb the maximum
amounts of water and mineral ions (and shoots grow downwards for light
powering photosynthesis).
If a root is tending to grow
sideways-horizontally, then, due to gravity it tends to have more auxin on its lower side
- unequal distribution of auxin.
BUT, excess auxin in the root tip can inhibit growth and so the
upper cells tend to elongate
faster the lower side cells, causing the root to bend round downwards and
become more firmly embedded in the soil.
By growing downwards the roots can
better access the soil for minerals and water.
Shoots will still grow upwards eve in
the dark - absence of light makes to difference in principle to positive
gravitropism.
A simple experiment to demonstrate
gravitropism
You can take maize seeds, plant
them, and allow them to germinate.
You then secure them in separate
containers e.g. petri dishes, and set them up so the growing roots
are horizontal.
You leave the roots to grow for
several hours and note the length and shape of them.
It's probably best to take time
lapse photographs e.g. every 30 mins, to monitor the growth
progress.
You should find the ends of the
roots will start to bend downwards and the tip eventually points in
a vertical, but downward direction.
Phototropism
- roots are negatively phototropic -
grow away from light
PHOTOTROPISM of
roots
Roots grow away from light if exposed to
it on or near the surface of soil, so roots are negatively phototropic.
If roots are exposed to light on or
near the soil surface more auxin concentrates on the more shaded
underside of the root.
In the root tip, this high concentration
of auxin inhibits growth on the underside i.e. inhibits cell elongation.
This
allows the greater elongation to occur on the upper side that is more
exposed to the light.
Consequently, the upper greater cell
elongation makes the root bend and grow downwards deeper into the soil.
All the roots deep in the ground tend
to grow downwards due to positive gravitropism.
However, they will also grow towards
a more concentrated area of water due to hydrotropism (described below).
By growing downwards the roots can
better access the soil for minerals and water.
Hydrotropism - roots are positively hydrotropic -
grow towards moisture - the best source of water
HYDROTROPISM
Roots growing towards moisture -
positive hydrotropism (positively hydrotropic)
If a root is exposed to an
uneven distribution of moisture i.e. one side of the root is more moist than
the other, more auxin concentrates on the side with the most moisture -
unequal distribution of auxin.
Consequently the increased auxin level inhibits growth on the moist side and
stimulates a greater growth rate on the least moist side to make the root bend
towards the moisture.
By growing towards moisture the roots
can better access the soil for water (and minerals too).
Summary of tropism advantages
Phototropism, gravitropism and
hydrotropism all increase the chance of a plant's survival e.g.
shoots growing towards light
increase the rate of photosynthesis - more light can be absorbed by
chlorophyll,
the root growing downwards can
find minerals and water in the soil AND be more firmly fixed in the
soil too.
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SUB-INDEXES
(c) The commercial use of plant hormones
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 derivatives and gibberellins.
-
Some plant growth controlling hormones can
be used as selective weed killers to disrupt the growth of weeds but leave
the crops unaffected. These
-
In high concentrations, auxins
disrupt cell metabolism and kill plants.
-
Therefore you can use synthetic
auxins as weedkillers or herbicides.
-
Desired crops of grasses and
cereals are narrow leafed plants but many weeds have broad leaves.
-
Selective weed killers have been
developed from auxins which only affect broad-leaved plants
compared to narrow leaved plants.
-
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
desired grass
and cereal crops with narrow leaves.
-
This enables farmers to kill
broad leafed weeds without harming fields of crops of barley or
wheat.
-
Gardeners can use these selective
weedkillers to remove dandelions and other plants regarded as weeds
- I don't mind the dandelions myself!
-
2,4-D (2,4-Dichlorophenoxyacetic acid) is a
synthetic auxin, which a type of plant growth regulators.
-
2,4-D is absorbed through the leaves and is
translocated to the meristems of the plant.
-
This causes uncontrolled, unsustainable growth
resulting in stem curl-over, leaf withering, and eventual plant
death.
-
A
plant cutting is a part cut
off from a plant e.g. the end of a branch with a few leaves on.
-
Some plant cuttings won't always
readily grow when planted in soil or compost without any extra help
-
By adding a rooting powder to the
compost containing a plant growth hormone like auxin, the growth of roots and
subsequent shoots are greatly encouraged so new good quality plants grow more rapidly.
-
It then enables a flower grower
or market gardener to rapidly produce lots of clones (exact copies) of a particular plant -
ideally, of the best quality plants.
-
Use of issue culture
-
You can use tissue culture to grow
lots of clones of a plant from just a few of its cells.
-
In order for the technique to work,
auxin hormones are added to the tissue growth medium, as well as
the nutrients any plant needs to grow.
-
The hormone auxin stimulates cell
division to promote growth and form roots and shoots.
-
Plant hormones like
gibberellins can be used to
control the ripening of fruit or produce seedless fruit.
-
There are several techniques
for controlling flower and fruit formation.
-
The ripening of fruits can be controlled
while the fruit are still on the tree/bush or during transport to the
warehouses/shops.
-
You can therefore pick
fruit before it is ripe and still quite firm - which means
the firmer unripe fruit is less easily damaged
in transport.
-
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
supermarket!
-
The hydrocarbon gas ethene
can act as a ripening hormone. If ethene is added to the
crop it will ripen on the way to supermarket and be just ready
for the shelves (more
on ethene in last section).
-
You can increase the size of
some fruits by spraying them with gibberellin 4 to 6 weeks
before they are due to be harvested - you can reduce flower
formation, fruit grows when they are pollinated - so instead of
having lots of small fruits, you can have a smaller number of
larger fruits of superior quality.
-
Producing seedless fruit
-
Most fruit plants with seeds in
the core, require pollination by insects, otherwise the fruits and seeds
will not grow.
-
If the flowers don't get
pollinated, the fruit and seeds can't grow.
-
By applying growth hormones
like gibberellins to
the unpollinated flowers of some fruit plants, the fruits grow BUT not the
seeds!
-
This is how you grow seedless
citrus fruits.- 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.
-
Sometimes the plant hormones are applied after pollination, but still
prevent the seeds developing.
-
Some seedless varieties like
grapes do not grow as large as seeded varieties, but application of
gibberellins will cause them to grow to their normal larger size.
-
Other uses of gibberellins
-
Gibberellin plant
growth hormones are used to stimulate seed germination, stem growth (taller) and
flowering.
-
Some seeds will not germinate
unless they have experienced certain conditions such as a dormant period or
lack of stimulation due to dryness or a period of coldness due to
low ambient temperatures.
-
Its good if you can control this
dormancy and gibberellins can stimulate germination..
-
Gibberellins can break this
period of dormancy, allowing the seeds to germinate.
-
If you treat these seeds with
gibberellins you can make them germinate at any time in the year
when they would not normally germinate. This is a technique used by
commercial plant growers.
-
This technique has the added
commercial advantage of making all the seed batches germinate and the
same time.
-
Germination is no longer
dependent on a 'dry' or 'cold' period.
-
You can produce a variety of
flower all the year round.
-
Gibberellins are also used to
reduce flower formation to improve fruit quality.
-
Fruit grow on
pollinated flowers and some fruit trees produce too many
flowers.
-
This causes too many fruits to form which the tree
struggles to support nutritionally and they tend to grow too
small.
-
Using hormone control you can reduce the number of
flowers and enable larger better quality fruit to grow.
-
Gibberellins also trigger a
plant response known as 'bolting' in response to a cold
spell or lack of water - both of the latter slow-inhibit plant
growth.
-
Commercial plant growers use
gibberellins to make plants flower earlier than they would
normally do, OR, under conditions where they wouldn't usually
flower - called induced flowering.
-
Some plants need certain
specific conditions to flower e.g. longer summer days or lower
temperatures in spring or autumn.
-
You can treat these plants
with gibberellins to make them flower without changing their
environmental conditions.
-
Also, if a variety of a dwarf plant is
treated with gibberellin, it can be made to grow much bigger flowers or a
taller variety!
-
It is possible to use
gibberellins to promote the flowering of some apple varieties in
a poor year due to poor weather.
-
Gibberellins are used in
the brewing industry - production of alcoholic drinks like
beer
-
Barley supplies the
necessary sugars and amino acids that yeast needs for
fermentation - converting sugars to 'alcohol' (ethanol) and
carbon dioxide.
-
The barley must first
germinate to do this and spraying it with water containing
gibberellin speeds up the germination.
-
Gibberellins are
naturally produced by seeds like barley, but enough for
commercial speed!
-
Complex growth control
TOP OF PAGE for
SUB-INDEXES
(d) Other
stimulants
used in controlling growing plants
Ethene
- promotes the chemistry of ripening
You normally come across ethene
when studying
alkene hydrocarbons and the
oil industry.
The hydrocarbon gas ethene is
produced by aging parts of plants e.g. leaves.
Ethene stimulates expansion of the cells
that connect the leaf to the stem or branches.
This expansion breaks
the cell walls and causes the leaves to fall off the plant.
Therefore, commercial growers
can, if so required, use ethene to remove leaves from a crop
e.g. in farming cotton, treat with ethene before its harvested.
You treat fruit bushes in the
same way with ethene to make the collection of fruits or seeds
easier i.e. without the waste leaves.
However, because ethene is a gas,
its difficult to apply to fruit crops to ripen them.
However, compounds have been
developed that can release ethene and a product called Ethrel
can be used to ripen fruit in the field.
Ethene
can act as a ripening hormone
Ethene affects the growth of
plants in several ways, including controlling cell division.
Ethene stimulates the
enzymes that cause fruit to ripen.
Bananas will ripen, even
when enclosed in a plastic bag.
They, like many other
fruits, release ethene and increase the rate of respiration
to ripen.
If you put a banana in a
bag of another fruit e.g. tomatoes, they will ripen more
quickly as the banana releases its ethene!
Some fruit only ripen on the
tree, but many other fruits only ripen after they are harvested.
These include apples,
bananas and tomatoes.
Therefore you can use ethene
to control the time when you want the fruit to ripen -
like speed things up!
You can do this when the
fruits are still on the plant OR during transportation to the depot, store or supermarket
- so some fruits can be picked unripened and transported to
ripen later under controlled conditions.
This offers a way of
picking unripe fruit which will be firmer and less easily
damaged in transportation.
You then add the ethene
to ripen the fruit on the way to the consumer so it arrives
in perfect 'ripened' condition at the shop or supermarket.
So it seems somewhat
unnatural, but very useful, to add ethene to an unripe crop
and ripen on the way to the supermarket and be just ready
for the shelves and you!
The fruit can be stored
in special rooms where ethene concentration, humidity and
temperature can be carefully monitored and controlled.
You can also, if required,
delay the ripening while the fruit is being stored before
distribution to shops and supermarkets.
You do this by chilling
or spraying with a
chemical inhibitor that block's ethene production during
transportation and storage - so you can work the effect of
ethene both ways!
Health
and safety note:
Ethene is a very flammable
gas and mixed with air can explode, so I presume great care is
taken in using it and with low levels in the atmosphere around
fruit I presume!
AND see
ALKENES - unsaturated hydrocarbons
for the 'usual' GCSE chemistry of ethene!
Practical investigation you may have
experienced
-
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
examination questions):
For hormones and human biology see
Hormone system - Introduction
to the endocrine system
and
Hormone Systems
- menstrual cycle
Summary of learning
objectives, keywords and phrases about plant hormones
Know the function of meristems in plants.
Be able to describe how hormones control plant functions,
particularly growth.
Know what controls the ripening of plants?
Know that auxins are plant hormones.
Know the use of ethene as a stimulant and
gibberellins which can have a positive or negative stimulating effect on
plant growth.
Know what is meant by phototropism (phototropic effect),
gravitropism (gravitropic effect) and geotropism (geotropic effect) in
plant growth as extra cells are formed.
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General PLANT BIOLOGY revision notes
Photosynthesis,
importance
explained, limiting factors affecting rate, leaf adaptations
Plant cells, transport, gas exchange in plants,
transpiration, absorption of nutrients, leaf & root structure
See also
Diffusion, osmosis, active transport, exchange of
substances - examples fully explained
Respiration - aerobic and anaerobic in plants (and
animals)
Plant diseases and defences against pathogens and pests
See also
Adaptations in plants - emphasis on extreme
environments
and a section on
Stem cells and uses - meristems in plants
