Homeostasis in
Plants
Plant Regulation


Regulation and coordination systems in plants are
much simpler than in animals
Homeostatic regulation of plants seeks to:



Maintain an adequate uptake of water and nutrients form
soil into leaves
Control stomatal opening so that water loss is minimised
and carbon dioxide is maximised
When plants respond to environmental conditions
such as high temperature or salinity, they are
balancing several conflicting demands
Regulation of Extracellular Fluid



The composition of extracellular fluids is not
precisely regulated in plants.
Plants are fairly tolerant of changes in the solute
concentration of the extracellular fluid providing the
solute concentration is hypotonic to the solute
concentration inside their cells.
If the solute concentration of the extracellular fluid is
hypertonic to the solute concentration of cytoplasm,
water diffuses out of the cytoplasm, resulting in
plasmolysis (shrinkage of the cytoplasm) and,
potentially cell death.
Regulation of Extracellular Fluid
Gaseous Exchange

In vascular plants the rate of movement of
water, carbon dioxide and oxygen between
atmosphere and internal spaces is regulated by
the degree of opening of stomata.
Stomata

Stomata are generally abundant on the surfaces of leaves, more
commonly on the underside.

Stomatal pores in the epidermis are bounded by two highly
specialised guard cells.
Guard Cells

Guard cells have three
structural features which
explain their function:



They are joined at their
ends in pairs
Their cell walls are thicker
on the side nearest to the
stomatal pore
Bands of inelastic cellulose
fibres run around each cell
Regulating Stomata


Stomal movement is the result of changes in
the turgor of the guard cells.
If water flows into the guard cells by osmosis,
their turgor increases and they expand. The
relatively inelastic inner wall makes them bend
and draw away from each other. This opens
the pore.
Why Regulate Stomata
Stomatal Opening
1.Potassium ions move into the vacuoles.
2.Water moves into the vacuoles, following
potassium ions.
3.The guard cells expand.
4.The stoma opens.
Stomatal Closing
1.Potassium ions move out of the vacuole and
out of the cells.
2.Water moves out of the vacuoles, following
potassium ions.
3.The guard cells shrink in size.
4.The stoma closes.
Communication in Plants


Communication between cells in different
parts of a plant is required to coordinate

the direction and timing of growth

water balance

other plant responses
Plants have no nervous system so internal
coordination is controlled by hormones
Hormonal Responses

Responses in plants are simple – no equivalent to endocrine
system of animals.

Hormone-producing cells in plants are not organised into
specialized tissues such as glands.

Hormones generally produced by the cells receiving the
appropriate environmental stimulus.

Responses are slower than in animals

Hormones are distributed throughout the plant in a variety of
ways:



Cell to cell
Through transport pathways (usually phloem)
Through air
Detecting Stimuli



Plants don’t monitor their internal environment as
animals do because there is no distinct difference
between their extracellular fluids and the external
environment.
Plants don’t have specialised receptors like those in
animals
Stimuli causes a sensitive cell to produce a particular
hormone, which then travels relatively slowly, usually
through the phloem, to reach responsive tissues
Detecting Stimuli
Stimuli to which plants respond include:
Physical factors:
 Direction and wavelength of light, day/night length
(photoperiod), gravity, temperature, touch
Chemical factors:
 Water, carbon dioxide and specific chemicals (e.g.
ethylene gas – ripens fruit)

Directionality is often an important aspect in plant
sensing and responding.
Plant responses



Plants respond to the physical parameters of their environment in different
ways:
Phototropism – growth in response to light
Geotropism – growth in response to gravity.







Negative geotropism – shoot grows up
Positive geothropism – roots grow down
Thigomotropism – tendency for climbing plants to wrap themselves around
a support
Heliotropism – tendency for some plants to follow the sun during the
course of the day
Photoperiodism – respond to changing day-length – this is the basis for
seasonal changes in plants
Vernalization – respond to periods of cold
When plants grow towards a stimulus it is referred to as a positive tropism,
and when plants grow away from a stimulus it is referred to as a negative
tropism.
What else do plant hormones
control?



Apical dominance – the inhibition of lateral
branches
Ripening of fruit – conversion of starches to
sugars
Abscission – shedding of leaves and fruit
Summary of the properties of
plant hormones
HORMONE
WHERE
PRODUCED
EFFECTIVE
SITE
ACTION
VISIBLE
EFFECT
Auxins
Shoot tip
(meristem)
Growing region
of shoot
Cells elongated
under turgor
pressure
Tip bends
towards light
Gibberellin
Fruits, seeds,
growing buds,
elongating stems
Whole plant
Growth of cells
Growth of plant,
germination of
seeds, flowering,
fruit enlargement
Cytokinins
Roots and
developing fruits
Branch and leaf
buds
Antagonises
auxins on leaf
buds, promotes
cell division and
differentiation
Growth of lateral
branches
Abscisic acid
Chloroplasts
Gene expression
in nuclei
Growth
inhibition
Seed dormancy,
vernalisation,
drought tolerance
Ethylene
Ripening fruits
and other parts of
plant
Cellular
metabolism
Fruit ripening,
leaf drop
Increased sugar
in fruit, leaf and
fruit drop
Auxins – The Good

Some auxins are used to stimulate root
development in stem cuttings and induce the
formation of lateral roots.

Spraying auxins can:




prevent natural pollination,
produce seedless vegetables such as tomatoes and
cucumbers
prevent fruit fall by delaying abscission
induce flowering in the pineapple family
Auxins – The Bad

Are both stimulators and inhibitors of growth.

Synthetic auxin-like chemicals 2,4-D and 2,4,5-T were used as
herbicides. (both contain trace levels of dioxin as a
contaminant)

The combination of these two chemicals was refered to as
Agent Orange during the Vietnam war and was used as a
defoliant as it causes such rapid, disproportionate growth that
leaves of treated plants shrivelled and died.

At the correct concentrations these chemicals are selective for
broad-leaved weeds and do not kill grasses.
IAA – An example of an auxin






Auxins are produced by the growing tips of plants.
Their site of production was first identified in germinating grass seeds. It
was found that the first leaves (coleoptiles) of these germinating seeds did
not grow if their tips were removed.
IAA is responsible for apical dominance. Apical dominance exists when
lateral buds on the stem close to the apex of a plant do not develop while
the growing tip at the apex of a plant grows and develops.
Development of the lateral buds is inhibited as a result of the action of IAA
that is produced by the terminal bud at the apex of the plant. The IAA
moves down the stem through the phloem and exerts an inhibitory effect.
When the bud at the apex is nipped off, the source of IAA is removed and
lateral buds lower down on the stem begin to develop.
Auxins are involved in the bending of plant shoots and roots in response to
light and gravity.
IAA – An example of an auxin

Auxins are water soluble chemicals produced
in the tip of the plant which promote
elongation of the cells below.
Auxins cause bending of plants

Auxin is evenly distributed throughout the tip and the
coleoptile grows straight up.

If light is concentrated to one side of a coleoptile then
auxin moves away from the light source to the darker
side of the tip and becomes more concentrated in the
cells in that region.

The increased concentration of auxin in these cells
means they grow more quickly than cells nearer the
light.

The uneven growth of cells results in bending of the
coleoptile.
Auxins cause bending of plants
Gibberellins

Can speed germination in spring by overcoming seed
dormancy and the requirement for light.

Can cause formation of giant flowers

Treating seedless grapes with gibberellin produces
larger juicer fruit.

Synthetic gibberellins may be used as herbicides by
producing abnormal growth of stems without
adequate root growth, or by stopping cell division.

Can be used to prevent root growth in potatoes,
thereby preserving the crop
Abscisic Acid (ABA)







Fruit that is about to fall from a plant, and
dormant buds, both contain high levels of
abscisic acid.
The separation of a plant part such as a leaf or
fruit from the parent plant is called abscission.
Before a leaf falls, a special zone called the
abscission zone forms at the base of the leaf
petiole or stalk.
This zone is a special layer of cells which
forms a barrier between the leaf and the plant
which marks where the leaf will break away
from the plant. The cells also form a protective
layer on the plant and inhibit entry of
parasites.
The presence of auxins in young leaves
inhibits abscission. As a leaf ages on a
deciduous plant, a number of changes occur,
including an increase in production of abscisic
acid. It was once thought that abscisic acid
was responsible for the formation of the
abscission layer, hence the similarity in names.
Recent work indicates that abscisic acid does not have this role.
ABA inhibits growth and also influences stomatal closure.
How does ABA work?

ABA the potential to help crop plants cope with
drought however it is expensive to produce and is
rapidly broken down by plants.

Xylem water, which contains ABA produced in
roots is drawn through stomata by transpiration.

As transpiration increases, levels of ABA increase,
causing the stomata to partially close.

This reduces transpiration, which causes ABA
levels to drop and stomata to open again.
Negative Feedback System!!!
Intervening in plant growth

Synthetic hormones are used by horticulturists
and home gardeners to:






Encourage root growth on cuttings
Discourage potatoes from sprouting
Make flowers set fruit
Delay fruit drop
Speed up ripening
Sometimes as herbicides
Tissue Culture:
Growing “cloned” plants






When large numbers of plants with a particular genetic make-up or of
particular economic importance are required, growth from cuttings or even
from a small group of cells is carried out in the laboratory using special
techniques.
The technique of tissue culture (or cloning) may be used to obtain large
numbers of plants in a relatively short time. Cloned plants are genetically
identical to the plant from which the original cells were taken.
When small groups of unspecialised cells are used, they are sterilised and
grown on agar in a test tube or other container. Each group is called a
callus.
The hormone cytokinin is added. High levels of cytokinin combined with
relatively low levels of auxin results in growth of shoots.
The shoots on each callus are then treated with auxin, leading to a relatively
high level of auxin and a relatively low level of cytokinin compared with
before. This results in the formation of roots.
Each callus, which started as a small group of cells, gives rise to a complete
new plant. By this technique many genetically identical plants can be
quickly produced from the one parent plant.
Plant hormones are used to promote growth when
new plants are cloned from unspecialised cells.