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 hormonesHORMONE 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)

Recent work indicates that abscisic acid does not have this role.

ABA inhibits growth and also influences stomatal closure.

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.

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.