Homeostasis in Plants

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Homeostasis in Plants
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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

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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.

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Regulation of Extracellular Fluid
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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.

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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.

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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

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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.

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Why Regulate Stomata
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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.

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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.

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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

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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

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Detecting Stimuli

• Plants dont monitor their internal environment
  as animals do because there is no distinct
  difference between their extracellular fluids and
  the external environment.
• Plants dont 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

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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.

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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.

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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

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Summary of the properties ofplant hormones
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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

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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.

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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.

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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.

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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.

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Auxins cause bending of plants
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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

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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.

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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!!!

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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

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Tissue CultureGrowing 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.

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Plant hormones are used to promote growth when
new plants are cloned from unspecialised cells.