Plant Responses to Abiotic Environment

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Plant Responses to Abiotic Environment
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Biological orientation of plants

• Tropisms when growth toward stimulus it is
  positive and vice versa.
• Taxes occur in algae which swim with flagella
  and swim toward light.
• Nastic responses turgor response to diffuse
  stimulus. Not fully understood.
• Tropisms are growth responses and are generally
  not reversible, nastys are reversible.

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Control of plant growth

• Achieved by hormones and chemicals.
• Auxin controls bending of plant due to light.
• Series of experiments done

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• Found that auxin produced in tip of shoot
• Is water soluble and diffuses through agar blocks
  but not through mica
• Causes elongation of cells thus promoting upward
  growth
• If light shone on side, cells on dark side
  elongate and grow faster so shoot bends.

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Low concentrations of auxins stimulate roots and
lateral buds, high conc. Inhibit. The reverse is
true for stems.

• Auxin migrates to dark side of stem
• Main auxin is IAA (indole acetic acid), it is
  turned into actively growing plant tissue.
• Artificial auxins can be used as weed killers.

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Apical Dominance

• Auxin at top of plant is produced in high
  concentrations so shoot grows fast but lateral
  buds stay dormant unless they are lower down the
  plant where the concentration is lower. This
  gives many plants and trees their triangular
  shape.
• Cytokinins from roots stimulate lateral buds
  making them grow more at the bottom of the plant.

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Geotropism in seeds

• Gravity causes an uneven movement of auxin to
  bottom of root and top of shoot to cause bend.
• Plants can tell up from down by using statoliths
  which move from side to side.

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Other tropisms

• Hydrotropism very strong in roots, stronger than
  geotropism as a root will grow to surface if that
  is where water is.
• Thigmotropisim tendrils of climbing plant bend
  round any object they touch, growth is slowed on
  the side that touches an object.
• Chemotropism roots often move towards or away
  from certain chemicals.

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Other effects of auxin

• Initiates root formation (used in powders to make
  roots form).
• Stops roots growing long
• Suppression of lateral buds
• Stimulate mitosis in stems
• Start flowering of some plants
• Stops premature abscission of leaves and fruit.

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Other plant hormones

• Gibberellins
• Produced in growing tip and cause rapid
  elongation, can promote germination.

• Cytokinins
• Work with auxin to promote cell division. High
  auxin low cytokinin roots, low auxin high
  cytokinin stems, equal amounts callus tissue.

• Ethene gas
• Ripens fruit

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• Abscisic acid (ABA)
• Tends to inhibit all other hormones. Induces
  leaf fall and winter dormancy in seeds.

• Etiolation
• Response to light, leaves grow small and yellow.

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Biological timing responses

• Daily rhythms (circadian)
• Some phytoplankton have three daily rhythms when
  agitated by waves it will glow but only at night,
  photosynthesis only occurs during the day, all
  cells divide during a one hour period just before
  dawn.
• Opening of flowers in day and closing at night.
• Nectar secreted only at certain times of day
• Flowers give out perfume at different times of
  day

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• Flowers which turn face to follow sun
• Leaves of many plants droop at night.

• Circamonthly
• No obvious ones in plants

• Circatidal
• Release of egg and sperm for reproduction.

• Circannual
• Flowering, dormancy, growth patterns, abscission
  of leaves

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Flowering of plants

• Many show photoperiodism the response to changes
  in day length such as flowering or dropping
  leaves. The most improtant factor in when a
  plant flowers is length of darkness not light.
  This means plants can be divided in 3 groups.
• Short Day plants require a short day and a long
  night, these plants flower in winter, early
  spring and autumn.

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• Long Day plants require a long day and short
  night so flower in summer.
• Day neutral plants relatively unaffected by
  daylight hours e.g. tomato.

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Phytochrome System

• The ability of plants to activate the photoperiod
  is controlled by the pigment phytochrome. This
  pigment detects light. It exisits in two forms
  coreresponding to the two wavelengths of light
  P665 and P725.

• When P665 absorbs red light it is quickly changed
  to P725.
• When P725 absorbs far red light it is quickly
  turned into P665.
• In the dark P725 is slowly converted into P665

• The significance is that P725 is biologically
  active while P665 is inactive. It seems that
  P665 is the way the plant stores the potentially
  active form of the phytochrome.

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Diag.
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Other plant responses to abiotic environment

• Vernalisation
• Seeds that require a period of cold before
  germinating

• Dormancy
• Seeds that are metabolically inactive after
  forming, dries out to 5 water of body weight
  then will only germinate in the right conditions

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• Stratification
• A moist seed exposed to low temperatures for many
  days breaks the dormancy.

• Scarrification
• Some seeds need coat scratched or burnt/treated
  to germinate.

• Ephemerals
• Many desert plants contain chemicals that stop
  germination and until there is enough rain to get
  rid of the chemicals there will be no growth..

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• Abcission
• Leaf fall prevents water loss in the cold months
  and is triggered by auxin at certain temperatures.

• Autumn colour in leaf
• In autumn the chlorophyll breaks down exposing
  the other pigments

• Growth forms of plants
• To cope with seasonal changes there are annuals,
  biennials, perennials (which overwinter by having
  bulbs etc).