BIO2202 Plant Physiology 1 Water Relations Lecture 3

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BIO2202 Plant Physiology 1 Water Relations -
Lecture 3

• Water Potential

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Osmotic Potential (?s)

• A measure of the effect of solutes on the free
  energy of water
• bears a negative sign, since solutes lower the
  free energy of their solvent
• ?s of the cell largely determined by vacuolar
  contents inorganic ions, sugars, amino acids,
  organic acids, proteins, secondary products etc.
• Generally between -0.5 to -3.0 MPa

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Osmotic Potential (?s)

• Defined by Vant Hoff equation
• ?s RTcs
• R universal gas constant
• T absolute temperature (oK)
• cs substrate concentration (molality)

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Pressure Potential (?P)

• Measures the effect of applied pressure.
• Usually bears a positive sign due to wall
  pressure on contents in turgid cells.
• In flaccid cells, value falls to zero.
• In xylem vessels, where contents are under
  tension, value is negative.

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Matric Potential (?M)

• A result of the interaction of water molecules
  with dry, hydrophilic or porous surfaces.
• A function of the distance between the water
  molecule and the surface.
• Matric potential is a measure of the ease with
  which the least tightly held water molecule can
  be removed.
• Important in two situations seed germination and
  water availability in soils.

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Gravitational Potential (?G)

• Only significant in tall trees when considering
  water potential differences over 5-10 m of
  vertical distance.
• Bears a positive sign.

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Total Water Potential (?W)

• ?W ?S ?P ?M ?G
• In hydrated vegetative tissue such as leaves,
  only the solute potential and the pressure
  potential significantly contribute to water
  potential in the cells.
• Water leaves or enters cells passively down water
  potential gradients

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Changes in Cell Volume

• Small changes in plant cell volume cause large
  changes in turgor pressure in turgid tissue
• ? ??P
• ?V/V
• ? volumetric elastic modulus - see TZ Fig
  3.10
• ??P change in pressure
• ?V/V relative change in volume

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Movement of Water in the Xylem

• In the very tallest of trees (Sequoia sp., E.
  regnans) the vertical distance from root tip to
  shoot tip may be 120 metres. How can xylem
  vessels support a very thin continuous water
  column of this height?
• Roots do not contain a pumping mechanism that can
  push water even to shrub height
• capillary forces fail by orders of magnitude

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The Cohesion Mechanism

• Water molecules show strong mutual attraction due
  to H bonding (thin columns show high tensile
  strength).
• Water molecules adhere strongly to the wettable
  surfaces found in thin xylem vessels.
• Thin columns of water under tension behave as if
  all molecules are connected and move towards the
  source of negative pressure.

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Requirements of a Cohesion Mechanism

• An adequate driving force (or suck) to draw the
  water up.
• The pathway must be fully hydrated the water
  column must be continuous and fully adhere to the
  walls.
• The conducting vessel must be strong enough to
  avoid collapse under negative pressures.