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The Importance of Water

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BISC 367 BISC 367 - Plant Physiology Lab Spring 2009 Plant Biology Fall 2006 Notices: O2 electrode data IRGA data Reading material (Taiz & Zeiger): – PowerPoint PPT presentation

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Title: The Importance of Water


1
BISC 367 - Plant Physiology Lab Spring 2009
Plant Biology Fall 2006
  • Notices
  • O2 electrode data
  • IRGA data
  • Reading material (Taiz Zeiger)
  • Chapter 3, Water and Plant Cells
  • Chapter 4, Water Balance of Plants

2
The Importance of Water
  • Physiological aspects

3
Movement of water in plants
  • Molecular diffusion
  • Water moves from an area of high free energy to
    area of low free energy
  • i.e. down a conc. gradient
  • Described by FICKS LAW Js -Ds dcs/dx
  • Js flux density for s (mol m-2 s-1)
  • Ds diffusion coefficient
  • dcs/dx difference in water conc. over distance
    x

4
Movement of water in plants
  • Bulk flow
  • Movement of water in response to a pressure
    gradient
  • Analogous to water flowing in a pipe
  • Affected by
  • Radius of pipe (r)
  • Viscosity of liquid (h)
  • Pressure gradient dyp/dx
  • Described by POISEUILLES equation
  • vol. flow rate (m3 s-1) (pr4/8h)(dyp/dx)

5
Movement of water into a plant cell occurs by
osmosis
  • 2 mechanisms
  • Diffusion across the membrane
  • Bulk flow across aquaporins (water filled pores)

6
Movement of water into a plant cell occurs by
osmosis
  • Water uptake is driven by a free energy gradient
    composed of
  • Concentration gradient
  • Pressure gradient
  • Free energy gradient for water movement is
    referred to as a Water Potential Gradient

7
Water Potential
  • Water potential (Yw) is equivalent to the free
    energy of water influenced by
  • Concentration (or activity)
  • Pressure
  • Gravity
  • Yw is the free NRG of water per unit volume (J
    m-3)
  • Divide chem. pot. of water (J mol-1) by the
    partial molal vol. (m3 mol-1)
  • Units equivalent to pressure (Pa)

8
Water Potential
  • Yw (Mpa) is a relative quantity and defined as
  • Chemical potential of water (in pressure units)
    compared to the chemical potential of pure water
    (at atm. pressure and temp.), which is set to zero

9
Water Potential
  • Yw Ys Yp Yg
  • Ys Solute component or osmotic potential
  • Result of dissolved solutes that dilute water
    (entropy effect)
  • Estimated using vant Hoffs eqtn (see p.44)
  • Yp Pressure component or pressure potential
  • Yp inside a cell is positive turgor pressure
  • Yp in the apoplast is negative
  • Note Yp of pure water is zero, therefore not
    a measure of absolute pressure

10
Water Potential

Yg Gravity component Ignored unless
considering vertical water movement gt 5
m Dependent on Height of water above ref.
state (h) density (rw) acceleration due to
gravity (g) Yg rwgh rwg
0.01MPa m-1
11
Plant Water Relations
  • Cell (protoplast) water relations

Cell wall (apoplast) water relations
Yw 0
Yw 0
Ys(a)
Ys(p)
Yp(a)
Yw(p)
Yw(a)
Yp(p)
Whole plant water relations
p protoplast a apoplast
Yw 0
Ys(a)
Yp(a)
Ys(p)
Yw(p)
Yw(a)
Yp(p)
12
?p is sensitive to small changes in cell volume
  • Relates to rigid cell wall, illustrated by Hofler
    diagram
  • Plot of Yw its components against relative cell
    vol.
  • Initial drop in cell vol (5) is accompanied by
    a sharp drop in Yp and Yw
  • As cell vol falls lt90, decreased Yw is accounted
    for by a lowered Ys as solute increases

13
?p is sensitive to small changes in cell volume
  • Slope of Yp curve yields the volumetric elastic
    modulus (e)
  • e is a function of the rigidity of the cell
    wall
  • High value indicates a rigid wall for which a
    small vol. change translates into a large drop in
    Yp
  • e decreases as Yp falls b/c walls are rigid
    only when Yp is high

14
Typical values for Yw
  • Yw -0.2 to -0.6 MPa
  • Plants are never fully hydrated due to
    transpiration
  • Ys -0.5 to -1.5 MPa
  • Plants living in saline or arid environments can
    have lower values
  • Yp 0.1 to 1.0 MPa
  • Positive values needed to drive growth and
    provide mechanical rigidity

15
Measuring Yw
Scholanders pressure bomb
  • A leaf or shoot is excised and placed in the
    chamber
  • Cutting the leaf breaks the tension in the xylem
    causing water to retreat into the surrounding
    cells
  • Pressurizing the leaf chamber returns water to
    the cut surface of the petiole
  • The amount of pressure to return water to the
    cut surface equals the tension (Yp) present in
    the xylem (but is opposite in sign) before
    excision
  • Values obtained approximate the tension in the
    xylem and are used as a measure of Yw
  • Strictly speaking to know the actual Yw some
    xylem sap should be collected to measure Ys

From Plant Physiology on-line (http//4e.plantphy
s.net/)
16
Measuring Yw
Relative water content
  • Assesses the water content of plant tissues as a
    fraction of the fully turgid water content
  • relevant when considering metabolic /
    physiological aspects of water deficit stress
  • Considered to be a better indicator of water
    status and physiological activity
  • Captures effects of osmotic adjustment
  • Osmotic adjustment lowers the Yw at which a
    given RWC is reached
  • Simple technique
  • Leaf disks are excised, weighed (W) then
    allowed to reach full turgidity and re-weighed
    (TW). Leaf disks are dried to obtain their dry
    weight (DW).
  • RWC () (W DW) / (TW DW) X 100
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