Transcript The Importance of Water

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
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BISC 367
The Importance of Water
• Physiological aspects
Movement of water in plants
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• 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
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 POISEUILLE’S equation:
vol. flow rate (m3 s-1) = (pr4/8h)(dyp/dx)
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Movement of water into a plant cell
occurs by osmosis
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• 2 mechanisms:
– Diffusion across the
membrane
– Bulk flow across
aquaporins (water
filled pores)
Movement of water into a plant cell
occurs by osmosis
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• 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
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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)
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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
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Water Potential
Yw = Ys + Yp + Yg
Ys = Solute component or osmotic potential
Result of dissolved solutes that dilute water
(entropy effect)
Estimated using van’t Hoff’s 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
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Water Potential
Yg = Gravity component
Ignored unless considering vertical water movement
>5m
Dependent on:
Height of water above ref. state (h)
density (rw)
acceleration due to gravity (g)
Yg = rwgh
rwg = 0.01MPa m-1
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Plant Water Relations
Cell (protoplast) water relations
Cell wall (apoplast) water relations
Yw = 0
Ys(p)
Yp(p)
Yw = 0
Ys(a)
Yp(a)
Yw(p)
Yw(a)
Whole plant water relations
p = protoplast
a = apoplast
Ys(a)
Yw = 0
Yp(a)
Ys(p)
Yp(p)
Yw(p)
Yw(a)
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Yp 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 <90%, decreased Yw
is accounted for by a lowered Ys as
[solute] increases
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Yp 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
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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
Measuring Yw
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Scholander’s 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
From Plant Physiology on-line (http://4e.plantphys.net/)
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
Measuring Yw
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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).
disks are dried to obtain their dry weight (DW) .
RWC (%) = [(W – DW) / (TW – DW)] X 100
Leaf