Transcript 002 Plant Water Relations

Plant Water
Relations
Prof. Dr. Muhammad
Ashraf
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What are Water Relations?
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A field of study in which one can observe plant
and environmental interactions with respect to
water
OR
Study of all mechanisms related to uptake of
water from soil by plants, its translocation from
root to shoot and evaporation through stomata
Movement of water and other substances from
soil to plant roots across membranes,
throughout the plant and between the plant
and its environment (Salisbury, 1992)
Water relations of a single cell
Nature of cellular water or distribution of water in cells
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Water is continuously present throughout the plant body
Some water is held in the micro-capillaries
Water exists in two systems
– Apoplast
– symplast
Cell wall water (matric
water) Apoplastic water
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5-40% of cell water occurs in the walls depending on the age, thickness and
composition of the walls
50% of cell wall volume is water
In thick leaves, walls are thick – low water is held in the walls by matric
forces including H-bonding to various constituents
The wall contains cellulose microfibrils, pectic substances, proteins and OH
and COOH groups which absorb water by H-bonding. Water is also held in
inter-micro-fibrillar spaces (matric water or imbibed water)
Water in the Cytoplasm
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In meristematic tissues, vacuole is small so
most water occurs in the cytoplasm.
However, in mature cells cytoplasm is as a
thin layer and consists of 5 to 10% of the
cell water
Vacuole
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50-80% or more of the cell water occurs in
the vacuoles. Cell sap consists of 2% solid
and 98% water. This water contains solutes
mainly sugars, salts and sometimes organic
acids. OP = -1.0 to -3.0 MPa.
In leaves of Eucalyptus 50% water in
vacuole
In wheat roots 80% or more in vacuoles
Cell Membrane
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Water is held by means of dipolar and H-bond.
The inner membrane spaces within proteins and lipids are
occupied by water molecules
Water is very dense so that they form semi-crystalline
structure.
Membrane surface is also covered by one molecular thick
layer of water
Water moves to the region of low water potential or low
energy
Water relations of plant
HISTORY of Plant Water Relations
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Tang and Wang (1941) first used the term
“water potential” to explain cell water
relations
Then it was used by Owen (1952) to explain
DPD (diffusion pressure deficit) which is
equivalent to water potential
DPD = OP -TP
Slatyer from Australia and Taylor from Utah
(1960) recognized this term
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Acceptance of the water potential concept was slow because of
the confusion regarding terminology, the lack of convenient
methods for measuring it, and the inadequate training of plant
physiologists in physical chemistry (Kramer 1995).
As a result, plant water status seldom was measured during the
second quarter of the 20th century.
Development of thermocouple psychrometers (Monteith and
Owen, 1958; Richards and Ogata, 1958; Spanner, 1951) and
pressure equilibration by Scholander and his colleagues
(1964,1965) made measurement of water potential relatively
easy, and they are the measurements used most often today to
characterize plant water status.
Chemical Potential
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Chemical potential (a thermodynamic term) is the amount of
energy per mole of a substance to do work
Chemical potential of water is water potential and it is a
measure of the free energy per unit volume available for
reaction or movement
a quantity that determines the transport of matter from one
phase to another: a component will flow from one phase to
another when the chemical potential of the component is
greater in the first phase than in the second.
Chemical potential depends upon concentration, pressure,
electric potential and gravity. e.g. molecules at high temp
move toward low temp regime.
Different Definitions of Water Potential
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Water potential is the chemical potential (Free energy) of
water in a system expressed in units of pressure and
compared to water potential of pure water i.e. 0
Free energy of water (water potential) in plants relates to
creating and breaking molecular bonds, moving ions
through the cellular organelles or moving water from soil
to root and leaf through different cells and xylem
Chemical Potential of water divided by partial molal
volume of water
It is the difference between matrically bound, pressurised
or osmotically held water and pure water
Chemical Potential and
Water Potential
Ψw = DPD = OP - TP
Ψw = μw - μºw
= RT ln e/ºe
Vw
Vw
μw = Chemical potential of water
μºw = Chemical potential of pure water
R = 0.00831 kg MPa mol-1 K-1 or 0.00831 kJ mol-1 K-1
T = Absolute temperature (K)
K = 273 + ºC
e = vapor pressure of water in a system
eº = vapor pressure of pure water
Vw =partial molal volume of water (e.g. volume of 1 mole of
water is 18 cm3 mol-1)
Factors affecting Chemical
potential
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Chemical activity (Solute conc.)
temperature contributes to chemical activity
(molecules with high temp will move toward low
temp regime)
electrical potential – only important for charged
substances
pressure – elastic cell walls allow plant cells to
develop significant hydrostatic pressure
gravitational pull – only applicable in tall trees
Units of Water Potential
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1 MPa = 10 bars
1 bar = 0.987 atm = 106 dynes cm-2 = 106 ergs cm-3
1 MPa = 1 kJ kg-1 = 1 J g-1
Water Potential-Example
w plant = s + p + m
- 0.8 MPa = - 0.9 + 0.3 - 0.2
w soil = s + m
- 0.6 MPa = - 0.2 + - 0.4
Water Potential
-3
-2
-1
0
1
2
3
Water Potential-Magnitude
w = 0 MPa
Pure Water
w = 0 to -1 MPa
Plant/Cell in
good condition
w = -1 to -2 MPa
Plant/Cell under
mild water stress
w < -2 MPa
Plant/Cell under
water stress
w ≈ -6 MPa
Desert soils
Water Potential-Flux
Water will flow from sites of high w (close
to zero) to sites of low w (more negative):
-0.3 MPa
Soil
-2 MPa
-1 MPa
Root
Stem
Leaf
-30 MPa
Air
-2 MPa
-1 MPa
0 MPa
Cell growth
Protein synthesis
Stomatal opening
Photosynthesis
Respiration
Pro/sugar accum.
Transport
Water Deficit-Effects
Photosynthesis
Respiration
Enlargement
75
50
25
-1.6
-1.2
-0.8
-0.4
Water Potential (MPa)
Water Deficit-Effects
Leaf
Stem
Root
0.9
0.6
0.3
-1.6
-1.2
-0.8
-0.4
Water Potential (MPa)
Westgate and Boyer (1985)
Components of Water Potential
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The components of water potential are
osmotic potential, turgor potential,
matric potential, and gravitational
potential
Ψw = Ψs + Ψp + Ψm+ Ψg
Values of Water Potential
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Water potential of pure water at standard
atmospheric
conditions
is
“0”
(the
maximum)
Water potential of a system is always
negative
More solute concentration, more negative
will be the water potential
It is zero when cell is fully turgid
Water potential becomes positive when
pressurized or compressed
Water Potential
Solution A
Unconfined system
Ψw = Ψs + Ψp
-10 = -10 + 0
-10 = -10
Ψw = -10
Solution A
Confined system
Ψw = Ψs + Ψp
-30 = -30 + 0 (Initial stage)
-10 = -30 + 20
-10 = -10
Ψw = -10
Osmotic Potential
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Osmotic Pressure is a pressure that a solution
develops to increase its chemical potential to that of
pure water or it is a hydrostatic pressure when
applied to a solution prevents the influx of water
Is based on concentration of solutes in water
Is potential developed by solutes in a system with
which influx of water occurs
Is always negative
Higher solute concentration, more negative will be
the value of osmotic potential
Is denoted by ψs
An isolated solution has no osmotic pressure but it
does have an osmotic potential.
Van’t Hoff Equation
Ψs = -miRT = -CiRT = -nRT/V
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i = ionization constant
C = concentration
m = molality
n = number of solutes
R = Gas constant
T = Absolute temperature
Effect of Temperature on Osmotic
Potentials of Same Solution
Ψs = -miRT
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Osmotic potential of 1 molal glucose solution at 30 oC
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Ψs = - (1.0 mol kg-1) 1.0 x (0.00831 kg MPa mol-1 K-1) x (273+30) K
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= -2.518 MPa at 30 oC
Osmotic potential of 1 molal glucose solution at 0 oC
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Ψs = - (1.0 mol kg-1) 1.0 x (0.00831 kg MPa mol-1 K-1) x (273+0) K
= -2.269 MPa at 0 oC
Turgor Pressure
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Turgor pressure is produced by the diffusion of
water into protoplasts enclosed in walls which resist
expansion
Turgor pressure is hydrostatic pressure of water
that is exerted on the liquid by the walls of a turgid
cell (pressure per unit area of liquid)
Is denoted by ψP
Is zero in open vessel
Is –ve in xylem of transpiring plant while it is
positive in guttating plants
Is zero when cell is flaccid
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Turgor pressure pushes the plasma
membrane against the cell wall of plant,
bacteria, and fungi cells as well as those of
protist cells which have cell walls. This pressure,
turgidity, is caused by the osmotic flow of
water from area of low solute concentration
outside of the cell into the cell's vacuole, which
has a higher solute concentration. Healthy plant
cells are turgid and plants rely on turgidity to
maintain rigidity.
Matric Potential
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Matric potential is due to the adhesive
characteristics of water when in contact
with surface or large macromolecules
Potential developed due to water held in
microcapillaries or bound on surfaces of cell
walls
Matric water increases as the cell water
decreases
Is negligible at high tissue hydration
If tissue hydration is low (60%), it should
be considered (Nobel et al., 1992)
Gravitational Potential
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It is the potential that is developed
due to gravitational pull
It is also negligible in crop plants while
in tall trees it influences the water
potential
Soil water potential
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ψT = ψ m + ψ s + ψ p + ψ z
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Ψm, matric potential resulting from the combined effects of capillarity
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and adsorptive forces within the soil matrix (Value –ve)
Ψs, Solute potential resulting from solutes present in water (Value –ve)
Ψp, pressure potential i.e. hydrostatic pressure exerted by unsupported
(free) water that tends to saturate the soil. Soil ψp is always positive
below a water table, or zero at or above the water table.
Ψz, gravitational potential is simply the vertical distance from a
reference level to the point of interest
If we consider no effect of gravity while considering soil water
potential at the same point so the overall equation will be
Ψw = ψm + ψs + ψp
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Soil water potential is measured by
Psychrometer
Soil matric potential by Tensiometer
Soil pressure potential by Piezometer
Soil solute potential by EC or
Osmometer