Transcript Water Potential

1
Botany: Part III
Plant Nutrition
Figure 36.2-1
H2O
Plant Nutrition and
Transport
Water and minerals in
the soil are absorbed
by the roots.
H2O
and
minerals
Transpiration, the loss
of water from leaves
(mostly through
stomata), creates a
force within leaves
that pulls xylem sap
upward.
Transpiration
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Getting Water Into The Xylem Of The Root
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Generation of Transpirational Pull
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In addition to apoplastic
and symplastic
movement, there are
newly discovered
channels called
aquaporins that allow
only water to move
across the membrane.
Water movement
through aquaporins is
quicker since no lipids
are involved.
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Movement of Minerals
Into The Root
Plants need minerals to
synthesize organic
compounds such as amino
acids, proteins and lipids.
Plants obtain these minerals
from the soil and are
transported by various
transport proteins.
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Macro- and Micro- Nutrients
Macronutrients are
required by plants in
relatively large amounts
and compose much of
the plant’s structure.
(C, N, O, P, S, H, K, Ca,
Mg, Si, etc. )
Micronutrients are
needed in very small
quantities. Typically
function as cofactors.
Mycorrhizae: A Mutualistic Relationship
Roots
Fungus
Figure 36.2-2
CO2
O2
H2O
• Gas exchange occurs
through the stomata.
• CO2 is required for
photosynthesis and O2
is released into the
atmosphere.
H2O
and
minerals
O2
CO2
• Roots exchange gases
with the air spaces in
the soil, taking in O2
and releasing CO2.
Figure 36.2-3
H2O
CO2
O2
Light
Sugar
• Sugars are produced
by photosynthesis in
the leaves.
• Phloem sap(green
arrows) can flow both
ways.
O2
H2O
and
minerals
CO2
• Xylem sap(blue
arrows) transport
water and minerals
upward from roots to
shoots.
Water Is In The Root, So Now What?
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 Root pressure is caused by active distribution of
mineral nutrient ions into the root xylem.
 Without transpiration to carry the ions up the stem,
they accumulate in the root xylem and lower the
water potential.
 At night in some plants,
root pressure causes
guttation or exudation
of drops of xylem sap
from the tips or edges
of leaves as pictured here.
Water Is In The Root, So Now What?
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 Water then diffuses from the soil
into the root xylem due to
osmosis.
 Root pressure is caused by this
accumulation of water in the
xylem pushing on the rigid cells.
 Root pressure provides a force,
which pushes water up the stem,
but it is not enough to account
for the movement of water to
leaves at the top of the tallest
trees.
Let’s Apply Some TACT To The Situation!
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A more likely scenario involves the Cohesion-Tension Theory
(also known as Tension-Adhesion-Cohesion-Transpiration or TACT Theory)
Tension: Water is a polar molecule.
 When two water molecules approach one
another they form an intermolecular
attraction called a hydrogen bond.
 This attractive force, along with other
intermolecular forces, is one of the
principal factors responsible for the
occurrence of surface tension in liquid
water.
 It also allows plants to draw water from the
root through the xylem to the leaf.
Let’s Apply Some TACT To The Situation!
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• Adhesion occurs
when water forms
hydrogen bonds
with xylem cell
walls.
• Cohesion occurs
when water
molecules hydrogen
bond with each
other.
Let’s Apply Some TACT To The Situation!
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• Transpiration: Water is
constantly lost by
transpiration in the leaf.
• When one water molecule is
lost another is pulled along by
the processes of cohesion and
adhesion.
• Transpiration pull, utilizing
capillary action and the
inherent surface tension of
water, is the primary
mechanism of water
movement in plants.
Generation of Transpiration Pull
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Ode To The Hydrogen Bond
Water Potential
 Water potential quantifies the tendency of free
(not bound to solutes) water to move from one
area to another due to osmosis, gravity,
mechanical pressure, or matrix effects such as
surface tension.
 Water potential has proved especially useful in
understanding water movement within plants,
animals, and soil.
 Water potential is typically expressed in
potential energy per unit volume and very often
is represented by the Greek letter psi,  .
(pronounced as “sigh” )
Water Potential
The addition of solutes to water lowers the
water's potential (makes it more negative),
just as the increase in pressure increases its
potential (makes it more positive).
Pure water is usually defined as having an
osmotic potential () of zero, and in this
case, solute potential can never be positive.
Free water moves from regions of higher
water potential to regions of lower water
potential if there is no barrier to its flow.
Water Potential
The word “potential” refers to water’s potential
energy which is water’s capacity to perform
work when it moves from a region of higher
water potential to a region of lower water
potential.
The water potential equation is  = S + P
where  is the water potential, S is the solute
potential (directly proportional to its molarity
and sometimes called the osmotic potential
and the S of pure water is zero) and P is the
pressure potential.
Water Potential
P is the physical pressure exerted on a
solution.
It can be either positive or negative relative to
the atmospheric pressure.
Water in a nonliving hollow xylem cells is under
a negative potential (tension) of less than
−2 MPa.
BUT the water in a living cell is usually under
positive pressure due to the osmotic uptake of
water.
Solutes have a negative
effect on  by binding
water molecules.

Pure water at equilibrium
H2 O
Positive pressure has a
positive effect on  by
pushing water.
Pure water at equilibrium
Pure water at equilibrium
H2 O
H2 O
Adding solutes to the
right arm makes  lower
there, resulting in net
movement of water to
the right arm:
Solutes and positive
pressure have opposing
effects on water
movement.
Applying positive
pressure to the right arm
makes  higher there,
resulting in net movement
of water to the left arm:
Negative pressure
(tension) has a negative
effect on  by pulling
water.
Pure water at equilibrium
H2 O
In this example, the effect
of adding solutes is
offset by positive
pressure, resulting in no
net movement of water:
Applying negative
pressure to the right arm
makes  lower there,
resulting in net movement
of water to the right arm:
Positive
pressure
Positive
pressure
Negative
pressure
Pure
water
Solutes
Membrane
H2 O
Solutes
H2 O
H2 O
H2 O
Solutes have a negative
effect on  by binding
water molecules.
Pure water at equilibrium
H2 O
Positive pressure has a
positive effect on  by
pushing water.

Pure water at equilibrium
Pure water at equilibrium
H2 O
H2 O
Adding solutes to the
right arm makes  lower
there, resulting in net
movement of water to
the right arm:
Solutes and positive
pressure have opposing
effects on water
movement.
Applying positive
pressure to the right arm
makes  higher there,
resulting in net movement
of water to the left arm:
Negative pressure
(tension) has a negative
effect on  by pulling
water.
Pure water at equilibrium
H2 O
In this example, the effect
of adding solutes is
offset by positive
pressure, resulting in no
net movement of water:
Applying negative
pressure to the right arm
makes  lower there,
resulting in net movement
of water to the right arm:
Positive
pressure
Positive
pressure
Negative
pressure
Pure
water
Solutes
Membrane
H2 O
Solutes
H2 O
H2 O
H2 O
Solutes have a negative
effect on  by binding
water molecules.
Pure water at equilibrium
H2 O
Positive pressure has a
positive effect on  by
pushing water.
Pure water at equilibrium

Pure water at equilibrium
H2 O
H2 O
Adding solutes to the
right arm makes  lower
there, resulting in net
movement of water to
the right arm:
Solutes and positive
pressure have opposing
effects on water
movement.
Applying positive
pressure to the right arm
makes  higher there,
resulting in net movement
of water to the left arm:
Negative pressure
(tension) has a negative
effect on  by pulling
water.
Pure water at equilibrium
H2 O
In this example, the effect
of adding solutes is
offset by positive
pressure, resulting in no
net movement of water:
Applying negative
pressure to the right arm
makes  lower there,
resulting in net movement
of water to the right arm:
Positive
pressure
Positive
pressure
Negative
pressure
Pure
water
Solutes
Membrane
H2 O
Solutes
H2 O
H2 O
H2 O
Solutes have a negative
effect on  by binding
water molecules.
Pure water at equilibrium
H2 O
Positive pressure has a
positive effect on  by
pushing water.
Pure water at equilibrium
Pure water at equilibrium
H2 O
H2 O
Adding solutes to the
right arm makes  lower
there, resulting in net
movement of water to
the right arm:
Solutes and positive
pressure have opposing
effects on water
movement.
Applying positive
pressure to the right arm
makes  higher there,
resulting in net movement
of water to the left arm:
Negative pressure
(tension) has a negative
effect on  by pulling
water.
Pure water at equilibrium
H2 O
In this example, the effect
of adding solutes is
offset by positive
pressure, resulting in no
net movement of water:
Applying negative
pressure to the right arm
makes  lower there,
resulting in net movement
of water to the right arm:
Positive
pressure
Positive
pressure
Negative
pressure
Pure
water
Solutes
Membrane
H2 O
Solutes
H2 O
H2 O

H2 O
Water Potential vs. Tonicity
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The green arrows indicate water moving OUT of
the cell.
The
the cell.
arrows indicate water moving INTO
Initial conditions: cellular  greater than environmental 
Initial flaccid cell:
P = 0
 S = −0.7
0.4 M sucrose solution:
 P= 0
 S = − 0.9
Plasmolyzed cell
at osmotic equilibrium
with its surroundings
 P= 0
 S = − 0.9
 = − 0.9 MPa
 = − 0.9 MPa
 = − 0.7 MPa
Initial conditions: cellular  less than environmental 
Initial flaccid cell:
 P= 0
 S = − 0.7
 = − 0.7 MPa
Distilled water:
 P= 0
 S= 0
 = 0 MPa
Turgid cell
at osmotic equilibrium
with its surroundings
 P = 0.7
 S = − 0.7
 = − 0 MPa
Wilting
 Turgor loss in plants causes wilting
 Which can be reversed when the plant is watered
Ascent of Xylem Sap
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Stomata Regulate Transpiration Rate
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•
When water moves into guard cells
from neighboring cells by osmosis,
they become more turgid.
•
The structure of the guard cells’ wall
causes them to bow outward in
response to the incoming water.
•
This bowing increases the size of the
pore (stomata) between the guard
cells allowing for an increase in gas
exchange.
Homeostasis and Water Regulation
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• By contrast, when the guard
cells lose water and become
flaccid, they become less
bowed , and the pore
(stomata) closes.
• This limits gas exchange.
Role Of Potassium Ion In Stomatal
Opening And Closing
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The transport of K+ (potassium ions, symbolized
here as red dots) across the plasma membrane and
vacuolar membrane causes the turgor changes of
guard cells.
H2O
K+
H2O
H2O
H2 O
H2O
H2O
H2O
H2O
H2O
H2O
Homeostasis and Water Balance
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• Trees that experience a
prolonged drought may
compensate by losing part of
their crown as a consequence
of leaves dying and being shed.
• Resources may be reallocated
so that more energy is
expended for root growth in
the “search” for additional
water.
Natural Selection and Arid
Environments
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Natural Selection and Arid
Environments
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Plants that have adapted to arid environments have
the following leaf adaptations:
1. Leaves that are thick and hard with few stomata
placed only on the underside of the leaf
2. Leaves covered with trichomes (hairs) which
reflect more light thus reducing the rate of
transpiration
3. Leaves with stomata located in surface pits which
increases water tension and reduces the rate of
transpiration
4. Leaves that are spine-like with stems that carry out
Natural Selection and Flooding
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• Plants that experience
prolonged flooding will have
problems.
• Roots underwater cannot
obtain the oxygen needed for
cell respiration and ATP
synthesis.
• As a result, leaves may dry out
causing the plant to die.
• Additionally, production of
hormones that promote root
synthesis are suppressed.
Adaptations to Water Environments
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Adaptations to Water Environments
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Plants that have adapted to wet environments have
the following adaptations:
1. Formation of large lenticels (pores) on the stem.
2. Formation of adventitious roots above the water
that increase gas exchange.
3. Formation of stomata only on the surface of the
leaf (water lilies).
4. Formation of a layer of air-filled channels called
aerenchyma for gas exchange which moves gases
between the plant above the water and the
submerged tissues.
Bulk Flow of Photosynthetic Products
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Vessel
Source cell
(leaf)
Sieve tube
(phloem)
(xylem)
H2O
1
Sucrose
1
H2O
Loading of sugar (green dots) into
the sieve tube at the source reduces
water potential inside the sieve-tube
members. This causes the tube to
take up water by osmosis.
2
Pressure flow
Transpiration stream
2
3
Sink cell
(storage root)
4
4
3
H2O
Sucrose
This uptake of water generates a
positive pressure that forces the
sap to flow along the tube.
The pressure is relieved by the
unloading of sugar and the
consequent loss of water from the
tube at the sink.
In the case of leaf-to-root
translocation, xylem recycles water
from sink to source.
Nutritional Adaptations in Plants
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Epiphytes- grow on other
plants, but do not harm
their host
Parasitic Plants-absorb
water, minerals, and
sugars from their host
Carnivorous Plantsphotosynthetic but
supplement their mineral
diet with insects and
small animals; found in
nitrogen poor soils
Halophytes
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Adaptations of Plants: Saline Environments
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• Soil salinity around the world is increasing.
• Many plants are killed by too much salt in the soil.
• Some plants are adapted to growing in saline
conditions (halophytes)
• Have spongy leaves with water stored that
dilutes salt in the roots
• Actively transport the salt out of the roots or
block the salt so that it cannot enter the roots
• Produce high concentrations of organic
molecules in the roots to alter the water
potential gradient of the roots