Xylem
Sap
Ascent
by
Bulk
Flow:
A
Review
• The
movement
of
xylem
sap
against
gravity
is
maintained
by
the
transpira7on‐cohesion‐tension
mechanism
• Transpira7on
lowers
water
poten7al
in
leaves,
and
this
generates
nega7ve
pressure
(tension)
that
pulls
water
up
through
the
xylem
• There
is
no
energy
cost
to
bulk
flow
of
xylem
sap
Stomata
regulate
the
rate
of
transpira<on
• Leaves
generally
have
broad
surface
areas
and
high
surface‐to‐volume
ra7os
• These
characteris7cs
increase
photosynthesis
and
increase
water
loss
through
stomata
Stomata:
Major
Pathways
for
Water
Loss
• About
95%
of
the
water
a
plant
loses
escapes
through
stomata
• Each
stoma
– Flanked
by
a
pair
of
guard
cells
– Control
the
diameter
of
the
stoma
by
changing
shape
1
Mechanisms
of
Stomatal
Opening
and
Closing
• Changes
in
turgor
pressure
in
guard
cells
open
and
close
stomata
– Primarily
from
the
reversible
uptake
and
loss
of
potassium
ions
• K+
co‐transported
by
proton
pumps
Fig. 36-17
Guard cells turgid/Stoma open Guard cells flaccid/Stoma closed
Radially oriented
cellulose microfibrils
Cell
wall
Vacuole
Guard cell
(a) Changes in guard cell shape and stomatal opening and
closing (surface view)
Guard cells turgid/Stoma open Guard cells flaccid/Stoma closed
H2 O
K
H2 O
H2 O
H2 O
H2 O
+
H2 O
H2 O
H2 O
H2 O
H2 O
(b) Role of potassium in stomatal opening and closing
S<muli
for
Stomatal
Opening
and
Closing
• Generally,
stomata
open
during
the
day
and
close
at
night
to
minimize
water
loss
• Stomatal
opening
at
dawn
is
triggered
by:
– Light
• Blue
light
receptors
s7mulate
K+
uptake
– CO2 deple7on
– Internal
“clock”
in
guard
cells
• Func7ons
even
in
the
dark
• circadian
rhythm
– 24‐hour
cycles
of
behavior
2
Effects
of
Transpira<on
on
Wil<ng
and
Leaf
Temperature
• Plants
lose
a
large
amount
of
water
by
transpira7on
– If
not
replaced
plant
will
wilt
• Transpira7on
also
results
in
evapora<ve
cooling
– Can
lower
the
temperature
of
a
leaf
and
prevent
denatura7on
of
various
enzymes
involved
in
photosynthesis
and
other
metabolic
processes
Adapta<ons
That
Reduce
Evapora<ve
Water
Loss
• Xerophytes
– plants
adapted
to
arid
climates
– Have
leaf
modifica7ons
that
reduce
the
rate
of
transpira7on
• Crassulacean
acid
metabolism
(CAM)
– Specialized
form
of
photosynthesis
where
stomatal
gas
exchange
occurs
at
night
Fig. 36-18
Ocotillo (leafless)
Oleander leaf cross section and flowers
Upper epidermal tissue
100 µm
Cuticle
Trichomes Crypt Stomata Lower epidermal
(“hairs”)
tissue
Ocotillo leaves
Ocotillo after heavy rain
Old man cactus
3
Sugar
Transport
• Transloca<on
– Process
of
transpor7ng
the
products
of
photosynthesis
through
phloem
Movement
from
Sugar
Sources
to
Sugar
Sinks
• Phloem
sap
– Aqueous
solu7on
that
is
high
in
sucrose
– Travels
from
a
sugar
source
to
a
sugar
sink
• Sugar
source
– An
organ
that
is
a
net
producer
of
sugar,
such
as
mature
leaves
• Sugar
sink
– An
organ
that
is
a
net
consumer
or
storer
of
sugar,
such
as
a
tuber
or
bulb
• A
storage
organ
can
be
both
a
sugar
sink
in
summer
and
sugar
source
in
winter
Movement
from
Sugar
Sources
to
Sugar
Sinks
• Sugar
– Must
be
loaded
into
sieve‐tube
elements
before
being
exposed
to
sinks
• Depending
on
the
species
– May
move
by
symplas7c
or
both
symplas7c
and
apoplas7c
pathways
• Transfer
cells
– Modified
companion
cells
that
enhance
solute
movement
between
the
apoplast
and
symplast
• Via
proton
pump
co‐transport
4
Fig. 36-19
High H+ concentration
Mesophyll cell
Cell walls (apoplast)
Plasma membrane
Companion
(transfer) cell
Proton
pump
Sieve-tube
element
Cotransporter
H+
S
Plasmodesmata
Key
ATP
Apoplast
Symplast
Mesophyll cell
Bundlesheath cell
Phloem
parenchyma cell
H+
H+
Low H + concentration
Sucrose
S
Fig. 36-19a
Mesophyll cell
Cell walls (apoplast)
Plasma membrane
Companion
(transfer) cell
Sieve-tube
element
Plasmodesmata
Key
Apoplast
Symplast
Mesophyll cell
Bundlesheath cell
Phloem
parenchyma cell
Movement
from
Sugar
Sources
to
Sugar
Sinks
• In
many
plants
– Phloem
loading
requires
ac7ve
transport
• Proton
pumping
and
cotransport
of
sucrose
and
H+
– Enable
the
cells
to
accumulate
sucrose
• At
the
sink
– Sugar
molecules
diffuse
from
the
phloem
to
sink
7ssues
• followed
by
water
5
Bulk
Flow
by
Posi<ve
Pressure
• Sap
moves
through
a
sieve
tube
by
bulk
flow
driven
by
posi7ve
pressure
– Called
pressure
flow
• Increasing
pressure
at
source
end
• Reduced
pressure
at
sink
end
Animation: Translocation of Phloem Sap in Summer
Animation: Translocation of Phloem Sap in Spring
Fig. 36-20
Vessel
(xylem)
Sieve tube Source cell
(phloem) (leaf)
H2O
1 Loading of sugar
Sucrose
1
H2O
Bulk flow by negative pressure
Bulk flow by positive pressure
2
2 Uptake of water
3 Unloading of sugar
Sink cell
(storage
root)
4 Water recycled
3
4
H2O
Sucrose
Movement
from
Sugar
Sources
to
Sugar
Sinks
• The
pressure
flow
hypothesis
– Explains
why
phloem
sap
always
flows
from
source
to
sink
• Experiments
have
built
a
strong
case
for
pressure
flow
as
the
mechanism
of
transloca7on
in
angiosperms
6
Fig. 36-21
EXPERIMENT
25 µm
Sievetube
element
Sap
droplet
Aphid feeding
Stylet
Sap droplet
Stylet in sieve-tube Separated stylet
element
exuding sap
The
symplast
is
highly
dynamic
• The
symplast
is
a
living
7ssue
and
is
responsible
for
dynamic
changes
in
plant
transport
processes
Plasmodesmata:
Con<nuously
Changing
Structures
• Plasmodesmata
– Can
change
in
permeability
(open
or
close)
in
response
to
• Turgor
pressure
• Cytoplasmic
calcium
levels
• Cytoplasmic
pH
• Plant
viruses
can
cause
plasmodesmata
to
dilate
– Normally
~2.5nm
• Dilate
to
>10nm
– Allowing
viruses
to
fit
through
7
Electrical
Signaling
in
the
Phloem
• The
phloem
allows
for
rapid
electrical
communica7on
between
widely
separated
organs
– Responsible
for
rapid
movements
in
touch
sensi7ve
plants
• Venus
flytrap
– Can
ini7ate
changes
in
• gene
expression
• Photosynthesis
• Respira7on
Phloem:
An
Informa<on
Superhighway
• Phloem
– is
a
passageway
for
systemic
transport
of
macromolecules
and
viruses
• Systemic
communica7on
– Helps
integrate
func7ons
of
the
whole
plant
• Ie
signals
for
conversion
of
vegeta7ve
meristems
to
floral
meristems
You
should
now
be
able
to:
1. Describe
how
proton
pumps
func7on
in
transport
of
materials
across
membranes
2. Define
the
following
terms:
osmosis,
water
poten7al,
flaccid,
turgor
pressure,
turgid
3. Explain
how
aquaporins
affect
the
rate
of
water
transport
across
membranes
4. Describe
three
routes
available
for
short‐distance
transport
in
plants
8
5. Relate
structure
to
func7on
in
sieve‐tube
cells,
vessel
cells,
and
tracheid
cells
6. Explain
how
the
endodermis
func7ons
as
a
selec7ve
barrier
between
the
root
cortex
and
vascular
cylinder
7. Define
and
explain
guha7on
8. Explain
this
statement:
“The
ascent
of
xylem
sap
is
ul7mately
solar
powered”
9. Describe
the
role
of
stomata
and
discuss
factors
that
might
affect
their
density
and
behavior
10. Trace
the
path
of
phloem
sap
from
sugar
source
to
sugar
sink;
describe
sugar
loading
and
unloading
9