Download Plant-Water Relations (1) Uptake and Transport

Plant-Water Relations (1)
Uptake and Transport
© 2014 American Society of Plant Biologists
Water is a major factor in plant
distribution
Yellow, orange, red and
pink indicate that water
is a key climate factor
limiting plant growth
Sunlight
Water
• Most plants experience occasional
water stress
• Losses in growth and yield due to
water stress are often cryptic because
no unstressed controls exist
Percent leaf enlargement
Temperature
15
30
10
20
5
10
Net photosynthesis
(mg / hr – 100 cm2)
Even mild water stress
can affect plant growth
Leaf water potential (bars)
Allen, C.D., et al., and Cobb, N. (2010). A global overview of drought and heat-induced tree mortality reveals emerging climate change risks for forests. Forest Ecol. Manage. 259: 660-684
with permission from Elsevier. Boyer, J.S. (1970). Leaf enlargement and metabolic rates in corn, soybean, and sunflower at various leaf water potentials. Plant Physiol. 46: 233-235.
© 2014 American Society of Plant Biologists
Plants have evolved several
strategies to manage water
Many bryophytes can tolerate
desiccation (i.e., drying to
equilibrium with the atmosphere )
Most
tracheophytes
cannot tolerate
desiccation –
they die
Some desert plants evade
drought. They survive the
dry season as seeds,
sprouting and flowering in
a brief period of rain
Some desert plants tolerate
dry conditions through
adaptations such as deep
roots, C4 photosynthesis,
succulence (water storage)
and tiny or absent leaves
Photo credits: Mary Williams; Amrum; Scott Bauer; James Henderson, Golden Delight Honey, Bugwood.org
© 2014 American Society of Plant Biologists
Outline: Plant - water relations.
Uptake and transport
• Brief history of the study of plant - water relations
• Water movement is governed by physical laws
• Aquaporins: Essential regulators of water
movement
• Water moves through the soil-plant-atmosphere
continuum (SPAC)
o Water uptake in roots
o Water movement through xylem
o The vulnerable pipeline
o Water movement within leaves
o Stomatal control of transpiration
• Hydraulic limits and drought
• Methods
© 2014 American Society of Plant Biologists
Brief history of the study of plant
water relations
1727 Stephen Hales
published experiments
on the nature of water
transport in plants
(1628)
William Harvey
showed that blood
circulates
Stephen Hales
(1677 – 1761)
Do plants pump sap
around their bodies?
Does sap circulate?
Is sap pushed up by
pressure from the
roots?
200 years later, in 1927,
the American Society of
Plant Biologists
established the Stephen
Hales Prize for
outstanding contributions
to plant biology
From Harvey, W, translated by Robert Willis (1908). An anatomical disquisition on the motion of the heart & blood in animals. Dent, London. See also Schultz, S.G. (2002). William Harvey and the Circulation of the
Blood: The Birth of a Scientific Revolution and Modern Physiology. Physiol. 17: 175-180, Stephen Hales, studio of Thomas Hudson, circa 1759 © National Portrait Gallery, London, used with permission.
© 2014 American Society of Plant Biologists
Malpighi and Grew saw plant
conducting tissues (1670 - 1680s)
Marcello Malpighi and Nehemiah Grew both recognized
that plant vascular tissues might be conducting tissues,
by analogy to similar-appearing structures in animals
Images courtesy of Biodiversity Heritage Library, from Malpighi, M. (1675). Anatome Plantarum. J. Martyn, London, and Nehemiah Grew, N. (1682) Plant Anatomy. W. Rawlins, London.
© 2014 American Society of Plant Biologists
Hales investigated water movement
in “Vegetable Staticks” (1727)
Hales measured
water uptake by
recording the
weight of a sealed
pot; weight loss
was caused by
water evaporating
from the plant
Hales
showed that
transpired
liquid is
essentially
water
Hales, S. (1727). Vegetable Staticks. W. and J. Innys, London. See also Mansfield, T.A., Davies, W.J. and Leigh, R.A. (1993). The transpiration stream: Introduction. Phil. Trans. Royal Soc. London B. 341: 3-4.
© 2014 American Society of Plant Biologists
Hales showed that water movement
requires leaves exposed to air
Water
movement
Little water
movement
Little water
movement
“These .. experiments all
shew, that .. capillary sap
vessels .. have little power
to protrude (moisture)
farther, without the
assistance of the
perspiring leaves, which
do greatly promote its
progress.”
The water is not pushed up from the roots, but
rather pulled up by transpiration from the leaves
Hales, S. (1727). Vegetable Staticks. W. and J. Innys, London.
© 2014 American Society of Plant Biologists
Dixon and Joly: The CohesionTension theory (1890s)
“… the all-sufficient cause
of the elevation of the sap
(is).. by exerting a simple
tensile stress on the liquid
in the conduits.”
The pressure in the chamber can be
elevated to determine tensile strength
Dixon and Joly (1895)
suggested that water
rising in the plant
occurs by an upwardspulling tension
Transpiration measured
by weight of water
removed from tube
Dixon, H.H. and Joly, J. (1895). On the Ascent of Sap. Phil. Trans. Roy. Soc. London. (B.). 186: 563-576. Böhm J (1893) Capillarität und Saftsteigen. Ber Dtsch Bot Ges 11:203–212. Dixon, H.H. (1914). Transpiration
and the Ascent of Sap in Plants. Macmillan & Co. Ltd. London.
© 2014 American Society of Plant Biologists
Water uptake and transport are
governed by physical laws
The flow of water up a
plant, driven by
evaporation, can be
replicated in a purely
physical model
“We may compare this
action to the action of a
porous vessel drawing
up a column of liquid to
supply the evaporation
loss at its surface”
– Dixon and Joly (1895)
Like Lego, water is very
cohesive, meaning that
the molecules stick
together and a column
of water can be pulled
to a great height
Dixon, H.H. and Joly, J. (1895). On the Ascent of Sap. Phil. Trans. Roy. Soc. London. (B.). 186: 563-576.
© 2014 American Society of Plant Biologists
Water has many unusual and
important properties
δ-
-
O
H
H
δ+
+
δ+
The arrangement of atoms in a water
molecule means that charge is
distributed asymmetrically; it is polar
and it has a high dielectric constant
100
Water’s high dielectric
constant means it is a
good solvent for polar
compounds
Boiling point
Melting point
50
δδ+
δ+
δδδ+
Water forms
intermolecular
hydrogen bonds and
so is very cohesive. It
sticks together and
has unusually high
boiling and melting
points
δ+
δ-
0
-50
-150
-250
CH4
NH3
H2O
HF
Exobiologists usually
consider water a
prerequisite for life
Photo credit: NASA; See Nobel, P. (2009) Physicochemical and Environmental Plant Physiology. Academic Press.
© 2014 American Society of Plant Biologists
Fick’s law describes the movement
of water by diffusion
dm
dc
DA
dt
dx
Fick’s Law
dm/dt = amount of substance
moved per unit time
D = diffusion coefficient
(property of substance and
matrix)
dc/dx = gradient of
concentration (change in
concentration per unit
of distance, dx, in a direction
perpendicular to the plane A)
[]
[]
dc
dx
x
A = area through
which flow occurs
© 2014 American Society of Plant Biologists
Diffusion rate is affected by area,
distance, and gradient
dm
dc
DA
dt
dx
Diffusion is faster when:
• The concentration
gradient dc/dx is steeper
• The area A is larger
• The distance x is smaller
• D is larger
[]
[]
dc
dx
x
A = area through
which flow occurs
© 2014 American Society of Plant Biologists
Diffusion rate is affected by the
properties of material and solvent
dm
dc
DA
dt
dx
The diffusion coefficient
D is affected by:
• Molecular shape
• Molecular size
• Molecular charge
• Solvent viscosity
• Solution properties as
salt concentration, pH,
temperature etc.
© 2014 American Society of Plant Biologists
Water moves down osmotic
gradients
Water moves across a
semipermeable membrane
into a compartment
containing salt
The water flows inwards
until the force of the
pressure inside balances
the osmotic driving force
The osmotic potential of pure water is 0. The addition
of solutes lowers the water potential. Water moves
towards a region with a lower water potential.
© 2014 American Society of Plant Biologists
Osmotic forces drive the movement
of water into and out of cells
Cells have a lower osmotic
potential than pure water,
(because of the salts and
proteins in them), so water
moves into them
Salt water
Fresh water
An animal cell might burst
Salt water
Fresh water
Plant cell walls prevent them
from bursting
Salt water has a lower
osmotic potential than
cells, so water flows
outwards
Osmotic potential is written as Ψπ and measured in MegaPascals (MPa)
For seawater, Ψπ is about -2.5 MPa, and for a typical cell, Ψπ is about -0.8 MPa
© 2014 American Society of Plant Biologists
Pressure can be positive or negative
Pressure
washer
15 MPa
Vacuum cleaner
-0.02 MPa
(household)
-0.1 MPa
(commercial)
Laboratory
vacuum
-0.01 MPa
Household
water
pressure
0.3 MPa
Car tire
pressure
0.25 MPa
Pressure
required to
blow up a
balloon
0.01 MPa
Inside typical plant cell
0.5 to 1.5 MPa
Human blood
pressure
< 0.02 MPa
Inside xylem:
From +1 MPa to
-3 MPa or lower
* These numbers are relative to atmospheric pressure (0.1 MPa), not absolute
Mschel; Image 14869 CDC/ Nasheka Powell
© 2014 American Society of Plant Biologists
Turgor pressure supports plant cells
and tissues
Young, non-woody
tissues are supported
by turgor pressure
Water limitation lowers
turgor pressure and
tissues wilt
Clemson University - USDA Cooperative Extension Slide Series ,R.L. Croissant, Bugwood.org
© 2014 American Society of Plant Biologists
The water potential equation
incorporates osmotic and pressure
potentials
The water potential (Ψw) is the sum of the pressure
The water
potential (Ψp) and the osmotic potential (Ψπ)
potential
equation
Ψw = Ψp + Ψπ
Water flows
from higher to
lower water
potential
© 2014 American Society of Plant Biologists
Water moves towards lower water
potential
Initial conditions:
Final conditions:
Ψπ = 0
+ Ψp = 0
Ψπ = -2
+ Ψp = 0
Ψw = 0
Ψw = -2
Water moves
towards lower
water potential
Ψπ = 0
+ Ψp = 0
Ψw = 0
Ψπ = -2
+ Ψp = 2
Ψw = 0
Water is at
equilibrium
© 2014 American Society of Plant Biologists
Water moves by diffusion and bulk
flow
Diffusion: Random movement of
individual molecules (1, 3)
ψw =
-100 MPa
3. Water
moves from
leaf to air
2. Water moves
through plant
Bulk flow: water in streams, tubes,
hoses, water column of xylem (1, 2)
ψw =
-0.3 MPa
1. Water
moves from
soil to root
© 2014 American Society of Plant Biologists
Diffusion rate is affected by the
properties of material and solvent
Cell membranes and walls
increase the diffusion
coefficient and can drastically
lower the rate of diffusion
Channels between cells called
plasmodesmata and water
channels called aquaporins can
accelerate intercellular diffusion
A small molecule can
diffuse across a 50 μm cell
in 0.6 s, but it would take 8
years to diffuse 1 m
© 2014 American Society of Plant Biologists
Long-distance water movement
occurs by bulk flow
Bulk flow is much faster
than diffusion, but requires
the input of energy
Human
circulatory
system:
Energy
provided by
mechanical
pump
(heart)
Waterfall:
Energy
provided by
gravity
Water
movement
up a plant:
Solar energy
drives
evaporation
Plastic Boy; Poco a poco
© 2014 American Society of Plant Biologists
Bulk flow through a tube is affected
by the diameter of the tube
 N r4
Flow  P
8 l
As the vessel radius decreases, the
cross-sectional area needed for the
same flow increases
Poiseuille’s Law
Where:
ΔP = hydraulic pressure gradient
η = viscosity of water
l = length of capillaries
N = number of capillaries
r = radius of capillary
Flow increases with the radius to the
fourth power!
Wider conduits are more conductive
Radius
40 μm
Radius
20 μm
4 x area
required
Each block conducts
the same flow
Radius 10 μm
16 x area required
Adapted from Tyree, M.T., Davis, S.D., and Cochard, H. (1994). Biophysical perspectives of xylem
evolution: Is there a tradeoff of hydraulic efficiency for vulnerability to dysfunction. IAWA 15: 335-360.
© 2014 American Society of Plant Biologists
Ohm’s law can model water flow,
whether diffusion or bulk flow
I=V/R
Ohm’s Law
Current (I) =
voltage (V) / resistance (R)
Ohm’s law as applied to water flow:
Water flow = Difference in ψ / resistance
The water potential difference (ψ) is
the difference between water
pressure and atmospheric pressure
Municipal
water
pressure
Atmospheric
pressure
Resistance is determined
by the position of the tap
As the tap opens,
resistance decreases
and so flow increases
Tyree, M.T. (1997). The Cohesion-Tension theory of sap ascent: current controversies. J. Exp. Bot. 48: 1753-1765.
© 2014 American Society of Plant Biologists
Flow rate is affected by conductance
of the root, stem and shoot
This model uses
conductance (K)
which is the inverse
of resistance:
Higher conductance
= higher flow
K=1/R
It is useful to
consider
separately
how the root,
stem and
leaves affect
conductance
We can also consider
the conductance of
individual branches
and leaves
Tyree, M.T. (1997). The Cohesion-Tension theory of sap ascent: current controversies. J. Exp. Bot. 48: 1753-1765 by permission of Oxford University Press.
© 2014 American Society of Plant Biologists
Resistance and conductance are
reciprocals & both affect flow
Example: Effect of conduit
size on resistance
The resistance is higher in
the sample with smallerdiameter conduits.
OPEN
CLOSED
Stomatal conductance (gs) is easily
measured as the flow of water
vapor out of the leaf. When the
stomata are open, conductance is
higher and resistance is lower
Stomata affect flow by
affecting resistance
The conductance is higher
in the sample with the
larger-diameter conduits.
Xylem anatomy affects
flow by affecting resistance
Image: Decagon Services
© 2014 American Society of Plant Biologists
Water flow is governed by the
conductance of 3 plant segments
2. Axial flow: Through
the roots and shoots into
the leaves via bulk flow
through the conducting
elements of the xylem
1. Root radial flow: From
soil to stele and the entry
point into the xylem system.
Dominated by diffusion and
permeability of membranes
and cell walls
3. Leaf radial flow: From
xylem through the
mesophyll, conversion to
water vapor, and finally
release into the
atmosphere through
regulated stomata
Adapted from Tyree, M.T. (1997). The Cohesion-Tension theory of sap ascent: current controversies. J. Exp. Bot. 48: 1753-1765
© 2014 American Society of Plant Biologists
Summary: Water movement is
governed by physical laws
• Water moves by diffusion and bulk flow
• Water moves towards lower water potential (ψw),
or lower pressure potential when no membranes
are involved
• Water potential is the sum of pressure and
osmotic potentials (and gravitational potential)
ψw = ψπ + ψp (+ ψg)
• Flow is determined by the difference in water
potential divided by resistance (Ohm’s Law)
I = V / R, which is analogous to the water
potential difference x conductance
Tyree, M.T. (1997). The Cohesion-Tension theory of sap ascent: current controversies. J. Exp. Bot. 48: 1753-1765 by permission of Oxford University Press.
© 2014 American Society of Plant Biologists
Aquaporins are regulated
membrane-bound water channels
Expression of an
aquaporin in a Xenopus
oocyte accelerates
water uptake
Relative volume
Cells with
aquaporin
Peter Agre was awarded the
Nobel Prize in 2003 for the
discovery of aquaporins
Cell with
aquaporin
Cell without
aquaporin
Cells without
aquaporin
Time (min)
Preston, G.M., Carroll, T.P., Guggino, W.B. and Agre, P. (1992). Appearance of water channels in Xenopus oocytes expressing red cell CHIP28 protein. Science.
256: 385-387; Maurel, C., Reizer, J., Schroeder, J.I., and Chrispeels, M.J. (1993). The vacuolar membrane protein γ-TIP creates water specific channels in Xenopus
oocytes. EMBO J. 12: 2241-2247; Maurel, C. and Chrispeels, M.J. (2001). Aquaporins. A molecular entry into plant water relations. Plant Physiol. 125: 135-138.
© 2014 American Society of Plant Biologists
Arabidopsis has 35 aquaporin genes
in four families
Small basic intrinsic
proteins
Nodulin-26-like
intrinsic proteins
Tonoplast intrinsic
proteins
Aquaporins have six
membrane-spanning domains,
and typically form tetramers
Plasma-membrane
intrinsic proteins
Luu, D.-T. and Maurel, C. (2005). Aquaporins in a challenging environment: molecular gears for adjusting plant water status. Plant Cell Environ. 28: 85-96, with permission from Wiley.
© 2014 American Society of Plant Biologists
Protoplast swelling assay of cell
permeability and aquaporin activity
How fast does the cell swell?
Wild-type cell
with many open
aquaporins
Loss-of-function of PIP1;2
The rate of volume change
after moving into a lower
water potential solution
indicates membrane water
permeability
Loss-of-function of aquaporin
PIP1;2 decreases protoplast
water permeability (Pf), as
measured by the change in
cell volume
Ramahaleo, T., Morillon, R., Alexandre, J. and Lassalles, J.-P. (1999). Osmotic water permeability of isolated protoplasts. modifications during development. Plant Physiol. 119: 885-896;
Postaire, O., et al. and Maurel, C. (2010). A PIP1 aquaporin contributes to hydrostatic pressure-induced water transport in both the root and rosette of Arabidopsis. Plant Physiol. 152: 1418-1430.
© 2014 American Society of Plant Biologists
Aquaporin activity is regulated at
many different levels, including
transcriptional regulation
mRNA abundance of
aquaporin transcripts
along length of
Arabidopsis (At) and
grape (Vv) roots
Gambetta, G.A., Fei, J., Rost, T.L., Knipfer, T., Matthews, M.A., Shackel, K.A., Walker, M.A. and McElrone, A.J. (2013). Water uptake along the length of grapevine fine
roots: Developmental anatomy, tissue-specific aquaporin expression, and pathways of water transport. Plant Physiol. 163: 1254-1265.
© 2014 American Society of Plant Biologists
Aquaporin activities are also
regulated post-transcriptionally
Phosphorylation
Shuttling between
internal and plasma
membranes
Gating
Reprinted from Li, G., Santoni, V. and Maurel, C. (2014) Plant aquaporins: Roles in plant physiology. Biochimica et Biophysica Acta (BBA) - General Subjects. (in press) with permission from Elsevier.
© 2014 American Society of Plant Biologists
Aquaporin channels are gated and
can be open or closed
Aquaporins are
tetramers but
each monomer
forms a channel
Closed
Open
Channel activity
is modified by
phosphorylation
and pH
Reprinted by permission from Macmillan Publishers Ltd from Tornroth-Horsefield, S., Wang, Y., Hedfalk, K., Johanson, U., Karlsson, M., Tajkhorshid, E., Neutze, R. and Kjellbom, P. (2006). Structural mechanism of
plant aquaporin gating. Nature. 439: 688-694. Reprinted from Maurel, C. (2007). Plant aquaporins: Novel functions and regulation properties. FEBS letters. 581: 2227-2236 with permission from Elsevier.
© 2014 American Society of Plant Biologists
Aquaporins facilitate rapid plant
movements
Cells that need a high rate of
water movement such as the
motor cells of the sensitive plant
can be enriched for aquaporins
Image credit: Sten Porse; Fleurat-Lessard, P., Frangne, N., Maeshima, M., Ratajczak, R., Bonnemain, J.L. and Martinoia, E. (1997). Increased
expression of vacuolar aquaporin and H+-ATPase related to motor cell function in Mimosa pudica L. Plant Physiol. 114: 827-834.
© 2014 American Society of Plant Biologists
Aquaporin expression and activity
affect hydraulic conductance
When aquaporin
activity is perturbed,
conductance and
water relations are
affected
Aquaporin genes are expressed
throughout the plant are involved in
leaf as well as root conductance
Siefritz, F., Tyree, M.T., Lovisolo, C., Schubert, A. and Kaldenhoff, R. (2002). PIP1 plasma membrane aquaporins in tobacco: From cellular effects to function in plants. Plant Cell.
14: 869-876. Heinen, R.B., Ye, Q. and Chaumont, F. (2009). Role of aquaporins in leaf physiology. J. Exp. Bot. 60: 2971-2985 by permission of Oxford University Press.
© 2014 American Society of Plant Biologists
Water moves through Soil – Plant –
Atmosphere Continuum (SPAC)
2. Transport
in the xylem
3. Transport
through the
leaf and into
the air
Stoma
Endodermis
1. Water
uptake in
roots
Vascular
cylinder
Outer root
layer
© 2014 American Society of Plant Biologists
The driving force for water flow is
evaporation powered by solar
energy
ψw =
-100 MPa
3. Water
moves from
leaf to air
2. Water moves
through plant
ψw =
-0.3 MPa
The water potential
of air is much lower
than that of soil, so
water is constantly
evaporating. The
plant harnesses
this energy and
channels water in,
up, and out of it
1. Water
moves from
soil to root
© 2014 American Society of Plant Biologists
Soil properties affect the movement
of water
Sand
Silt
Clay
Soils are made up of
particles of various
sizes, which affect
how water moves and
is retained in them
Large pore space (sandy soil)
Gravitational pull dominates
Small pore space (clayey soil)
Capillary action contributes to
lateral movement
Whiting, D., Card, A., Wilson, C., Moravec, C., and Reeder, J. (2010). Managing Soil Tilth: Texture, Structure, and Pore Space. Colorado Master Gardener Program, Colorado State University Extension.
© 2014 American Society of Plant Biologists
Molecular interactions between
water and soil affect water uptake
Increasing amount of water added to soil
Wilting point: The point at
which most plants wilt and
will not recover at night
Field capacity: The level
above which water drains off
© 2014 American Society of Plant Biologists
The type of soil particle (shape, size)
affects soil interactions with water
Increasing
water
added to
soil
Other factors that affect
water uptake from soil
include organic matter,
microbes, and salinity
Field capacity
Wilting point
Sand
2 mm
Sandy
loam
Loam
Silt
loam
Decreasing particle size
Clay
loam
Clay
.002 mm
Adapted from Kramer, P.J., and Boyer, J.S. (1995). Water
Relations of Plants and Soils. Academic Press. San Diego.
© 2014 American Society of Plant Biologists
Soil-Plant-Atmosphere Continuum 1.
Water uptake in roots: From soil to stele
Flow 
SPAC segment
flow equation
soilroot interface  root xylem
R soilroot interface root xylem
Endodermis
Ψsoil-root interface
R
Ψxylem
Vascular
cylinder
Factors affecting water
flow into the root:
Ψw at soil-root interface
Ψw at root xylem
R soil-root interface to root
xylem
Resistance of
water path
© 2014 American Society of Plant Biologists
Under water deficit, root growth is
maintained relative to shoot growth
Water
content:
Percent of
control
Ψw (MPa)
100%
-0.03
16%
-0.20
4%
-0.81
2%
-1.60
As water becomes scarce, the
plant’s resources are preferentially
allocated to the root
Hsiao, T.C. and Xu, L.K. (2000). Sensitivity of growth of roots versus leaves to water stress: biophysical analysis and relation to water transport. J. Exp.Bot. 51: 1595-1616, by permission of Oxford
University Press Sharp, R.E., Silk, W.K. and Hsiao, T.C. (1988). Growth of the maize primary root at low water potentials: I. Spatial distribution of expansive growth. Plant Physiol. 87: 50-57.
© 2014 American Society of Plant Biologists
Water flow into roots is affected by
root architecture and conductance
Optimal architecture
depends on soil type and
water depth, as well as
distribution of nutrients
and microorganisms
Root architecture (length of
roots, branching, angle) affects
and is affected by water
Conductance is
affected by root
anatomy and
morphology
Steudle, E. (2000). Water uptake by roots: effects of water deficit. J. Exp. Bot. 51: 1531-1542; Frensch, J. and Steudle, E. (1989). Axial and radial hydraulic resistance to roots of maize (Zea
mays L.). Plant Physiol. 91: 719-726. Weaver, J.E. (1925). Investigations on the root habits of plants. Am. J. Bot. 12: 502-509 with permission from Botanical Society of America.
© 2014 American Society of Plant Biologists
The distribution of root growth is
affected by water availability
Winter wheat grown in regions
with different amounts of rainfall
26 – 32
inches
21 – 24
inches
16 – 19
inches
Total root length at
0 to 0.2 m depth
Shallower root systems
can be more effective at
catching limited rainfall
Non-irrigated soybean
plants produce more root
mass deeper in the soil
On the other hand,
when water is
withheld, deeper roots
can be more effective
at reaching water
Total root
length at
0.6 to 0.8 m
depth
Irrigated
Nonirrigated
1.2 to 1.4 m depth
Weaver, J.E. (1925). Investigations on the root habits of plants. Am. J. Bot. 12: 502-509; Hoogenboom, G., Huck, M.G. and Peterson, C.M. (1987). Root growth rate of soybean as affected by drought stress. Agron. J. 79: 607-614.
© 2014 American Society of Plant Biologists
Root growth can respond to water
gradients (hydrotropism)
Growth of a plant in a
uniform water
environment is
dominated by gravity
When there is a water
potential gradient, the
root can exhibit
hydrotropic growth –
growth towards water
Growth medium at
the bottom of the
plate has a very
low water potential
Eapen, D., Barroso, M.L., Ponce, G., Campos, M.E. and Cassab, G.I. (2005). Hydrotropism: root growth responses to water. Trends in Plant Science. 10: 44-50 with
permission from Elsevier; See also Cassab, G.I., Eapen, D. and Campos, M.E. (2013). Root hydrotropism: An update. American Journal of Botany. 100: 14-24.
© 2014 American Society of Plant Biologists
Root conductance is determined by
radial and axial conductance
Radial conductance:
From soil to stele
Radial conductance is
affected by anatomy,
morphology, cell wall
permeability, activity of
water channels, etc.
Axial conductance:
Through the xylem
Axial
conductance is
affected by the
number,
diameter and
structure of the
xylem conduits,
as well as
formation of
embolisms
Reprinted from Li, H.-F., et al. (2011). Vessel elements present in the secondary xylem of Trochodendron
and Tetracentron (Trochodendraceae). Flora. 206: 595-600 with permission from Elsevier.
© 2014 American Society of Plant Biologists
Water likely moves from soil to stele
through multiple paths
plasmodesmata
Apoplastic path
(through wall):
No crossing of plasma
membrane
Symplastic path
(through plasmodesmata):
Plasma membrane crossed once
wall
Transcellular path:
Plasma membrane
crossed many times
Adapted from Steudle, E. and Peterson, C.A. (1998). How does water get through roots? J. Exp. Bot. 49: 775-788.
© 2014 American Society of Plant Biologists
Conductance of cell walls and
membranes affects water uptake
The exodermis and endodermis are cell layers
Key factors that may
affect movement through with Casparian bands or strips that restrict
their conductance of water and solutes
the root cortex:
• The endodermis,
exodermis and water
Xylem
impermeable Casparian
strip
• Aquaporins (membranebound water channels)
• Plasmodesmata
Plasmodesmata are small
plasma membrane–lined
cytoplasmic bridges
connecting cells
Aquaporins are regulated
water channels in cell
membranes
Adapted from Steudle, E. (2000). Water uptake by roots: effects of water deficit. J. Exp. Bot. 51: 1531-1542.
© 2014 American Society of Plant Biologists
The Casparian strip forces water to
cross a plasma membrane
Casparian strip
cortex
stele
xylem
Water forced to
cross a plasma
membrane is
effectively filtered
Na+
Na+
OUT
Water can enter
the root through
the apoplast (cell
wall spaces) until
it reaches the
Casparian strip,
which blocks
apoplastic water
flow
IN
Photo credit Michael Clayton; Adapted from Grebe, M. (2011). Plant biology: Unveiling the Casparian strip. Nature. 473: 294-295.
© 2014 American Society of Plant Biologists
The root endodermis acts as a filter
for incoming water
Na+
Na+
Na+
Na+
The endodermis produces a
water-impermeable layer,
the Casparian strip, that
provides selectivity
Na+
Na+
Na+
Photo credit Michael Clayton
© 2014 American Society of Plant Biologists
Robert Caspary (1865) recognized
the significance of this barrier
Cell A
Casparian strip
Cell B
xylem
cortex
Endodermis stele
showing
Casparian
strip
Robert Caspary’s original 1865
drawing showing endodermal
cells surrounded by the
Casparian strip
Caspary R. (1865). Bemerkungen über die Schutzscheide und die Bildung des Stammes und der Wurzel. Jahrb. Wiss. Bot. 4:101–124, as described in Geldner, N. (2013). The endodermis. Annu. Rev. Plant Biol. 64: 531558; Reprinted by permission from Macmillan Publishers Ltd: Roppolo, D., De Rybel, B., Tendon, V.D., Pfister, A., Alassimone, J., Vermeer, J.E.M., Yamazaki, M., Stierhof, Y.-D., Beeckman, T. and Geldner, N.
(2011). A novel protein family mediates Casparian strip formation in the endodermis. Nature. 473: 380-383; Adapted from Grebe, M. (2011). Plant biology: Unveiling the Casparian strip. Nature. 473: 294-295; .Photo
courtesy of Royal Danish Archive.
© 2014 American Society of Plant Biologists
Summary: Movement of water into
and through roots
Flow 
soilroot interface  root xylem
rsoilroot interface root xylem
The flow of water into the root is affected by the
difference in water potential from the soil-root
interface and the root xylem, and the resistance of
root tissue to water flow
Root radial conductance, from soil to stele, is
usually lower than axial conductance, and is
affected by permeability of membranes and
walls, particularly aquaporin activity
© 2014 American Society of Plant Biologists
Soil – Plant – Atmosphere Continuum 2:
Flow of water through the xylem
Once the water enters the
transpiration stream of the xylem,
it moves through the plant by bulk
flow, pulled by the tension
produced by evaporation at the
leaf surface.
• How does the structure of the
xylem cells affect the flow of
water?
• Why is the xylem a
“vulnerable pipeline”?
© 2014 American Society of Plant Biologists
Transport in bryophytes – some
specialized transport cells
Bryophytes take up water from the air and substrate
Water moves along
external surfaces,
within cells, between
cells, and through
specialized waterconducting cells or
hydroids (h)
Hydroids (h) in
cross section
Some bryophytes
have foodconducting cells
or leptoids (l)
The end walls
between foodconducting cells can
have enlarged
plasmodesmataMicrographs from Ligrone, R., Duckett, J.G. and Renzaglia, K.S. (2000). Conducting tissues and phyletic relationships of bryophytes. Philosophical
Transactions of the Royal Society of London. Series B: Biological Sciences. 355: 795-813 by permission of the Royal Society.
© 2014 American Society of Plant Biologists
Hollow conducting tissues allow
greater height and CO2 assimilation
Lycophytes
(~ 1200 species)
Ferns
Gymnosperms
(~ 13,000 species) (~ 1000 species)
Angiosperms
(~ 350,000 species)
Bryophytes
Seeds
Vascular tissue (> 400 million years ago)
Photos by Tom Donald
© 2014 American Society of Plant Biologists
Fossil record: Xylem conduits
became wider and more complex
There also has
been an increase
in proportion of
lignified,
degradationresistant cell wall
material
Tracheid diameter (μm)
150
100
50
0
420
395
380
Million years ago
Adapted from Sperry, J.S. (2003). Evolution of water transport and xylem structure. Int. J. Plant Sci. 164: S115–S127; Reprinted by permission from Macmillan Publishers Ltd. from Kenrick, P. and Crane, P.R. (1997).
The origin and early evolution of plants on land. Nature. 389: 33-39. Pittermann, J. (2010). The evolution of water transport in plants: an integrated approach. Geobiology. 8: 112-139, adapted from Niklas, K.J. (1985).
The evolution of tracheid diameter in early vascular plants and its implications on the hydraulic conductance of the primary xylem strand. Evolution. 39: 1110-1122.
© 2014 American Society of Plant Biologists
The secondary walls are reinforced by
lignin, a complex, hydrophobic polymer
Lignin-strengthened
cell walls are one of
the major features
that lets plants get big
Reprinted from Höfte, H. (2010). Plant cell biology: How to pattern a wall. Curr. Biol. 20: R450-R452 with permission from Elsevier.
Weng, J.-K. and Chapple, C. (2010). The origin and evolution of lignin biosynthesis. New Phytol. 187: 273-285.
© 2014 American Society of Plant Biologists
Lignified xylem provides structural
support for vascular plants
115 m Sequoia sempervirens
St. Paul’s Cathedral 111 m
Statue of Liberty 93 m
The tallest living trees
tower over many
familiar monuments
Sydney Opera House 65 m
Taj Mahal 65 m
© 2014 American Society of Plant Biologists
Development and differentiation of
tracheary elements
Nieminen, K.M., Kauppinen, L. and Helariutta, Y. (2004). A weed for wood? Arabidopsis as a genetic model for xylem development. Plant Physiol. 135: 653-659.
© 2014 American Society of Plant Biologists
Tracheary elements have been
described as “functional corpses”
Mature
tracheary
element
Procambial
cell
Elongation
Secondary
wall
deposition
Degradation
of vacuole
and
organelles
Reprinted from Roberts, K., and McCann, M.C. (2000). Xylogenesis: the birth of a corpse. Current Opinion in Plant Biology 3: 517-522 with permission from Elsevier
© 2014 American Society of Plant Biologists
Most seed plants (and a few others)
can produce secondary xylem
Lycopods
+
+
Ferns and
relatives
Most cannot
produce
secondary
xylem, but
there are
exceptions
The vascular cambium is a lateral meristem
that produces new xylem and phloem
Phloem
Cambium
Xylem
Gymnosperms
+
Angiosperms
Monocots
Seed plants
Most can
produce
secondary
xylem
The xylem produced by
the vascular cambium
allows the plant to grow
in the radial dimension
and replaces older, less
functional xylem
Adapted from Spicer, R. and Groover, A. (2010). Evolution of development of vascular cambia and secondary growth. New Phytol. 186: 577-592; Photo credit MPF.
© 2014 American Society of Plant Biologists
General Sherman Tree
The Wawona Tunnel Tree,
Yosemite National Park
(thought to be the largest living tree by
volume with a diameter of more than 7 m)
Secondary thickening
produces new xylem
and lets plants
increase in girth
These trees are
Sequoiadendron
giganteum, giant
sequoias
Yosemite Research Library, National Parks Service; Jim Bahn
© 2014 American Society of Plant Biologists
How does xylem structure affect its
function?
• Xylem conductance is a function of
– Distribution of diameters of elements
– Element length
– Number of vessels participating
– Resistance to transfer between elements
Brodersen, C.R., McElrone, A.J., Choat, B., Lee, E.F., Shackel, K.A. and Matthews, M.A. (2013). In vivo visualizations of drought-induced embolism spread in Vitis vinifera. Plant Physiol. 161: 1820-1829; McCully, M.E.
(1999). Root xylem embolisms and refilling. Relation to water potentials of soil, roots, and leaves, and osmotic potentials of root xylem sap. Plant Physiol. 119: 1001-1008; Pittermann, J., Choat, B., Jansen, S., Stuart, S.A.,
Lynn, L. and Dawson, T.E. (2010). The relationships between xylem safety and hydraulic efficiency in the Cupressaceae: The evolution of pit membrane form and function. Plant Physiol. 153: 1919-1931.
© 2014 American Society of Plant Biologists
Xylem anatomy: Hollow, thickened
conducting cells
Vessel
Vessel
Vessels
Length
1 cm - > 1 m
Diameter
15 - 500 μm
Vessel
Tracheids (gymnosperm)
are narrow, up to 1 cm in
length, and perforated by
complex pit membranes
Tracheids
Length
0.1 – 1 cm
Diameter
5 -80 μm
Vessels
(angiosperm) are
made from vessel
elements, which are
wide, short,
perforated by simple
pit membranes, and
have open or
perforated end walls
Vessel
element
Vessel
Choat, B., Cobb, A.R. and Jansen, S. (2008). Structure and function of bordered pits: new discoveries and impacts on whole-plant hydraulic function. New Phytol. 177: 608-626 with permission from Wiley; Adapted
from Myburg, A.A, Lev‐Yadun, S., and Sederoff, R.R. (Oct 2013) Xylem Structure and Function. In: eLS. John Wiley & Sons Ltd, Chichester.
© 2014 American Society of Plant Biologists
Tracheids and vessels are quite
different in size
Conifer tracheids are
significantly shorter and
narrower (but with some
overlap in the width) than
angiosperm vessels,
Poiseuille’s Law: The
flow of fluid through a
tube scales to the fourth
power of the radius of
the tube (Flow ∝ r4)
What other factors
are involved in
hydraulic
conductivity?
Sperry, J.S., Hacke, U.G. and Pittermann, J. (2006). Size and function in conifer tracheids and angiosperm vessels. Am. J. Bot. 93: 1490-1500 with permission from Botanical Society of America.
© 2014 American Society of Plant Biologists
Xylem is a “vulnerable pipeline”,
prone to embolism
The ability to lift water
depends on a
continuous column of
water
Air can enter a conduit
and expand to form an
air-vapor blockage that
breaks the water
column, a condition
known as embolism
©INRA/Hervé Cochard Used with permission
© 2014 American Society of Plant Biologists
Cavitation describes a condition
wherein an air bubble moves into a
vessels
Embolized vessel
Pressure = 0 MPa
Sap-filled vessel
Pressure = -6 MPa
Adapted from Jacobsen, A.L., Ewers, F.W., Pratt, R.B., Paddock, W.A. and Davis, S.D. (2005). Do xylem fibers affect vessel cavitation resistance? Plant Physiol. 139: 546-556.
© 2014 American Society of Plant Biologists
Embolisms can spread between
vessels
Early
drought
Embolisms (dark; water-vapor filled conduits)
in an intact grapevine stem, made visible by
high-resolution computed tomagraphy
Late
drought
Embolisms spread from conduit to
conduit; a pathway connecting the
embolized vessels is shown in yellow
Brodersen, C.R., McElrone, A.J., Choat, B., Lee, E.F., Shackel, K.A. and Matthews, M.A. (2013). In vivo visualizations of drought-induced embolism spread in Vitis vinifera. Plant Physiol. 161: 1820-1829.
© 2014 American Society of Plant Biologists
Pit membranes permit water flow
and limit embolism spread
ANGIOSPERM
Uniform pit
membrane
GYMNOSPERM
Central,
impermeable
torus
surrounded by
permeable
margo
Choat, B., Cobb, A.R. and Jansen, S. (2008). Structure and function of bordered pits: new discoveries and impacts on whole-plant hydraulic function. New Phyt. 177: 608-626 with permission from Wiley.
© 2014 American Society of Plant Biologists
Some pits are more embolismresistant than others
No embolism:
Water present on
both sides of pit
membrane
Embolism-resistant
pits block embolism
movement between
conduits
Pits with a large
torus can seal the
pit membrane
Embolismsensitive
membrane pits
Many desert plants
have vestured pits
with wall elaborations
Thick pit membranes
can resist embolism
movement
Lens, F., Tixier, A., Cochard, H., Sperry, J.S., Jansen, S. and Herbette, S. (2013). Embolism resistance as a key mechanism to understand adaptive plant strategies. Curr. Opin. Plant Biol. 16: 287-292, with permission from Elsevier.
© 2014 American Society of Plant Biologists
Vulnerability curves characterize the
sensitivity of a plant to embolism
In most plants the xylem
can either have a high
hydraulic efficiency or
be resistant to embolism
formation, but not both,
and the functionality of
the xylem is correlated
with the environment a
plant is adapted to
100
% Loss xylem conductance
Increasing proportion
of cavitated conduits
Different species and tissues
produce different
“vulnerability curves”
50
0
-10
-8
-6
-4
-2
Xylem water pressure (MPa)
0
Increasing tension
(decreasing ψp)
Adapted from Tyree, M.T., Davis, S.D., and Cochard, H. (1994). Biophysical perspectives of xylem evolution — Is there a
tradeoff of hydraulic efficiency for vulnerability to dysfunction? IAWA J. 15: 335 – 360 with permission of IAWA.
© 2014 American Society of Plant Biologists
Many plants experience a seasonal
loss and recovery of conductivity
Xylem
Pressure
Temp.
(kPa)
(°C)
Winter –
Freezing-induced embolisms
Early spring –
Root-pressure refilling
Late spring –
New xylem formation
100
Percentage loss
of hydraulic
conductivity
Freezing-induced
embolisms
decrease
conductivity
Spring
activation of
cambium
increases
conductance
80
60
40
20
Root
pressure
refills xylem
and increases
conductance
0
30
0
20
0
Jan
Feb Mar
Apr May
Jun
Cochard, H., Lemoine, D., Améglio, T. and Granier, A. (2001). Mechanisms of xylem recovery from winter embolism in Fagus sylvatica. Tree Physiol. 21: 27-33 by permission of Oxford University Press.
© 2014 American Society of Plant Biologists
Vulnerability to embolism is
ecologically relevant
Increasing proportion
of cavitated conduits
Dry habitats
Wet habitats
Salt water
(mangrove)
Ψ50 is a measure of
the plants’ sensitivity
to embolism
Increasing tension
(decreasing ψp)
A comparison of
vulnerability curves for
six species from diverse
climates:
Ceanothus megacarpus
Juniperus virginiana (red cedar)
Rhizophora mangle (mangrove)
Acer saccharum (sugar maple)
Thuja occidentalis (white cedar)
Populus deltoides (cottonwood)
In general, plants from
drier climates tend to have
lower Ψ50 and be more
resistant to cavitation
Adapted from Tyree, M.T., Davis, S.D., and Cochard, H. (1994). Biophysical perspectives of xylem evolution — Is there a
tradeoff of hydraulic efficiency for vulnerability to dysfunction? IAWA J. 15: 335 – 360 with permission of IAWA.
© 2014 American Society of Plant Biologists
LA
Paris
Miami
Tokyo
Singapore
Juneau, Alaska
Solomon Islands
Cairo
Minimum xylem pressure measured
under natural conditions
There is a correlation between
habitat and cavitation resistance
Mean annual precipitation (mm)
Less resistance to
cavitation (↑) is
correlated with
wetter climate (→)
Less resistance to cavitation (→) is correlated
with less negative normal xylem pressure
experienced (↑). Gymnosperms tend to have
larger “hydraulic safety margins”
Reprinted by permission from Macmillan Publishers Ltd. From Choat, B., et al. (2012). Global convergence in the vulnerability of forests to drought. Nature. 491: 752755; See also Zanne et al (2014) Three keys to the radiation of angiosperms into freezing environments. Nature
© 2014 American Society of Plant Biologists
The environment affects xylem
tension
Scholander observed that in spite
of their roots being immersed in
seawater, the xylem sap of
mangroves contains very little salt.
He hypothesized that the xylem pressure
must be very low to draw water in
against its concentration gradient
ROOT
ψπ = -2.5 MPa
Na+
Na+
Na+
Na+
Seawater
Root cortex
ψπ = -0.25 MPa
Endodermis
ψπ = -2.5 MPa
90% of the salt in
seawater is
filtered out
before it reaches
the xylem
Xylem
© 2014 American Society of Plant Biologists
Scholander showed that tension in
the xylem drives water uptake
ROOT
ψπ = -2.5 MPa
ψp = 0 MPa
ψw = -2.5 MPa
Na+
Na+
Na+
Na+
Seawater
Root cortex
ψπ = -0.25 MPa
ψp = -2.3 MPa
ψw = -2.55 MPa
Endodermis
ψπ = -2.5 MPa
ψp = 0 MPa
ψw = -2.5 MPa
Xylem
© 2014 American Society of Plant Biologists
Tension in the xylem is balanced by
solute accumulation in the leaves
ψπ = -2.6 MPa
ψp = 0.1 MPa
ψw = -2.7 MPa
Water in leaf
cells usually
has a lower
osmotic
potential than
seawater!
ROOT
ψπ = -2.5 MPa
ψp = 0 MPa
ψw = -2.5 MPa
Na+
Na+
Na+
Na+
Seawater
Root cortex
ψπ = -0.25 MPa
ψp = -2.3 MPa
ψw = -2.55 MPa
Endodermis
ψπ = -2.5 MPa
ψp = 0 MPa
ψw = -2.5 MPa
90% of the salt in
seawater is
filtered out
before it reaches
the xylem
Xylem
© 2014 American Society of Plant Biologists
Does xylem conductance limit the
height of trees?
Ψw = Ψ p + Ψπ + Ψg
Xylem pressure (MPa)
Gravitational potential lowers
Ψw by -0.01 MPa per m height
Predawn
Midday
Height above ground (m)
Leaf morphology by height
Open question:
Does water limitation and its
effects on photosynthesis
prevent trees from reaching
more than ~130 m?
Reprinted by permission from Macmillan Publishers Ltd from Koch, G.W., Sillett, S.C., Jennings, G.M., and Davis, S.D. (2004). The limits to tree height. Nature 428: 851 – 854.
© 2014 American Society of Plant Biologists
Xylem under pressure: Root
pressure and grapevine refilling
Winter
Summer
In the winter, the
xylem vessels of
grapevines are
filled with air
In the summer,
transpiration causes
tension in the fluid-filled
xylem vessels
Howard F. Schwartz, Colorado State University; Howard F. Schwartz, Colorado State University, Bugwood.org; Virtual Grape
© 2014 American Society of Plant Biologists
White bars = pressure
Black bars = vine length
Hollow vessels fill with water in the
spring due to root pressure
In the spring, before
the leaves emerge,
the hollow vessels fill
with water, driven by
pressure generated
in the roots
Pressures are so
high, cut vines
exude water
copiously and are
said to “bleed”
In the summer,
when the leaves
are out and the
plant is transpiring,
xylem pressure is
negative (tension)
Scholander, P.F., Love, W.E., and Kanwisher, J.W. (1955). The rise of sap in tall grapevines. Plant Physiol. 30: 93 – 104.
© 2014 American Society of Plant Biologists
Stem pressure and the flow of sap in
maple trees (Acer saccharum)
In the early spring, when the nights are below freezing
and the days are above freezing, in the daytime sweet
sap flows from holes drilled into the bark of sugar
maples (Acer saccharum)
People have been
collecting the sugar-rich
sap from maple trees in
the north east of North
America for hundreds of
years. Where does it
come from?
Photo credit: University of Minnesota; T. Davis Sydnor, The Ohio State, Bugwood.org
© 2014 American Society of Plant Biologists
Clues to the source of maple sap
Maple sap
flows in early
spring, before
leaves emerge,
suggesting it is
not driven by
evaporative
transpiration
When a branch is cut, sap flows from the
leafy end, not the root end, suggesting
sap flow is not driven by root pressure
John Ruter, University of Georgia, Bugwood.org
© 2014 American Society of Plant Biologists
As long as water is provided, sap
flow does not require roots or crown
Cut branches
set onto dry
surfaces,
indicating that a
source of water
is required
Stevens, C.L., and Russell, L. Eggert. (1945). Observations on the causes of the flow of sap in red maple. Plant Physiol. 20: 636 - 648.
© 2014 American Society of Plant Biologists
Experimental set up to
monitor sap flow into an
out of excised branch
under varying temperature
Temp °C
Negative flow =
into branch
0
-5
-10
+5
Positive flow =
out of branch
0
-2
5
0
-5
Time, min
0
15
30
45
60
75
90
Flow rate cm3/ hr
As air temp
rises above
freezing, sap
flows out of the
branches
5
-5
Temp °C
As air temp
drops below
freezing, sap
flows into the
branches
0
Flow rate cm3/ hr
Sap flow requires temperatures
alternating above and below freezing
105
Tyree, M.T. (1983). Maple sap uptake, exudation, and pressure changes correlated with freezing exotherms and thawing endotherms. Plant Physiol. 73: 277-285.
© 2014 American Society of Plant Biologists
1983 sap flow model: Freezing
compresses vapor in xylem fibers
Vessels
The Milburn & O’Malley model
Cooling:
Vapor
compresses,
water drawn into
fibers
Fibers
Frozen:
Ice traps
vapor at high
pressure
Warming:
Thawing ice
releases
pressurized
vapor and forces
sap down
In sugar maple the xylem fibers
surrounding the vessel elements
are usually filled with air
Fiber
Vessel
Rays
Milburn, J.A. and O'Malley, P.E.R. (1984). Freeze-induced sap absorption in Acer pseudoplatanus: a possible mechanism. Can. J. Bot. 62: 2101-2106. Image source: Université Laval
© 2014 American Society of Plant Biologists
2008 model: Contribution of
sugars in minimally pitted vessels
Pressure from trapped gasses aren’t
sufficient. Osmotic pressure due to sucrose
in the vessel, trapped by minimal pit
connections to fibers, can explain the
continued flow of sap after thawing
Ray
Bordered pits P[b] and half-bordered pits
P[hb] could allow sugar (orange arrows) to
move into but not out of the vessel
elements, and water (blue) to pass freely
Vessel element
Vessel elements without pits
Net influx of
water
generates
pressure
Cirelli, D., Jagels, R. and Tyree, M.T. (2008). Toward an improved model of maple sap exudation: the location and role of osmotic barriers in sugar maple, butternut and white birch. Tree Physiol. 28: 1145-1155 by permission
of Oxford University Press; See also Ceseri, M. and Stockie, J. (2013). A mathematical model of sap exudation in maple trees governed by ice melting, gas dissolution, and osmosis. SIAM J. Appl. Math. 73: 649-676.
© 2014 American Society of Plant Biologists
Summary: Water flow through
xylem, a low-resistance pathway
• Xylem evolution reflects
trade-offs between safety
and efficiency
• Angiosperms sometimes
have larger conduits
• Gymnosperms can move
water from conduit-toconduit more efficiently
• Preventing embolisms is
critical to plant success
© 2014 American Society of Plant Biologists
Soil – Plant – Atmosphere
Continuum 3: From leaf to air
Stoma
Three elements to water
flow in leaves:
1. Through veins
2. From vein to stomata
3. Out through stomata
1
2
3
Resistance diagram for water flow
in a leaf. From the end of the vein
to the stomatal pore, water can
move through an apoplastic or
symplastic pathway
Cochard, H., Venisse, J.-S., Barigah, T.S., Brunel, N., Herbette, S., Guilliot, A., Tyree, M.T. and Sakr, S. (2007). Putative role of aquaporins in variable hydraulic conductance of leaves in response to light. Plant Physiol. 143: 122-133.
© 2014 American Society of Plant Biologists
Leaf conductance (Kleaf): Xylem and
out-of-xylem conductance
Stoma
Kx is determined by
xylem conduit
diameter, abundance,
anatomy, etc.
Leaf conductance
(Kleaf) is affected by
xylem conductance
(Kx) and out-of-xylem
conductance (Kox)
Kox is determined by the
conductance of the nonvascular cells, the path
length from vein to
stomata, and stomatal
density and conductance
See Sack, L. and Scoffoni, C. (2013). Leaf venation: structure, function, development, evolution, ecology and applications in the past, present and future. New Phytol. 198: 983-1000.
© 2014 American Society of Plant Biologists
Leaf xylem anatomy affects leaf
hydraulic properties
Dicot leaves with
reticulate vein
patterns, showing
increasing vein
densities
Dichotomously
branching veins of
ferns
Monocot
leaves with
parallel veins
at two different
densities
Sack, L. and Scoffoni, C. (2013). Leaf venation: structure, function, development, evolution, ecology and applications in the past, present and future. New Phytol. 198: 983-1000, with permission from Wiley.
© 2014 American Society of Plant Biologists
The distance from the end of the
conducting cells to the air matters
Angiosperms
The high Kleaf of angiosperms is
correlated with a short distance
from the end of the vein xylem to
the gas-exchange epidermis (Dm)
Conifers
Lycopods
Ferns
Bryophytes
Sample
measurements
to calculate Dm
Brodribb, T.J., Feild, T.S. and Jordan, G.J. (2007). Leaf maximum photosynthetic rate and venation are linked by hydraulics. Plant Physiol. 144: 1890-1898.
© 2014 American Society of Plant Biologists
Water conducting sclereids shorten
Dm and increase Kleaf
Some gymnosperms (circled)
produce lignified transfusion tissue
(a.k.a. water-conducting sclereids)
that serve as hydraulic shortcuts
The water-conducting sclereids
(stained blue) provide a hydraulic
shortcut from the vein (V) to the
stoma (*) in this Podocarpus
dispermis
Brodribb, T.J., Feild, T.S. and Jordan, G.J. (2007). Leaf maximum photosynthetic rate and venation are linked by hydraulics. Plant Physiol. 144: 1890-1898.
© 2014 American Society of Plant Biologists
Leaf and small branch xylem can act
as “disposable fuses”
Increased vulnerability to
embolism in small (< 6 mm
diameter) vs large (> 6 mm)
branches in Acer saccharum
Leaf
Stem
Increased vulnerability
to embolism in leaves
(●) vs stems (○) in two
tree species
Adapted from Tyree, M.T., Snyderman, D.A., Wilmot, T.R. and Machado, J.-L. (1991). Water relations and hydraulic architecture of a tropical tree (Schefflera morototoni) : Data, models, and a comparison with two
temperate species (Acer saccharum and Thuja occidentalis). Plant Physiol. 96: 1105-1113 and Tyree, M.T. and Ewers, F.W. (1991). The hydraulic architecture of trees and other woody plants. New Phytol. 119: 345-360.
Johnson, D.M., McCulloh, K.A., Meinzer, F.C., Woodruff, D.R. and Eissenstat, D.M. (2011). Hydraulic patterns and safety margins, from stem to stomata, in three eastern US tree species. Tree Physiol. 31: 659-668.
© 2014 American Society of Plant Biologists
In conifer tracheids, xylem collapse
in the needles protects stem xylem
100 μm
Stem
PLC
Xylem pressure 0MPa
Onset of
cavitation in
needles
Xylem pressure -5 MPa
Cochard, H., Froux, F., Mayr, S. and Coutand, C. (2004). Xylem wall collapse in water-stressed pine needles. Plant Physiol. 134: 401-408; See also
Brodribb, T.J. and Holbrook, N.M. (2005). Water stress deforms tracheids peripheral to the leaf vein of a tropical conifer. Plant Physiol. 137: 1139-1146.
© 2014 American Society of Plant Biologists
Kox: Water leaving the xylem passes
through bundle sheath cells
Water leaving the xylem moves into
bundle sheath cells, and from there into
the symplast or other cells. Bundle sheath
cells may contribute to controlling water
and solute flow into the leaf tissues.
Bundle
sheath
cells
Mesophyll cells
Image courtesy Biodiversity Heritage Library from Strasburger, E., Noll, F., Schenck, H., and Schimper, A.F.W. (1898). A Text-Book of Botany. Macmillan and Co.,
London. Kinsman, E.A. and Pyke, K.A. (1998). Bundle sheath cells and cell-specific plastid development in Arabidopsis leaves. Development. 125: 1815-1822.
© 2014 American Society of Plant Biologists
Aquaporins contribute to radial
conductance in the leaf
The flow of water from bundle
sheath cells to the stomata is
affected by aquaporin activities
Aquaporin activity regulates
resistance of the outside-ofxylem pathway
Heinen, R.B., Ye, Q. and Chaumont, F. (2009). Role of aquaporins in leaf physiology. J. Exp. Bot. 60: 2971-2985 by permission of Oxford University Press; Cochard, H., Venisse, J.-S., Barigah, T.S.,
Brunel, N., Herbette, S., Guilliot, A., Tyree, M.T. and Sakr, S. (2007). Putative role of aquaporins in variable hydraulic conductance of leaves in response to light. Plant Physiol. 143: 122-133.
© 2014 American Society of Plant Biologists
Guard cells are the gatekeepers of
transpiration
Water flow requires tension
produced by water exiting the leaf,
and water can only* exit the leaf
when the guard cells are open
Closed stomata – no
transpiration, no flow
(*with an effective cuticle very little
water is lost via non-stomatal
transpiration)
Stoma
As an analogy, it
doesn’t matter how
quickly you can jump
out of your airplane
seat, until the door
opens, you’re not
going anywhere
Open stomata – flow
regulated by other factors
© 2014 American Society of Plant Biologists
Guard cells are the portals through
which CO2 enters and H2O exits
CO2C
Sirichandra, C., Wasilewska, A., Vlad, F., Valon, C., and Leung, J. (2009a). The guard cell as a single-cell model towards understanding
drought tolerance and abscisic acid action. J. Exp. Bot. 60: 1439-1463. by permission of Oxford University Press.
© 2014 American Society of Plant Biologists
Guard cells movements are
controlled by ion currents
INNER WALL
K+
AK+
H2O
K+
A-
A-
K+
H2O
H2O
K+
K+
K+
A-
OPEN
A-
When signalled to close, ion channels in
the vacuolar and plasma membranes
open, releasing ions from the cell
AH2O
H2O
H2O
K+
A-
K+
A-
H2O K+
K+
H2O
H2O
K+
© 2014 American Society of Plant Biologists
Exiting ions draw water out of the
cell
INNER WALL
K+
H2O
H2O
K+ K+
A-
OPEN
K+
H2O
A-
Water follows
by osmosis
H2O
A- A-
H2O
K+
K+
K+
A© 2014 American Society of Plant Biologists
The guard cells lose turgor pressure
and relax, closing the pore
K+
H2O
A-
H2O
A-
CLOSING
H2O
H2O
K+
K+
H2O
A-
K+
K+
The cell volume shrinks and the cell
shape changes, closing the stomatal pore
© 2014 American Society of Plant Biologists
What regulates stomatal aperture?
CO2C
Guard cells respond to the
hormone abscisic acid (ABA),
light, and [CO2]
Guard cells also respond to hydraulic
signals and a change in ψw
Sirichandra, C., Wasilewska, A., Vlad, F., Valon, C., and Leung, J. (2009a). The guard cell as a single-cell model towards understanding
drought tolerance and abscisic acid action. J. Exp. Bot. 60: 1439-1463. by permission of Oxford University Press.
© 2014 American Society of Plant Biologists
Stomatal phylogeny, anatomy and
morphology affect function
Stomata
Stomatal crypts increase humidity
outside stomata, reducing flow
when the stomata are open
In some grasses,
subsidiary cells
participate in guard
cell movement and
make the pores
more efficient
Image James Mauseth, University of Texas; Franks, P.J. and Farquhar, G.D. (2007). The Mechanical diversity of stomata and its significance in gas-exchange control. Plant Physiol. 143: 78-87.
© 2014 American Society of Plant Biologists
From leaf to air: Summary
Water movement in
leaves occurs through
three distinct segments:
• Movement through the
vascular tissues
• Movement through the
non-vascular tissues
• Movement through the
stomata
© 2014 American Society of Plant Biologists
Drought, hydraulic failure and what
it all means
Plants are well adapted
for their own
environment, but may
not tolerate a drier one
As an example, two species
of Quercus (oak) showing
different sensitivities to water
deficit. Quercus robur is
drought sensitive and in
Europe shows a more
northern distribution
Total plant
water loss
Leaf
transpiration
Stomatal
conductance
CO2 assimilation
rate
From Urli, M., Porté, A.J., Cochard, H., Guengant, Y., Burlett, R. and Delzon, S. (2013).
Xylem embolism threshold for catastrophic hydraulic failure in angiosperm trees. Tree
Physiology. 33: 672-683 with permission of Oxford University Press.
© 2014 American Society of Plant Biologists
A disturbing and widespread
increase in tree mortality is
occurring
By Region
Pacific
Northwest
By Size
Interior
(Stem Diameter)
By Elevation
By Genus
California
From van Mantgem, P.J., Stephenson, N.L., Byrne, J.C., Daniels, L.D., Franklin, J.F., Fulé, P.Z., Harmon, M.E., Larson, A.J., Smith, J.M., Taylor, A.H. and
Veblen, T.T. (2009). Widespread increase of tree mortality rates in the Western United States. Science. 323: 521-524 Reprinted with permission from AAAS.
© 2014 American Society of Plant Biologists
This trend is taking place world-wide
Dots show locations
of documented
mortality that have
been associated with
drought since 1970
Examples shown in Korea,
China and Turkey
Reprinted from McDowell, N.G., Beerling, D.J., Breshears, D.D., Fisher, R.A., Raffa,
K.F. and Stitt, M. (2011). The interdependence of mechanisms underlying climatedriven vegetation mortality. Trends Ecol. Evol. 26: 523-532 with permission from
Elsevier, and Allen, C.D., et al., and Cobb, N. (2010). A global overview of drought and
heat-induced tree mortality reveals emerging climate change risks for forests. Forest
Ecol. Manage. 259: 660-684 with permission from Elsevier.
© 2014 American Society of Plant Biologists
Drought has pleiotropic effects and
increases vulnerability to pests
Tree deaths caused by bark beetle
The anticipated combination of drier and hotter
climate will be particularly detrimental to plants
Adapted from McDowell, N.G. (2011). Mechanisms linking drought, hydraulics, carbon metabolism, and vegetation mortality. Plant Physiol. 155: 1051-1059. Allen, C.D., et al., and Cobb, N. (2010). A global
overview of drought and heat-induced tree mortality reveals emerging climate change risks for forests. Forest Ecol. Manage. 259: 660-684. Dave Powell, USDA Forest Service, Bugwood.org
© 2014 American Society of Plant Biologists
Water Uptake and Transport:
Summary
From dynamic root growth
patterns, to
regulated aquaporin
channels,
with an amazing
integration of xylem
structure and function
and
sophisticated leaf anatomy,
the mechanisms by which
plants take up and transport
water are spectacular
Weaver, J.E. (1925). Investigations on the root habits of plants. Am. J. Bot. 12: 502-509 with permission from Botanical
Society of America; Maurel, C. and Chrispeels, M.J. (2001). Aquaporins. A molecular entry into plant water relations.
Plant Physiol. 125: 135-138; McCully, M.E. (1999). Root xylem embolisms and refilling. Relation to water potentials of
soil, roots, and leaves, and osmotic potentials of root xylem sap. Plant Physiol. 119: 1001-1008
© 2014 American Society of Plant Biologists
Methods used in plant-water
relations research
Transpiration rate be
measured with a
porometer, which
measures water vapor
release from the leaf
Transpiration rate can be
determined by water loss from
a sealed pot (by weighing), or
measuring water removed
from a source using a
potometer (“drink” meter)
Araus, J.L., Serret, M.D. and Edmeades, G. (2012). Phenotyping maize for adaptation to drought. Front. Physiol. 3: 305.Benedikt.Seidl
© 2014 American Society of Plant Biologists
Pressure to balance xylem tension
can be measured in pressure bomb
The Scholander
“pressure bomb”
The total water potential of leaf
cells is transduced into tension
at the cut surface
A single
xylem
conduit
showing
water
pulling
away
from the
cut
surface
Using the pressure bomb, the
internal tension of the xylem
can be measured by the
balancing pressure required
to extrude water from the cut
surface
Scholander, P.F., Hammel, H.T., Hemmingsen, E.A. and Bradstreet, E.D. (1964). Hydrostatic pressure and osmotic potential
in leaves of mangroves and some other plants. Proc. Natl. Acad. Sci. 52: 119-125; Peggy Greb, USDA.
© 2014 American Society of Plant Biologists
A pressure probe can also measure
xylem tension, but it is difficult
The probe records
tension when the
xylem is impaled
Wei, C., Tyree, M.T. and Steudle, E. (1999). Direct measurement of xylem pressure in leaves of intact maize plants. A test of the Cohesion-Tension Theory taking hydraulic architecture into consideration. Plant Physiol.
121: 1191-1205. See also Balling, A. and Zimmermann, U. (1990). Comparative measurements of the xylem pressure ofNicotiana plants by means of the pressure bomb and pressure probe. Planta. 182: 325-338.
© 2014 American Society of Plant Biologists
Flow rate can be measured by the
rate at which heat moves
Heat-based methods can
determine the rate of xylem
flow. The rate of heat
movement from a source is
correlated with flow rate
Plant Sensors
© 2014 American Society of Plant Biologists
Conductance can be measured as
flow through a segment
Conductance = Flow / Δ Pressure
For a given pressure, more flow (more
water passed through the segment)
indicates more conductance
Stem
segment
Flow calculated by volume
of water moving through
the segment
The height of the
water source can be
raised or lowered to
change the pressure
Balance to
weigh water
Cochard, H. (2002). A technique for measuring xylem hydraulic conductance under high negative pressures. Plant Cell Environ. 25: 815 – 819. Sperry, J.S. (1986). Relationship of xylem embolism to
xylem pressure potential, stomatal closure, and shoot morphology in the Palm Rhapis excelsa. Plant Physiol. 80: 110-116. Adapted from Sperry lab: http://bioweb.biology.utah.edu/sperry/methods.html
© 2014 American Society of Plant Biologists
Protoplast-swelling assay measures
cell membrane water conductance
How fast does the cell swell?
Wild-type cell
with many open
aquaporins
Loss-of-function of PIP1;2
The rate of volume
change in response to a
change in water potential
indicates membrane water
permeability
Loss-of-function of aquaporin
PIP1;2 decreases protoplast
water permeability (Pf), as
measured by the change in
cell volume
Ramahaleo, T., Morillon, R., Alexandre, J. and Lassalles, J.-P. (1999). Osmotic water permeability of isolated protoplasts. modifications during development. Plant Physiol. 119: 885-896;
Postaire, O., et al. and Maurel, C. (2010). A PIP1 aquaporin contributes to hydrostatic pressure-induced water transport in both the root and rosette of Arabidopsis. Plant Physiol. 152: 1418-1430.
© 2014 American Society of Plant Biologists
Percentage loss of conductivity can
be measured by lowering ψw
Dry habitats
Wet habitats
Salt water
(mangrove)
A comparison of
vulnerability curves for
six species from diverse
climates:
Ceanothus megacarpus
Juniperus virginiana (red cedar)
Rhizophora mangle (mangrove)
Acer saccharum (sugar maple)
Thuja occidentalis (white cedar)
Populus deltoides (cottonwood)
Plants from drier climates
tend to lose conductivity at
lower ψw, indicating less
vulnerability to embolism
formation
Adapted from Tyree, M.T., Davis, S.D., and Cochard, H. (1994). Biophysical perspectives of xylem evolution — Is there a
tradeoff of hydraulic efficiency for vulnerability to dysfunction? IAWA J. 15: 335 – 360 with permission of IAWA.
© 2014 American Society of Plant Biologists
% Loss xylem conductance
Increasing proportion
of cavitated conduits
Xylem ψw can be lowered by drying
or centrifugation
100
The original method involved letting the
segment slowly dehydrate. Flow was
measured repeatedly as the stem dried
and xylem water pressure decreased
50
0
-10 -8 -6 -4 -2 0
Xylem water pressure (MPa)
The spinning method allows
measurements to be carried
out more quickly. A stem
segment is rotated at various
speeds, exerting tension on
the xylem sap
Stem segments in
centrifuge rotor
Cochard, H. (2002). A technique for measuring xylem hydraulic conductance under high negative pressures. Plant Cell Environ. 25: 815 – 819. Sperry, J.S. (1986). Relationship of xylem embolism to xylem
pressure potential, stomatal closure, and shoot morphology in the palm Rhapis excelsa. Plant Physiol. 80: 110-116. See also Cochard, H., Badel, E., Herbette, S., Delzon, S., Choat, B. and Jansen, S. (2013).
Methods for measuring plant vulnerability to cavitation: a critical review. J. Exp. Bot.64: 4779 – 4791. Image courtesy of John Sperry, University of Utah.
© 2014 American Society of Plant Biologists
Maximal conductance can be
measured after refilling embolisms
Step 1. Measure
flow in partially
embolized segment
Step 2. Push water
through segment at
high pressure to
clear embolisms
Stem
segment
Step 3. Measure flow
in de-embolized
segments
Balance to
weigh water
Cochard, H. (2002). A technique for measuring xylem hydraulic conductance under high negative pressures. Plant Cell Environ. 25: 815 – 819. Sperry, J.S. (1986). Relationship of xylem embolism to
xylem pressure potential, stomatal closure, and shoot morphology in the Palm Rhapis excelsa. Plant Physiol. 80: 110-116. Adapted from Sperry lab: http://bioweb.biology.utah.edu/sperry/methods.html
© 2014 American Society of Plant Biologists
Embolism visualization by Cryo-SEM
and dye uptake studies
Cryo-SEM involves freezing
tissue and then slicing it to see
if air or ice is in the conduits
Ice
Ultrasonic
acoustic
emissions
– sound
produced
when
embolisms
form
Air
(indicates
embolism)
Dye is drawn into
functioning conduits
Dye does not enter
embolized conduits
Mayr, S., Cochard, H., Améglio, T. and Kikuta, S.B. (2007). Embolism formation during freezing in the wood of Picea abies. Plant Physiol. 143: 60-67.
© 2014 American Society of Plant Biologists
3D visualizations using high
resolution computed tomography
3D and 4D images of vascular tissues
can be obtained using X-rays.
2D scans are reconstructed into 3D
forms using a computer
Brodersen, C.R., McElrone, A.J., Choat, B., Matthews, M.A. and Shackel, K.A. (2010). The dynamics of embolism repair in xylem: In vivo visualizations using high-resolution computed tomography. Plant Physiol.
154: 1088-1095.Video stills from ALS-Micro-CT (http://youtu.be/-r_Ndy5zqoE?t=16s, http://www.youtube.com/watch?v=QpRx_dWtq1w) – See also Brodersen, C.R., Lee, E.F., Choat, B., Jansen, S., Phillips, R.J.,
Shackel, K.A., McElrone, A.J. and Matthews, M.A. (2011). Automated analysis of three-dimensional xylem networks using high-resolution computed tomography. New Phytol. 191: 1168-1179. Brodersen, C.R., Roark,
L.C. and Pittermann, J. (2012). The physiological implications of primary xylem organization in two ferns. Plant Cell Environ. 35: 1898-1911.
© 2014 American Society of Plant Biologists