24/01/2014 the american society of plant biologists · the american society of plant biologists ......

The Plant Cell, January 2014 © 2014The American Society of Plant Biologists

24/01/2014

www.plantcell.org/cgi/doi/10.1105/tpc.114.tt0114 1

© 2014 American Society of Plant Biologists

Plant-Water Relations (1)Uptake and Transport


Water is a major factor in plant distribution

Temperature

Sunlight Water

Yellow, orange, red and pink indicate that water is a key climate factor limiting plant growth

• Most plants experience occasional water stress

• Losses in growth and yield due to water stress are often cryptic because no unstressed controls exist

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: 660-684with 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.

Per

ce

nt

lea

f e

nla

rge

me

nt

15

10

5

Net

ph

oto

syn

thes

is

(mg

/ h

r–

100

cm2 )

30

20

10

Leaf water potential (bars)

Even mild water stress can affect plant growth


Plants have evolved several strategies to manage water

Many bryophytes can tolerate desiccation (i.e., drying to equilibrium with the atmosphere )

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

Most tracheophytes cannot tolerate desiccation –

they die

Photo credits: Mary Williams; Amrum; Scott Bauer; James Henderson, Golden Delight Honey, Bugwood.org


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 rootso Water movement through xylem

o The vulnerable pipelineo Water movement within leaves o Stomatal control of transpiration

• Hydraulic limits and drought• Methods


Brief history of the study of plant water relations

(1628)William Harvey

showed that blood circulates

Stephen Hales (1677 – 1761)

1727 Stephen Hales published experiments on the nature of water

transport in plants

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.


Malpighi and Grew saw plant conducting tissues (1670 - 1680s)

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.

Marcello Malpighi and Nehemiah Grew both recognized that plant vascular tissues might be conducting tissues, by analogy to similar-appearing structures in animals


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, 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.

Hales showed that transpired liquid is essentially water


Hales showed that water movement requires leaves exposed to air

“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.”

Water movement

Little water movement

Little water movement

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.


Dixon and Joly: The Cohesion-Tension theory (1890s)

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.

“… the all-sufficient cause of the elevation of the sap (is).. by exerting a simple tensile stress on the liquid in the conduits.”

Dixon and Joly (1895) suggested that water rising in the plant occurs by an upwards-pulling tension

Transpiration measured by weight of water removed from tube

The pressure in the chamber can be elevated to determine tensile strength


24/01/2014



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

Like Lego, water is very cohesive, meaning that the molecules stick together and a column of water can be pulled to a great height“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)

Dixon, H.H. and Joly, J. (1895). On the Ascent of Sap. Phil. Trans. Roy. Soc. London. (B.). 186: 563-576.


Water has many unusual and important properties

δ-

δ-δ-

δ-

δ+

δ+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

OH H

δ-

δ+δ+

-

+

-250

-150

-50

50

0

100Boiling pointMelting point

H2OCH4 NH3 HF

Water forms intermolecular

hydrogen bonds and so is very cohesive. It

sticks together and has unusually high boiling and melting

points

Water’s high dielectric constant means it is a good solvent for polar

compounds

Photo credit: NASA; See Nobel, P. (2009) Physicochemical and Environmental Plant Physiology. Academic Press.

Exobiologists usually consider water a prerequisite for life


Fick’s law describes the movement of water by diffusion

dx

dcAD

dt

dm

dm/dt = amount of substance moved per unit timeD = diffusion coefficient (property of substance and matrix)dc/dx = gradient of concentration (change in concentration per unitof distance, dx, in a direction perpendicular to the plane A)

A = area through which flow occurs

x

dcdx

[ ] [ ]Fick’s Law


Diffusion rate is affected by area, distance, and gradient

Diffusion is faster when:• The concentration

gradient dc/dx is steeper• The area A is larger• The distance x is smaller• D is larger

A = area through which flow occurs

x

dcdx

[ ] [ ]dx

dcAD

dt

dm


Diffusion rate is affected by the properties of material and solvent

The diffusion coefficient D is affected by: • Molecular shape• Molecular size• Molecular charge• Solvent viscosity• Solution properties as

salt concentration, pH, temperature etc.

dx

dcAD

dt

dm


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.


Osmotic forces drive the movement of water into and out of cells

Salt water

Fresh waterFresh water

Salt water

An animal cell might burst Plant cell walls prevent them from bursting

Cells have a lower osmotic potential than pure water, (because of the salts and proteins in them), so water moves into them

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


Mschel; Image 14869 CDC/ Nasheka Powell

Household water

pressure 0.3 MPa

Car tire pressure 0.25 MPa

Pressure required to blow up a balloon 0.01 MPa

Vacuum cleaner -0.02 MPa

(household)-0.1 MPa

(commercial)Laboratory

vacuum -0.01 MPa

Human blood pressure < 0.02 MPa

Inside typical plant cell 0.5 to 1.5 MPa

Pressure washer 15 MPa

Inside xylem:From +1 MPa to -3 MPa or lower

* These numbers are relative to atmospheric pressure (0.1 MPa), not absolute

Pressure can be positive or negative


Turgor pressure supports plant cells and tissues

Clemson University - USDA Cooperative Extension Slide Series ,R.L. Croissant, Bugwood.org

Young, non-woody tissues are supported

by turgor pressure

Water limitation lowers turgor pressure and

tissues wilt


24/01/2014



The water potential equation incorporates osmotic and pressure

potentials

Ψw = Ψp + Ψπ

The water potential (Ψw) is the sum of the pressure potential (Ψp) and the osmotic potential (Ψπ)The water

potential equation

Water flows from higher to lower water potential


Water moves towards lower water potential

Initial conditions: Final conditions:

Ψπ = 0Ψp = 0

Ψw = 0

+

Ψπ = -2Ψp = 0

Ψw = -2

+

Water moves towards lower water potential

Ψπ = 0Ψp = 0

Ψw = 0

+

Ψπ = -2Ψp = 2

Ψw = 0

+

Water is at equilibrium


Water moves by diffusion and bulk flow

ψw = -100 MPa

ψw =-0.3 MPa 1. Water

moves from soil to root

2. Water moves through plant

3. Water moves from leaf to air

Bulk flow: water in streams, tubes, hoses, water column of xylem (1, 2)

Diffusion: Random movement of individual molecules (1, 3)


Diffusion rate is affected by the properties of material and solvent

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

Cell membranes and walls increase the diffusion coefficient and can drastically lower the rate of diffusion


Long-distance water movement occurs by bulk flow

Plastic Boy; Poco a poco

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


Bulk flow through a tube is affected by the diameter of the tube

Where: ∆P = hydraulic pressure gradient η = viscosity of waterl = length of capillaries

N = number of capillariesr = radius of capillary

l8

rN 4

PFlow

Flow increases with the radius to the fourth power! Wider conduits are more conductive

Radius 40 μm Radius

20 μm4 x area required

Radius 10 μm 16 x area required

As the vessel radius decreases, the cross-sectional area needed for the

same flow increases

Each block conducts the same flow

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.

Poiseuille’s Law


Ohm’s law can model water flow, whether diffusion or bulk flow

I = V / ROhm’s Law

Current (I) = voltage (V) / resistance (R)

Tyree, M.T. (1997). The Cohesion-Tension theory of sap ascent: current controversies. J. Exp. Bot. 48: 1753-1765.

Ohm’s law as applied to water flow:Water flow = Difference in ψ / resistance

As the tap opens, resistance decreases and so flow increases

Municipal water pressure

Atmospheric pressure

The water potential difference (ψ) is the difference between water pressure and atmospheric pressure

Resistance is determined by the position of the tap


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.

This model uses conductance (K) which is the inverse of resistance: Higher conductance = higher flow

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

Flow rate is affected by conductance of the root, stem and shoot

K = 1 / R


Resistance and conductance are reciprocals & both affect flow

The resistance is higher in the sample with smaller-diameter conduits.

The conductance is higher in the sample with the larger-diameter conduits.

Example: Effect of conduit size on resistance

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

OPEN

CLOSED

Xylem anatomy affects flow by affecting resistance

Stomata affect flow by affecting resistance

Image: Decagon Services


24/01/2014



Water flow is governed by the conductance of 3 plant segments

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

2. Axial flow: Through the roots and shoots into the leaves via bulk flow through the conducting elements of the xylem

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


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.


Aquaporins are regulated membrane-bound water channels

Re

lati

ve v

olu

me

Cell with aquaporin

Cell without aquaporin

Cells with aquaporin

Cells without aquaporinPeter Agre was awarded the

Nobel Prize in 2003 for the discovery of aquaporins

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 Xenopusoocytes. 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.

Expression of an aquaporin in a Xenopusoocyte accelerates water uptake

Time (min)


Arabidopsis has 35 aquaporin genes in four families

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.

Plasma-membrane intrinsic proteins

Tonoplast intrinsic proteins

Nodulin-26-like intrinsic proteins

Small basic intrinsic proteins

Aquaporins have six membrane-spanning domains,

and typically form tetramers


Protoplast swelling assay of cell permeability and aquaporin activity

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.

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


Aquaporin activity is regulated at many different levels, including

transcriptional regulation

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.

mRNA abundance of aquaporin transcripts along length of Arabidopsis (At) and grape (Vv) roots


Aquaporin activities are also regulated post-transcriptionally

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.

Phosphorylation

Gating

Shuttling between internal and plasma membranes


Aquaporin channels are gated and can be open or closed

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.

Aquaporins are tetramers but each monomer forms a channel

Closed Open

Channel activity is modified by phosphorylation and pH


Aquaporins facilitate rapid plant movements

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.

Cells that need a high rate of water movement such as the motor cells of the sensitive plant can be enriched for aquaporins


24/01/2014



Aquaporin expression and activity affect hydraulic 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.

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


Stoma

Outer root layer

Endodermis

Vascular cylinder

Water moves through Soil – Plant –Atmosphere Continuum (SPAC)

2. Transport in the xylem

3. Transport through the leaf and into the air

1. Water uptake in roots


The driving force for water flow is evaporation powered by solar

energy

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

ψw = -100 MPa

ψw =-0.3 MPa 1. Water

moves from soil to root

2. Water moves through plant

3. Water moves from leaf to air


Soil properties affect the movement of water

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.

Large pore space (sandy soil)Gravitational pull dominates

Small pore space (clayey soil)Capillary action contributes to

lateral movement

Sand

Silt

Clay

Soils are made up of particles of various sizes, which affect

how water moves and is retained in them


Molecular interactions between water and soil affect water uptake

Increasing amount of water added to soil

Field capacity: The level above which water drains off

Wilting point: The point at which most plants wilt and will not recover at night


The type of soil particle (shape, size) affects soil interactions with water

Wilting point

Field capacity

Increasing water added to soil

Sand Loam ClaySandy loam

Clay loam

Silt loam

Decreasing particle size

2 mm .002 mm

Other factors that affect water uptake from soil include organic matter, microbes, and salinity

Adapted from Kramer, P.J., and Boyer, J.S. (1995). Water Relations of Plants and Soils. Academic Press. San Diego.


xylemrootinterfacerootsoil


RFlow

Endodermis

Vascular cylinder

Factors affecting water flow into the root:Ψw at soil-root interfaceΨw at root xylemR soil-root interface to root xylem

Ψsoil-root interface

Ψxylem Resistance of water path

R

SPAC segmentflow equation

Soil-Plant-Atmosphere Continuum 1. Water uptake in roots: From soil to stele


Under water deficit, root growth is maintained relative to shoot growth

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.

Water content:Percent of controlΨw (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


Water flow into roots is affected by root architecture and conductance

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.

Root architecture (length of roots, branching, angle) affects and is affected by water

Optimal architecture depends on soil type and water depth, as well as distribution of nutrients and microorganisms

Conductance is affected by root anatomy and morphology


24/01/2014



The distribution of root growth is affected by water availability

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.

Winter wheat grown in regions with different amounts of rainfall

26 – 32 inches

21 – 24 inches

16 – 19 inches

Shallower root systems can be more effective at catching limited rainfall

IrrigatedNonirrigated

Total root length at 0.6 to 0.8 m depth

Total root length at 0 to 0.2 m depth

1.2 to 1.4 m depth

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


Root growth can respond to water gradients (hydrotropism)

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.

Growth medium at the bottom of the plate has a very low water potential

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


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

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.

Axial conductance is affected by the number, diameter and structure of the xylem conduits, as well as formation of embolisms


Water likely moves from soil to stele through multiple paths

Adapted from Steudle, E. and Peterson, C.A. (1998). How does water get through roots? J. Exp. Bot. 49: 775-788.

Apoplastic path (through wall):No crossing of plasma membrane

Symplastic path (through plasmodesmata): Plasma membrane crossed once

Transcellular path:Plasma membrane crossed many times

plasmodesmata

wall


Conductance of cell walls and membranes affects water uptake

Adapted from Steudle, E. (2000). Water uptake by roots: effects of water deficit. J. Exp. Bot. 51: 1531-1542.

Key factors that may affect movement through the root cortex:• The endodermis,

exodermis and water impermeable Casparian strip

• Aquaporins (membrane-bound water channels)

• Plasmodesmata

Xylem

The exodermis and endodermis are cell layers with Casparian bands or strips that restrict their conductance of water and solutes

Plasmodesmata are small plasma membrane–linedcytoplasmic bridges connecting cells

Aquaporins are regulated water channels in cell membranes


The Casparian strip forces water to cross a plasma membrane

Photo credit Michael Clayton; Adapted from Grebe, M. (2011). Plant biology: Unveiling the Casparian strip. Nature. 473: 294-295.

Na+

Na+

OUT IN

Water can enter the root through the apoplast (cell wall spaces) until it reaches the Casparian strip, which blocks apoplastic water flow

Water forced to cross a plasma membrane is effectively filtered

stele

xylem

cortex

Casparian strip


The root endodermis acts as a filter for incoming water

Na+

Na+

Na+

Na+

Na+Na+

Na+

The endodermis produces a water-impermeable layer, the Casparian strip, that provides selectivity

Photo credit Michael Clayton


Robert Caspary (1865) recognized the significance of this barrier

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: 531-558; 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.

Cell A

Cell B

Casparian strip

Robert Caspary’s original 1865 drawing showing endodermal cells surrounded by the Casparian strip

stele

xylem

cortexEndodermis showing Casparian strip


Summary: Movement of water into and through roots



rFlow

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

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


24/01/2014



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”?

Soil – Plant – Atmosphere Continuum 2: Flow of water through the xylem


Transport in bryophytes – some specialized transport cells

Hydroids (h) in cross section

Water moves along external surfaces, within cells, between cells, and through specialized water-conducting cells or hydroids (h)

Bryophytes take up water from the air and substrate

Some bryophytes have food-conducting cells or leptoids (l)

The end walls between food-conducting cells can have enlarged plasmodesmata-

Micrographs 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.


Hollow conducting tissues allow greater height and CO2 assimilation

Bryophytes

Vascular tissue

Lycophytes (~ 1200 species)

Ferns(~ 13,000 species)

Gymnosperms (~ 1000 species)

Angiosperms (~ 350,000 species)

Seeds

Photos by Tom Donald(> 400 million years ago)


Fossil record: Xylem conduits became wider and more complex

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.

Tra

che

id d

iam

ete

r (μ

m)

0

50

100

150

420 395 380Million years ago

There also has been an increase in proportion of lignified, degradation-resistant cell wall material


The secondary walls are reinforced by lignin, a complex, hydrophobic polymer

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.

Lignin-strengthened cell walls are one of the major features that lets plants get big


Lignified xylem provides structural support for vascular plants

115

m S

eq

uo

ias

em

pe

rvir

en

s

Sydney Opera House 65 m Taj Mahal 65 m

Statue of Liberty 93 m

St. Paul’s Cathedral 111 m

The tallest living trees tower over many familiar monuments


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.


Tracheary elements have been described as “functional corpses”

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

Procambial cell

Secondary wall

deposition

Degradation of vacuole

and organelles

Mature tracheary element

Elongation


Most seed plants (and a few others) can produce secondary 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.

Seed plantsMost can produce secondary xylem

Ferns and relatives

Lycopods

+

-

+

+

Gymnosperms

Angio-sperms

Monocots

Most cannot produce secondary xylem, but there are exceptions

The vascular cambium is a lateral meristem that produces new xylem and phloem

Cambium

Phloem

Xylem

The xylem produced by the vascular cambium allows the plant to grow in the radial dimension and replaces older, less functional xylem


24/01/2014



The Wawona Tunnel Tree, Yosemite National Park

Yosemite Research Library, National Parks Service; Jim Bahn

General Sherman Tree(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 Sequoiadendrongiganteum, giant

sequoias


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.


Xylem anatomy: Hollow, thickened conducting cells

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.

Tracheids (gymnosperm) are narrow, up to 1 cm in length, and perforated by complex pit membranes

Vessels (angiosperm) are made from vessel elements, which are wide, short, perforated by simple pit membranes, and have open or perforated end walls

TracheidsLength0.1 – 1 cmDiameter5 -80 μm

VesselsLength1 cm - > 1 mDiameter15 - 500 μm

Vessel

Vessel element

Vessel

Vessel

Vessel


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.

Tracheids and vessels are quite different in size

Poiseuille’s Law: The flow of fluid through a tube scales to the fourth power of the radius of the tube (Flow ∝ r4)

Conifer tracheids are significantly shorter and narrower (but with some overlap in the width) than angiosperm vessels,

What other factors are involved in

hydraulic conductivity?


Xylem is a “vulnerable pipeline”, prone to embolism

©INRA/Hervé Cochard Used with permission

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


Cavitation describes a condition wherein an air bubble moves into a

vessels

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.

Embolized vesselPressure = 0 MPa

Sap-filled vesselPressure = -6 MPa


Embolisms can spread between vessels

Embolisms (dark; water-vapor filled conduits) in an intact grapevine stem, made visible by high-resolution computed tomagraphy

Early drought

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.


Pit membranes permit water flow and limit embolism spread

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.

ANGIOSPERM GYMNOSPERM

Uniform pit membrane

Central, impermeable

torus surrounded by

permeable margo


Some pits are more embolism-resistant than others

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.

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

Many desert plants have vestured pits with wall elaborations

Thick pit membranes can resist embolism movement

Embolism-sensitive

membrane pits


24/01/2014



Vulnerability curves characterize the sensitivity of a plant to embolism

Incr

ea

sin

g p

rop

ortio

n

of

cavi

tate

d c

on

duits

% L

oss

xyle

m c

ondu

ctan

ce

Xylem water pressure (MPa)0-2-4-6-8-10

0

50

100

Different species and tissues produce different “vulnerability curves”

Increasing tension (decreasing ψp)

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

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.


Many plants experience a seasonal loss and recovery of conductivity

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.

Freezing-induced embolisms decrease conductivity

Root pressure refills xylem and increases conductance

Spring activation of cambium increases conductance

Tem

p.

(°C

)

0

20

Xyl

emP

res

su

re(k

Pa)

0

30

Pe

rce

nta

ge

lo

ss

o

f h

ydra

uli

c

co

nd

uc

tivi

ty

20

40

0

60

100

80

Jan Feb Mar Apr May Jun

Winter –Freezing-induced embolismsEarly spring –Root-pressure refillingLate spring –New xylem formation


Vulnerability to embolism is ecologically relevant


Dry habitats Wet habitats

Salt water (mangrove)

Ψ50 is a measure of the plants’ sensitivity to embolism

In general, plants from drier climates tend to have lower Ψ50 and be more resistant to cavitation

Incr

ea

sin

g p

rop

ort

ion

o

f ca

vita

ted

co

nd

uits

Increasing tension (decreasing ψp)

A comparison of vulnerability curves for six species from diverse climates:Ceanothus megacarpusJuniperus virginiana (red cedar)Rhizophora mangle (mangrove)Acer saccharum (sugar maple)Thuja occidentalis (white cedar)Populus deltoides (cottonwood)


There is a correlation between habitat and cavitation resistance

Reprinted by permission from Macmillan Publishers Ltd. From Choat, B., et al. (2012). Global convergence in the vulnerability of forests to drought. Nature. 491: 752-755; See also Zanne et al (2014) Three keys to the radiation of angiosperms into freezing environments. Nature

Mean annual precipitation (mm)

Solomon Islands

MiamiTokyoParis

SingaporeJuneau, AlaskaLA

Cairo Min

imu

m x

yle

m p

res

su

re m

ea

su

red

u

nd

er n

atu

ral

con

dit

ion

s

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”


The environment affects xylem tension

Seawater Root cortex XylemEn

do

de

rmisNa+

Na+

Na+

Na+

90% of the salt in seawater is filtered out before it reaches the xylem

He hypothesized that the xylem pressure must be very low to draw water in against its concentration gradient

Scholander observed that in spite of their roots being immersed in seawater, the xylem sap of mangroves contains very little salt.

ψπ = -2.5 MPa ψπ = -2.5 MPa ψπ = -0.25 MPa

ROOT


Scholander showed that tension in the xylem drives water uptake


do

de

rmisNa+

Na+

Na+

Na+

ψπ = -2.5 MPaψp = 0 MPaψw = -2.5 MPa


ψπ = -0.25 MPaψp = -2.3 MPaψw = -2.55 MPa

ROOT


Tension in the xylem is balanced by solute accumulation in the leaves


do

de

rmisNa+

Na+

Na+

Na+

90% of the salt in seawater is filtered out before it reaches the xylem

ψπ = -2.6 MPaψp = 0.1 MPaψw = -2.7 MPa

Water in leaf cells usually has a lower osmotic potential than seawater!



ψπ = -0.25 MPaψp = -2.3 MPaψw = -2.55 MPa

ROOT


Does xylem conductance limit the height of trees?

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.

Ψw = Ψp + Ψπ + Ψg

Gravitational potential lowers Ψw by -0.01 MPa per m height

Predawn

Midday

Height above ground (m)

Xyl

em

pre

ss

ure

(M

Pa

)

Leaf morphology by height

Open question:Does water limitation and its effects on photosynthesis prevent trees from reaching more than ~130 m?


Xylem under pressure: Root pressure and grapevine refilling

Howard F. Schwartz, Colorado State University; Howard F. Schwartz, Colorado State University, Bugwood.org; Virtual Grape

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


24/01/2014



Hollow vessels fill with water in the spring due to root pressure

Scholander, P.F., Love, W.E., and Kanwisher, J.W. (1955). The rise of sap in tall grapevines. Plant Physiol. 30: 93 – 104.

Bla

ck b

ars

= v

ine

len

gth

Wh

ite

bar

s =

pre

ssu

re

In the spring, before the leaves emerge, the hollow vessels fill with water, driven by pressure generated in the roots

In the summer, when the leaves are out and the plant is transpiring, xylem pressure is negative (tension)

Pressures are so high, cut vines exude water copiously and are said to “bleed”


Stem pressure and the flow of sap in maple trees (Acer saccharum)

Photo credit: University of Minnesota; T. Davis Sydnor, The Ohio State, Bugwood.org

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?

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)


Clues to the source of maple sap

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

Maple sap flows in early spring, before leaves emerge, suggesting it is not driven by evaporative transpiration


As long as water is provided, sap flow does not require roots or crown

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.

Cut branches set onto dry surfaces, indicating that a source of water is required


Sap flow requires temperatures alternating above and below freezing

Tyree, M.T. (1983). Maple sap uptake, exudation, and pressure changes correlated with freezing exotherms and thawing endotherms. Plant Physiol. 73: 277-285.

Tem

p °

C

Flo

w r

ate

cm

3 / h

r

Tem

p °

C

Flo

w r

ate

cm

3 / h

r

Time, min

0

0

-5

-10

0

0-2

+5

-5

5

-5

5

0 15 30 45 60 75 90 105

As air temp drops below freezing, sap flows into the branches

As air temp rises above freezing, sap flows out of the branches

Experimental set up to monitor sap flow into an out of excised branch under varying temperature

Negative flow = into branch

Positive flow = out of branch


1983 sap flow model: Freezing compresses vapor in xylem fibers

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

The Milburn & O’Malley model

In sugar maple the xylem fibers surrounding the vessel elements are usually filled with air

Vessels

Fibers

Rays

Cooling:Vapor compresses, water drawn into fibers

Frozen: Ice traps vapor at high pressure

Warming: Thawing ice releases pressurized vapor and forces sap down

Fiber Ves

sel


2008 model: Contribution ofsugars in minimally pitted vessels

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.

Vessel element

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

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

Vessel elements without pits

Net influx of water generates pressure


Summary: Water flow through xylem, a low-resistance pathway

• Xylem evolution reflects trade-offs between safetyand efficiency

• Angiosperms sometimes have larger conduits

• Gymnosperms can move water from conduit-to-conduit more efficiently

• Preventing embolisms is critical to plant success


Soil – Plant – Atmosphere Continuum 3: From leaf to air

Stoma

Three elements to water flow in leaves:1. Through veins 2. From vein to stomata3. Out through stomata

1 23

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

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


24/01/2014



Leaf conductance (Kleaf): Xylem and out-of-xylem conductance

Stoma

Leaf conductance (Kleaf) is affected by xylem conductance (Kx) and out-of-xylem conductance (Kox)

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.

Kx is determined by xylem conduit diameter, abundance, anatomy, etc.

Kox is determined by the conductance of the non-vascular cells, the path length from vein to stomata, and stomatal density and conductance


Leaf xylem anatomy affects leaf hydraulic properties

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.

Dicot leaves with reticulate vein patterns, showing increasing vein densities

Monocot leaves with parallel veins at two different densities

Dichotomously branching veins of ferns


The distance from the end of the conducting cells to the air matters

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.

The high Kleaf of angiosperms is correlated with a short distance from the end of the vein xylem to the gas-exchange epidermis (Dm)

Sample measurements to calculate Dm

Angiosperms

Conifers

FernsBryophytes

Lyc

op

od

s


Water conducting sclereids shorten Dm and increase Kleaf

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.

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


Leaf and small branch xylem can act as “disposable fuses”

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.

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


In conifer tracheids, xylem collapse in the needles protects stem xylem

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.

Xylem pressure 0MPa

Xylem pressure -5 MPa

100 μm

Stem PLC

Onset of cavitation in needles


Kox: Water leaving the xylem passes through bundle sheath cells

Bundle sheath cells

Mesophyll cellsImage 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.

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.


Aquaporins contribute to radial conductance in the leaf

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.

Aquaporin activity regulates resistance of the outside-of-xylem pathway

The flow of water from bundle sheath cells to the stomata is affected by aquaporin activities


Guard cells are the gatekeepers of transpiration

Stoma

Water flow requires tension produced by water exiting the leaf, and water can only* exit the leaf when the guard cells are open (*with an effective cuticle very little water is lost via non-stomatal transpiration)

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

Closed stomata – no transpiration, no flow

Open stomata – flow regulated by other factors


24/01/2014



Guard cells are the portals through which CO2 enters and H2O exits

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.

CCO2


Guard cells movements are controlled by ion currents

INNER WALL

A-K+ K+K+

When signalled to close, ion channels in the vacuolar and plasma membranes open, releasing ions from the cell

K+

OPENK+

K+

K+

K+

K+

K+

A-

A-

A-

A-

A-

K+

K+

A-A-

H2O

H2O

H2O

H2O

H2O

H2OH2O

H2O

H2O


Exiting ions draw water out of the cell

INNER WALL

A-K+ K+ K+

H2OH2O

H2OWater follows by osmosis

OPEN

K+ A-K+ A-

K+A-H2O

K+A-H2O


A-K+ K+K+

H2OH2OH2O

CLOSING

The cell volume shrinks and the cell shape changes, closing the stomatal pore

K+A-H2O

K+A-H2O

The guard cells lose turgor pressure and relax, closing the pore


What regulates stomatal aperture?

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.

CCO2

Guard cells also respond to hydraulic signals and a change in ψw

Guard cells respond to the hormone abscisic acid (ABA), light, and [CO2]


Stomatal phylogeny, anatomy and morphology affect function

Stomatal crypts increase humidity outside stomata, reducing flow when the stomata are open

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.

In some grasses, subsidiary cells participate in guard cell movement and make the pores more efficient

Stomata


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


Drought, hydraulic failure and what it all means

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.

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

Plants are well adapted for their own environment, but may not tolerate a drier one


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.

A disturbing and widespread increase in tree mortality is

occurringBy Region By Elevation

By Size (Stem Diameter) By Genus

Pacific Northwest

California

Interior


24/01/2014



This trend is taking place world-wide

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 climate-driven 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.

Dots show locations of documented mortality that have been associated with drought since 1970

Examples shown in Korea, China and Turkey


Drought has pleiotropic effects and increases vulnerability to pests

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

Tree deaths caused by bark beetle

The anticipated combination of drier and hotter climate will be particularly detrimental to plants


Water Uptake and Transport: Summary

From dynamic root growth patterns, to

regulated aquaporin channels,

with an amazing integration of xylem

structure and functionand

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


Methods used in plant-water relations research

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

Transpiration rate be measured with a porometer, which measures water vapor release from the leaf


Pressure to balance xylem tension can be measured in pressure bomb

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.

The Scholander “pressure bomb”

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

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


A pressure probe can also measure xylem tension, but it is difficult

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.

The probe records tension when the xylem is impaled


Flow rate can be measured by the rate at which heat movesHeat-based methods can determine the rate of xylem flow. The rate of heat movement from a source is correlated with flow rate

Plant Sensors


Conductance can be measured as flow through a segment

Balance to weigh water

Stem segment

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

Conductance = Flow / ∆ PressureFor a given pressure, more flow (more water passed through the segment) indicates more conductance

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


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

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.

Loss-of-function of aquaporin PIP1;2 decreases protoplast water permeability (Pf), as measured by the change in cell volume


24/01/2014



Percentage loss of conductivity can be measured by lowering ψw

Dry habitats Wet habitats

Salt water (mangrove)

Plants from drier climates tend to lose conductivity at lower ψw, indicating less vulnerability to embolism formation


A comparison of vulnerability curves for six species from diverse climates:Ceanothus megacarpusJuniperus virginiana (red cedar)Rhizophora mangle (mangrove)Acer saccharum (sugar maple)Thuja occidentalis (white cedar)Populus deltoides (cottonwood)


Xylem ψw can be lowered by drying or centrifugation

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

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.

Incr

ea

sin

g p

rop

ort

ion

o

f ca

vita

ted

con

du

its

% L

oss

xyle

m c

ondu

ctan

ce

Xylem water pressure (MPa)0-2-4-6-8-10

0

50

100

The original method involved letting the segment slowly dehydrate. Flow was measured repeatedly as the stem dried and xylem water pressure decreased

Stem segments in centrifuge rotor


Maximal conductance can be measured after refilling embolisms

Balance to weigh water

Stem segment

Step 1. Measure flow in partially embolized segment

Step 2. Push water through segment at high pressure to clear embolisms

Step 3. Measure flow in de-embolized segments

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


Embolism visualization by Cryo-SEM and dye uptake studies

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.

Cryo-SEM involves freezing tissue and then slicing it to see

if air or ice is in the conduits

IceAir

(indicates embolism)

Dye does not enter embolized conduits

Dye is drawn into functioning conduits

Ultrasonic acoustic emissions – sound produced when embolisms form


3D visualizations using high resolution computed tomography

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.

3D and 4D images of vascular tissues can be obtained using X-rays.

2D scans are reconstructed into 3D forms using a computer

24/01/2014 the american society of plant biologists · the american society of plant biologists ......

Documents