Dr. T logo BIO 405/505 Plant Physiology
Xylem Transport

The problem

•  The tallest plants, such as Sequoias, can be as tall as 100 meters or more. However, the barometric height of water is only about 10 meters. This means that a simple suction pump explanation of xylem water movement will not "hold water." (pun intended)

Possible mechanisms

•  Root pressure forces water up the xylem

•  What is root pressure?

•  Pressure can develop in the xylem due to osmosis in roots. Water moves into the vascular tissue across the endodermis building up pressure that forces water up through the xylem.

•  In some plants this causes guttation

•  Guttation is the loss of liquid water from vein endings in leaves due to root pressure.

•  Problems with the root pressure model

•  Does not occur in all plant species

•  Happens only when soil moisture is high and transpiration is low.

•   But water flow is most rapid when transpiration is high.

•  Pressure is typically 0.5 MPa (5 bars) or less.

•  This is not great enough to move water to the height of a tall tree

•  Water in the xylem is usually under tension, not pressure

•  Capillary action creates a force that pulls water up xylem.

•  What is capillary action?

•  Liquids can be drawn up in small tubes because of the high surface tension of the air/water interface.

•  Problems with the capillary action explanation

•  Not strong enough

•  With a tube diameter of 80 ┬Ám (the diameter of a typical tracheid), water is only drawn up 38 cm.

•  No air/water interface

•   Xylem cells are completely full of water so there is no interface at which surface tension can develop.

•  Water may actively "pumped."

•  This hypothesis suggests that living cells can create a positive pressure in the xylem by some kind of active transport utilizing cellular energy.

•  Problems

•  Tracheary elements are dead when functional

•  Living cells of a stem can be killed, but transport will still occur.

•  However, the leaf cells must remain alive.

•  Direct measurements show that water in the xylem is usually under tension (negative pressure), not positive pressure.

Anatomy of xylem

•  Stem

Helianthus stem, x.s.
Cross section of Helianthus (sunflower) stem with major tissues labeled
Micrograph by Biodisc

•  In stems, xylem and phloem occur together in vascular bundles.

•  Root

Ranunculus root, x.s.
Cross section of Ranunculus root
Micrograph by Biodisc

•  In most roots, the vascular system is a single strand with xylem at the center.

•  Leaf

Ligustrum leaf, x.s.
Cross section of a typical dicot leaf. Note upper and lower epidermis, palisade and spongy mesophyll, veins, stomata.
Micrograph by John Tiftickjian

•  In the leaf, the vascular bundles are called veins.

•  Each vein contains xylem and phloem.

Tracheary elements

•  Transport of water in the xylem occurs in elongate cylindrical cells known as tracheary elements.

•  Tracheary elements are dead when mature. The lumen provides a tube that water can move through.

•  The tube-like form can be easily seen in longitudinal sections of vascular bundles.

Helianthus stem, l.s.
Helianthus stem, l.s. showing xylem, phloem, and fibers
Micrograph by John Tiftickjian

•  Tracheids

•  Present in all vascular plants

•  Elongate cells with tapering end walls

Pinus wood
Transverse, radial, and tangential sections of pine wood
Micrograph by Biodisc

•  Water moves from from cell to cell through pits.

•  Tracheids have continuous cell walls (no perforations)

•  Vessel members

•  Found in nearly all angiosperms and rarely in a few other taxa.

•  Generally shorter and wider than tracheids

•  Vessel members stack up in vertical series called vessels.

•  The end walls of each vessel member contain openings (perforation plates)

•  Perforation plates are complete openings (not pits)

•  Before cell dies, end walls are digested to produce perforations.

•  Vessels in cross section and longitudinal views

Quercus wood
Transverse, radial, and tangential sections of wood oak wood
Micrograph by Biodisc

•  Perforation plates can be clearly seen in a whole vessel member from macerated wood.

Quercus vessel member
Quercus (oak) wood maceration showing a vessel member.
Micrograph by John Tiftickjian

Mechanism of transport: cohesion/tension

•  Transport depends on water potential gradient

•  Development of water potential gradient in the xylem

•  Water evaporates from leaf (transpiration).

•  As water leaves mesophyll cells, their solute potentials and pressure potentials decrease.

•  Xylem cells now have higher water potential than leaf cells so water moves by osmosis from xylem to leaf cells.

•  Pressure potential in xylem decreases and becomes negative (negative pressure = tension)

•  Tension is transmitted from the leaf xylem to the root xylem through the continuous water column that fills the tracheary elements.

•  Tension in root xylem reduces water potential below that of root parenchyma cells so water enters xylem from parenchyma cells.

•  Water enters root cells from the soil.

•  Summary of cohesion-tension mechanism

Xylem transport occurs because a series of events occurs that result in a water potential gradient being formed from the leaves, through the xylem, to the roots. Move the cursor over the numbered steps to reveal what is happening in each location in order for the water potential gradient to develop.

•  So flow of water depends on:

•  Tension developed by osmosis.

•  Cohesion and adhesion of water molecules

•  These forces are possible because of water's ability to form numerous hydrogen bonds.

•  Cohesion - attraction of water molecules to each other

•  Adhesion - attraction of water molecules to cell walls

•  Cohesive and adhesive forces must be strong enough to keep water columns intact. If water columns break (cavitation), then water flow stops.

•  Structure of the tracheary elements must be such that they:

•  Resist collapse under tension

•  Provide an open channel for water to move through

•  Present as little resistance to water movement as possible

•  Prevent the spread of embolisms (air bubbles) if cavitation occurs

•  Structure of tracheary elements is a compromise related to these physiological requirements.

•  As tracheary element diameter increases:
•  Resistance to water flow decreases - GOOD
•  Tension that can be maintained decreases (cavitation more likely) - BAD
•  As cell wall thickness increases:
•  Resistance to collapse increases - GOOD
•  More energy expended to produce wall materials - BAD
•  Vessel elements completely remove their end walls (discussed below)
•  Resistance to water flow decreases - GOOD
•  If cavitation happens, all cells in the whole vessel stop working - BAD

Evidence for the cohesion/tension mechanism

•  Tension can be demonstrated in the xylem

•  The amount of tension can be measured with a pressure bomb.

•  Tension is greatest during the day when transpiration is most rapid

•  Cohesive force is strong enough to withstand tensions greater than those that develop

•  Z-tube experiments have show that water can withstand tensions of more than 200 atm (20 MPa)

Cavitation and embolisms

•  When gas bubbles form in a xylem element, they may coalesce into larger bubbles. This bubble formation is called cavitation.

•  The large bubbles caused by cavitation are called embolisms.

•  Embolisms cause a loss of tension and stop water transport in any tracheids or vessels in which they occur.

•  Embolisms happen during times of water stress when xylem tensions are high.

•  Mechanisms of reducing the effects of embolisms.

•  Restricting spread of embolisms - pits (discussed below)

•  Reformation of water columns by root pressure

•  Production of new xylem to replace older non-functioning xylem

Pits and cavitation control

•  Remember that pits are openings in the secondary wall between two adjacent cells.

•  Simple pits

Simple pit
A simple pit seen in cross-sectional view.

•  The pit appears as a simple hole in face view.

•  Pits help prevent spread of embolisms.

•  Because of their small diameter, it is difficult for air to pass through from one cell to the next.

•  The high surface tension resist the air bubble deforming enough to squeeze through the opening.

•  Bordered pits are most effective in preventing embolism spread.

•  Bordered pits

•  Secondary wall arches over forming a pit chamber. In face view, this area appears as a border around the inner aperture.

Bordered pit
A bordered pit seen in cross-sectional view. [Drawing by John Tiftickjian]

•  Outer pit aperture is wide and gives more surface area for water to move through

•  Inner pit aperture is narrow making it difficult for air to pass through.

•  Bordered pits in conifers are even more elaborate. The pit membrane is swollen to form the torus.

Bordered pit with torus
Drawing of bordered pit with torus, common in conifers. The middle of the pit membrane is modified to form the torus.
Drawing by John Tiftickjian

•  When an embolism forms in the tracheid on one side of the pit, the tension of the water on the uncavitated side pulls the torus against the border blocking the flow of air into the tracheid that is still functional.

Aspirated bordered pit
The valve-like action of the torus in a bordered pit prevents the spread of cavitation.
Drawing by John Tiftickjian

•  Micrographs of bordered pits (with tori) in tracheids

•  Face view

Bordered pits, face
Circular bordered pits of pine tracheids seen in face view
Micrograph by John Tiftickjian

•  Sectional view

Bordered pits, section
Bordered pits of pine tracheids seen in cross sectional view
Micrograph by John Tiftickjian

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