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 5 atm 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 µm (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

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

•  Root

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

•  Leaf

•  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.

•  Tracheids

•  Present in all vascular plants

•  Elongate cells with tapering end walls

•  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

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

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.

•  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

•  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.

•  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.

•  Some bordered pits (especially in gymnosperms) have a torus that acts as a check valve preventing spread of air.

•  Micrographs of bordered pits (with tori) in tracheids

•  Face view

•  Cross sectional view

Delta State University > DSU Sciences > Dr. Tiftickjian > Plant Physiology > Xylem Transport