Water flow in xylem

• Uptake and transport of water with dissolved nutrients is essential for the life of plants. In most land plants, water is continuously lost to the atmosphere and taken up from the soil.

• Water moves through the plant body partly by diffusion, by osmosis across membranes, but mainly by bulk flow in the xylem in response to a pressure difference.

• The water status of a plant and water transport are evaluated by water potential, which is the sum of solute concentration (osmotic potential) and hydrostatic pressure (turgor), and to a smaller extent matrix potential.

• The bulk flow is carried by conducting elements of xylem, vessel elements with perforation plates and partly by tracheids with pits.

• The ascent of xylem sap through xylem is explained by cohesion-tension properties of water and negative pressure generated by water evaporation from the leaf surface, unless the column of liquid is interrupted by air bubbles in a process known as cavitation or embolism.

• The driving force of transpiration is the difference in water vapor concentration in the leaf and in the surrounding air (together with diffusional resistance of this pathway).

Root hairs absorb water with nutrients (example radish)

Large surface area (density/cm² root surface in rye 25,000)

Q: What is the mechanism that drives water through the root, generating “root pressure”?

Water potential in cells

Water potential: free energy of water (potential to perform work), measured as chemical potential of water divided by partial molal volume of water

sSolute potential (osmotic potential): the effect of dissolved solutes on water potential, expressed as osmolality (moles of dissolved solutes per litre of water, mol L^-1)

 Pressure potential (hydrostatic pressure): Positive pressure, turgor; negative pressure, tension; unit MPa

_g Gravity and_m Matrix (minor components)

_w = _s + _p (+_g + _m)

Water potential of pure water is
0 MPa

It becomes more negative with

increasing solute concentration

Water transport mechanism 1. Diffusion along concentration gradient
Very slow, inversely proportional to the square of distance (eg. 50 m distance, 2.5 s; 1 m distance, 32 years)

Water transport mechanism 2. Osmosis (diffusion through semi-permeable membrane)
Water enters cells along a water potential gradient

Aquaporins facilitate
osmosis-driven water flow across membranes

Note: water flow is a passive process (along concentration gradient), although it may be coupled to active solute transport

Aquaporin
structure

Space-filling atomic structure
(left)

Ribbon diagram
(right)

Pathways of water uptake by roots:
Apoplastic: cell wall matrix (except Casparian strip)
Symplastic: via plasmodesmata
Transmembrane: across membranes and through cytoplasm

“Root pressure” is generated by accumulation of solutes (and consequently water) in xylem

• Roots absorb ions from soil solution and transport them into xylem

• A build-up of solutes in the xylem sap leads to lowering of osmotic potential (_s)

• Lowering xylem_s provides a driving force for water absorption, generating positive hydrostatic pressure

• This pressure results in the formation of droplets along leaf edges (“guttation”) when relative humidity is high; Similarly, xylem sap exudes from a cut stem near the soil surface

What drives water up the stem?

Water transport mechanism 3. Bulk flow
through xylem (accounts for >99% transport)
Figure from Strasburger ~1890, ‘vitalists’ x ‘physicists’
(see www.plantphys.net)

Cohesion-Tension Theory

Water rises by evapo-transpiration

Water evaporates from the pores in the clay cup or leaves and is replaced by water “pulled up” by capillary forces

Water flows through ‘tracheary’ elements

Vessels with annular or helical wall thickening


Differentiation involves programmed cell death

Differentiation of a tracheary element in Zinia

Tracheary elements
Typical vessels with open ends and pits in side walls

Types of tracheary elements
Vessel elements transport water by bulk flow; Transport in tracheids involves osmosis through pit membranes

Bulk flow in xylem vessels with perforated end walls (or open ends)

Xylem pits allow osmosis

Cavitation (air bubbles) forms by degassing of liquid under tension and by ‘air seeding’ from cell walls
Cavitation can be by-passed through pits; gas can dissolve back into solution at night

Negative pressure (tension) in xylem is generated by evapo-transpiration from cell surfaces

Driving force: Vapour Pressure Deficit between leaf interior and ambient air

Evaporation from moist cell walls increases the surface tension of water at the interface
Cell wall acts like a fine capillary wick soaked with water
As the water layer is depleted deeper into the wall, the radius of curved air-water interfaces decreases, increasing the surface tension of water

Bulk flow based on Cohesion-tension theory
can explain water transport up 120 m-tall trees


(Sequoia, Eucalyptus)

Measuring water transport in tall trees

Rope climbing, or cranes above tree-tops

Water potential of a twig can be measured with a pressure ‘bomb’
as gas pressure at which xylem exudate appears at the cut stem

With increasing height in a tree, xylem _w becomes more negative and photosynthesis decreases

Summary of Cohesion-Tension model

• Root pressure is too small (<0.1 MPa) to push water up a tall plant; Instead, large tension at the top pulls water up through xylem

• Water within a plant forms a continuous network of liquid columns in xylem from roots to leaves, forming hydraulic continuity, which depends on high tensile strength of water

• The driving force for water movement is generated by surface tension at the evaporating surfaces, forming small water menisci in pores among cellulose microfibrils, and lowering the _w of adjacent xylem elements (0.12 m radius can hold a column 120 m high)

• Typical _w values are -3 MPa in crop species, -4 MPa in tress, and -10 MPa in desert species

• Water flows through the xylem according to vapour-pressure deficit between intercellular spaces in leaves and ambient air: at large VPD, transpiration is high and xylem sap is under tension, cavitation may occur; at zero VPD transpiration stops and the root develops positive pressure (root pressure)