Water transport in the xylem• In most plants, the xylem

constitutes the longest part of the whole plant water transport pathway

• For a 1 m tall, more than 99.5% of the water transport pathway will occur within the xylem.

• Compared to the root, xylem water transport is a fairly simple low resistance transport pathway.

• We will be considering now:– The role of negative pressures

in xylem transport– And the importance of xylem

transport safety under negative pressure

Negative or positive pressure driven water flow?

• Pressure gradients needed to move water through the xylem could come about in two ways:– Positive pressures- hypothetically positive pressure could drive water

to the tops of trees, but • root pressures are generally less than 0.1 MPa (lifts water only 1

m above the ground) and pressures of at least 2 MPa are requiredto supply some of the tallest trees on the planet

• Root pressure is energetically expensive...• Also, evaporation will easily collapse positive pressure

gradients...• Negative pressures - Solar power! Instead plant’s use the evaporative

power of solar energy to pull water through the vascular system.• So instead, water at the top’s of trees develops a large tension to pull

water through the xylem

Cohesion-tension theory of sap ascent• The well-supported theory of sap

ascent is that water moves up plants under tension

• Requires that the cohesive properties of water sustain large tensions in the xylem water column

• Despite the attractiveness of the theory, still continues to generate controversy; mainly along whether tension exists in the xylem.

• Pressure probe studies failing to find high tensions

• Later refinements

Xylem water transport under tension is risky

• The large tensions that develop in the xylem of trees can create some problems

• First, tension results in an INWARD pull on the tracheary cell system. The development of lignified walls is necessary to allow resistance to implosion from this force...

Metastable state of water under tension; air entry problems

• A second problem is that water is METASTABLE and very sensitive to slight changes in gas-content

• Recall that pure degassed water is very strong, but with gas added, the water column can become increasingly easily broken

• As tensions increase, there is an increased tendency for air to be pulled in from microscopic pores in the xylem wall that contain air (from respiring living cells or just close to lenticels etc.)

• This is called air-seeding. • Another way that air can be introduced into the xylem is when the

sap freezes (bubbles form) and if this occurs when the xylem is under tension, then after thawing the bubbles will expand under tension

Embolism or cavitation

• This phenomenon of bubble formation is called cavitation or embolism.

• Analogous to a vapor lock in a fuel line or an air bubble in a blood vessel

• Breaks like this in the water column are not all that unusual• However, these breaks in xylem water continuity, if not repaired,

end up blocking xylem water transport in the plant

Cavitation blocks xylem water transport

• Note that water can detour, but as the number of blocked vessels or tracheids increases, transpirational surfaces of the plant are supplied with less and less water.

Plants attempt to minimize the consequences of xylem cavitation

• Gas bubbles can be eliminated from the xylem– At night by root pressure, by dissolving gas back into solution

of the xylem– Recent data suggests that bubbles may be collapsed even

under tension; but mechanism is not known. • Or, plants may “grow out” of the effects of cavitation; add new

sapwood to replace older embolized vessels; we see this in ring-porous plants where the early wood vessels become blocked and are not refilled year to year

Root pressure and in vivo embolism refilling

• From Holbrook et al. (2001) Plant Physiology

Water evaporation in the leaf generates a negative pressure on the xylem

• IMPORTANT: The tensions need to pull water through the xylem are the result of evaporation of water from leaves.

• This tension develops at the surface of the cell walls in the leaf that are in contact with the air.

• The analogous to the situation in soil

As evaporation increases and leaf water content declines, the motive force for water transport increases

• Curved water surfaces at the cellulose microfibrils in the leaf cells become smaller and smaller.

• The Ψp can be estimated as: Ψp = -2 T/r– where T is surface tension of

water (7.28 x 10-8 MPa m) and r is the radius of the curvature at an air water interface

Note how the curvature changes

Water movement from the leaf to the atmosphere

• After water has evaporated from the leaf, diffusion is the primary means of water movement out of the leaf.

• The rate of diffusion out the leaf is controlled by three interacting pathways– cuticular– stomatal– boundary layer

Diffusion and the water vapor concentration gradient

• Because movement of water out of the leaf is based on diffusion, the gradient in water vapor concentration from inside the leaf intercellular air spaces to outside controls the rate of diffusion.

• The atmosphere gradient is strongly dependent on temperature

Diffusional water loss regulated resistors along the pathway

• Cuticular– direct diffusion through

the cuticle, generally very small (~ 5% of total flux)

• Stomatal and boundary layer– These two processes

interact, stomata generally control 95% of the water loss by changes in their aperture; but the boundary layer can become important

Stomatal control and non-control of leaf

water loss rate

Measurements of leaf water loss rates

When can boundary layer resistance override stomatal resistance to leaf

water loss?• Still air condition• Plants with big leaves• Plants that have stomata covered or embedded (sunken deep) in

the leaf– generally thought to be drought-avoid structures– But remember waxy stomatal plugs in Drimys?

Stomatal function

• Guard cells function as multisensory hydraulic valves– respond to light,

temperature, humidity, intercellular CO2, and water potential of the leaf

– An increase in guard cell turgor causes stomatal pores to open.

Cellulose microfibril orientation and guard cell bending

• A distinctive feature of guard cells are their cell wall structure

• Portions of the wall are thickened

• Also, the alignment of cellulose microfibrils, which recall determine cell shape, are oriented radially and this plays an important role in their movement

• cellulose microfibrils are oriented like steel-belts in a radial tire

Overview of the soil-plant-atmospheric continuum

OK, a brief word about phloem transport: the pressure flow hypothesis

• In lab 8