How do changes in the components of  w affect each other and the total value of  w ?

Turgor of living cells changes depending on solute concentration and total water potential. (.1M, or MPa) cell solution

Reducing cell volume concentrates solutes and reduces  S.

Water potential gradient tells us which direction water will move, but how do we understand how rapidly water moves? Several concepts for understanding speed of water flow 1.Diffusion rates 2.Bulk flow 3.Ideal flow through tubes: Hagen-Poiseuille equation 4.Hydraulic conductivity

1. Back to diffusion - net movement of molecules from regions of higher concentration to lower concentration. Diffusive flux = diffusion coefficient x concentration gradient J = -D s  C s /  x Fick’s first law J is flux rate, moles per m 2  C s /  x is the concentration gradient, moles m -3 /m D s is the diffusion coefficient, m 2 s -1

Values of D depend on the type of molecule and the medium Larger, heavier molecules have lower D. D values are higher in air than water D CO2 in air = 1.51 x m 2 s -1 D O2 in air = 1.95 x m 2 s -1 D H2O in air = 2.42 x m 2 s lower in water!

How effective is diffusion for transport across membranes? from roots to leaves? Diffusion time = L 2 /D s Double the distance means 4X the time. Compare 50µm membrane to 1 m long corn leaf D glu in water is m 2 s -1 (50 x m) 2 /10 -9 m 2 s -1 = 2.5 seconds!

(1 m) 2 /10 -9 m 2 s -1 = 10 9 seconds About 32 years! So how does water move long distances through plants? Bulk flow

Let’s build an equation that describes the various influences on the rate of liquid moving through a straw. “Volume flow rate” m 3 s -1 =

Hagen - Poiseuille Equation m 3 s -1 =  r 4  P 8   x

Water flow in xylem “pipes” Pressure gradients Diameter of tracheids or vessel elements The viscosity of xylem fluid - does it vary?

Conductive Vessel Element in Mountain Mahogany Wood (SEM x750). This image is copyright Dennis Kunkel at

Water movement in the soil-plant-atmosphere continuum. Chapter 4 Water moves from higher to lower water potential, so  atmos <  leaf <  stem <  root <  soil Fig. 4.1

Soil water potential

Soil water adheres to soil particles of different sizes and kinds. This adhesion represents a “tension”, or  P < 0. In most soil solutions, solutes are dilute so  S ≈ 0. Exceptions: saline soils, salt marshes.  W =  S +  P +  g  S ≈ 0  P < 0  g ≈ 0 So, for most soils  W =  P Fig. 4.2

What determines the value of  w (  P ) of soils?

What dries out faster, a bucket of sand or a bucket of clay? Why?

Soils differ in characteristic particle size. How might particle size affect soil water potential?

The more contact a volume of water has with the soil surface, the greater the tension with which it is held.

Water is held more tightly in small crevices.  P = -2T/r  Where r = radius (m) of curvature of meniscus, and  T = the surface tension of water,7.28 x MPa m r1r1 r2r2

 P = -2T/r 1.As soils dry, water is held in small pore spaces (r decreases) so soil water potential decreases 2.Soils with smaller characteristic particle size (e.g. clay vs. sand) tend to have lower water potential. 3.More difficult for plants to extract water from clay than sand

.  P = -2T/r

Example: calculate  P for r = 1 x m and 1 x m. About -0.15MPa for 1µm, and -1.5 MPa for 0.1 µm.

Getting water from the soil into the plant.  root <  soil What is the pathway for water movement into the xylem of the roots?

Water can travel from the soil to the root xylem by two distinct pathways - the symplastic and apoplastic pathways. Fig. 4.3

The less-suberized growing tips of roots have higher water uptake rates than older portions of the root. Fig. 4.4

What is the pathway for water movement from roots to leaves?

Water flows from roots to leaves via the xylem, a network of specialized cells called tracheary elements. Gymnosperms have tracheids. Angiosperms have vessel elements & sometimes tracheids. Note special anatomical features. Fig. 4.6

Xylem cavitation Embolisms that stop water transport can form in tracheary elements when xylem pressure is sufficiently negative to pull in air through a pit. Fig. 4.7

May 17, 2003 North of San Francisco Peaks

September 20, 2003 North of San Francisco Peaks PJ Woodland Juniper Woodland

The xylem network is extremely intricate in leaves. Fig. 4.8

OK, we’ve got water from the soil, into the roots, and up to to the leaves. Where does water evaporate inside leaves? How does water at sites of evaporation have a lower water potential than xylem “upstream”?

The wet walls of leaf cells are the sites of evaporation. Fig. 4.9

As for soils, a more negative  P develops as leaf cell walls dehydrate and water is held in smaller pore spaces. Fig. 4.9  P = -2T/r

Putting it all together: A model for water movement through the plant. The Cohesion-Tension Model

The most widely accepted model of water transport through the xylem is the “cohesion-tension model”. (note web essay) 1.A negative pressure or tension is generated in leaf cell walls by evaporation (transpiration). 2. The cohesive property of water means this tension is transmitted to water in adjacent xylem and throughout the plant to the roots and soil.