EFB530 Plant Physiology

Water and Water Potential

Properties of water

• water is polar, which makes it an excellent solvent (H bonds)
• high heat of vaporization (energy to move from liquid to gas - the highest of any liquid)
• water has cohesion (attraction between water mols.) and adhesion (to surface)
• surface tension to minimize air-water surface area
• capillarity=movement up a tube due to adhesion, cohesion, surface tension
• tensile strength due to cohesion (force to pull water apart)

Chemical potential

m = m^* + 2.3RTlogC + zFE + VP + mgh

• when talking about water, we can eliminate the term referring to electrical charge

Water potential = m - m^* , then solved for V, =Y

• replace the concentration term with Y_s = osmotic potential = -RTCi
• the pressure term becomes Y_p = hydrostatic pressure or pressure potential (turgor pressure when positive; tension when negative)
• gravity becomes Y_g = gravity potential

Y_w = Y_s + Y_p + Y_g

expressed in terms of pressure (MPa, 1 MPa=10 atm)

Water will move from regions of higher water potential (less negative) to regions of lower water potential (more negative)

What are some values and how do these formulas help us to analyze water movement?

pure water: Y_s =0 MPa (osmotic potential), open to the atmosphere, Y_p =0 MPa

therefore, Y_w = Y_s + Y_p = 0

add sucrose to 0.1 M (add 0.1 mol to 1 L of water):
Y_s = -0.244 MPa, Y_p =0 MPa, therefore Y_w = - 0.244 MPa + 0 = -0.244 MPa

flaccid cell (no internal pressure, no turgor pressure):
solute concentration of its contents=0.3 M, so Y_s = -0.732 MPa; Y_p = 0 MPa
therefore Y_w = - 0.732 MPa + 0 = -0.732 MPa

put this cell into the beaker with 0.1 M sucrose:
there is a DY_w: Y_w(outside) - Y_w(inside) = 0.488 MPa, a tiny amount of water will flow to the more negative side (which is inside the cell)
as water flows in:

• the volume will increase slightly
• hydrostatic pressure will push against the cell wall (increasing Y_p)

this will continue until DY_w =0 (the cell is in equilibrium with the sucrose)

• Y_w(cell) = Y_w(soln) = -0.244 MPa

assume very little water entered the cell, so the internal concentration (Y_s) didn't change

• Y_p = Y_w - Y_s = -0.244 - (-0.732) = 0.488 MPa
  

transfer the cell to a 0.3 M solution of sucrose:
Y_w(soln) = -0.732 MPa
Y_w(cell) = -0.244 MPa
a very tiny amount of water will flow out, cell volume will decrease, and Y_p will decrease
Y_w(soln) = Y_w(cell) = -0.732 MPa, therefore Y_p = Y_w - Y_s = -0.732 - (-0.732) = 0 MPa

apply pressure to the cell, reducing its volume, doubling the concentration (doubling Y_s =-1.464)
in equilibrium with a 0.1 M solution Y_w = -0.244
therefore Y_p = -0.244 - (-1.464) = 1.22 MPa

Transport

1) Diffusion

• Fick's Law J_s=-D_s (DC_s / Dx)

J_s=flux (mol m^-2 s^-1)
D_s=diffusion coefficient (how easily substance moves)
DC_s=concentration gradient
Dx=distance between two points

diffusion is . . .

• fast over short distances, slow over long distances
• driven by concentration gradient (concentration difference over a distance)

• the time a molecule takes to diffuse is proportional to x²
• for a small molecule (glucose) to cross a cell=2.5 sec, to cross a corn leaf (1 m)=32 years

2) Bulk flow

• movement driven by a pressure gradient (like water through a garden hose)
• dependent on radius of the pipe to the fourth power, viscosity, & the pressure gradient

Pressure-driven bulk flow is the main mechanism for water movement in the xylem and in the soil

3) Osmosis

• movement of a solvent across a biological membrane (selectively permeable)
• movement is driven by the sum of a concentration gradient and pressure gradient
• total force=free energy gradient of water=water potential gradient
• water can move across the lipid bilayer portion of a membrane or many membranes have water-specific channel proteins that facilitate osmosis (called aquaporins)

resistance to flow: conductance = 1/resistance

ie resistance of a membrane= hydraulic conductivity (L_p) = permeability of the membrane

total conductance of a membrane = L = area x L_p

therefore flow rate = L x DY_w

bulk flow through a pipe

• rate depends on frictional drag against the walls of the pipe, also viscosity
• resistance is dependent on length and radius {R=(1/r⁴)(length)(viscosity)}

Conductivity

We have been comparing cell to cell, but we can apply these principles to whole tissues, if we consider the limitations

Plant water status

Y_w serves 2 functions:

• 1) to predict patterns of H₂O movement
• 2) monitor plant H₂O status

1) It is the DY that drives water movement through plants

• this is a result of physical forces, no direct energy input into moving water

2) Measurements of Y_w give a measure of water status

leaves of well-watered plants: Y_w = -0.2 - -0.6 MPa
leaves of plants in arid climates Y_w = -2 - -5 MPa

1 atm = 0.1 MPa = 14.7 psi
car tire inflated to 30 psi = 0.2 MPa
home water pressure = 40-50 psi =0.3 MPa

Plants can change the Y_s of the cell

• there is an upper limit ~-0.5 MPa; typically Y_s =-0.8 - -1.2 MPa
• can increase osmotic concentration (make Y_s more negative) in order to lower Y_w to allow the plant to extract water
• in response to drought stress or in halophytes
• plants often synthesize higher levels of proline, glycine betaine, or sorbitol as a stable osmoticum (also called compatible solutes)

Values for Y_p are rarely fully equal (but opposite) to values for Y_s

• therefore Y_w is always negative (or zero) for living cells & tissues

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