Water Potential

Contents

Water potential

I hated this particular topic when I was at university and school, as I was told about a million different and conflicting things by different and conflicting people, so here's what I hope is the definitive version of water potential…

Plants need water.

A fairly obvious statement, but important nonetheless. They need it as a solvent, to dissolve chemicals; as a chemical reactant in hydrolysis reactions; for support (by keeping cells turgid); to increase their size without investing in cytoplasm (by having large, cheap, fluid-filled vacuoles); and last, but not least to 'pay for' water lost by the transpiration that is made necessary by having to absorb CO2 for photosynthesis.

Water potentials (ψ) are a way of measuring the free-energy (G) of water. Water will flow spontaneously from a high potential to a low potential, like a ball rolling down a hill.

Water flows down its potential gradient.

Water potentials are usually negative, so we're talking about going from e.g. −1 to −4, rather than 4 to 1 in most cases.

Most plants get their water from the soil. It enters the roots and moves through the plant down a gradient of water potential. The water potential at any given point, ψ, is the combined effect of all the factors that make water move. Like any chemical, water moves because of four factors:

These factors sum together to give the overall free energy of a mass of water. For historical and practical reasons, the potential of water is not generally given as a Gibbs free energy value (a value of G or similar in kJ mol−1), instead it is given as a pressure (a value of ψ in MPa), which is called the water potential. The conversion factor is simply the molar volume of water:

ψ = G ⁄ Vw

where Vw is the molar volume of water, 18 × 10−6 m3 mol−1.

So, water potential is simply free energy expressed in MPa rather than J mol−1. Water will flow spontaneously from high potential to low potential. In the same way that chemical reactions are spontaneous if ∆G (Gafter − Gbefore) is negative, water flow is spontaneous if ∆ψ (ψafter - ψbefore) is negative.

Like free energy and enthalpy, we cannot easily define an absolute value of water potential. Instead, we make a relative measurement to the water potential of pure water (ψ0). At 25OC and 101 325 Pa (STP), the potential of pure water is arbitrarily defined as zero. In biological systems water potential is seldom zero and its value is due to five main components.

ψ = ψ0 + ψp + ψπ + ψg + ψv + ψm

1. Pressure potential (ψp).

ψp = Pabs − Patm

Pressure potential is the difference between the absolute pressure in the water and atmospheric pressure: since the reference potential ψ0 is defined for water at STP, only pressure differences from atmospheric contribute to pressure potential.

Water under pressure has a higher capacity to do work (like knocking over football hooligans at the end of water cannon). Pressure potential may be positive (physical pressure): water in a hosepipe has a high pressure potential. It may also be negative (tension): water being 'stretched' in a sealed syringe has a low pressure potential.

Note that cells that are turgid (left) have a large positive pressure potential (the water presses out on the cell wall, and the wall presses back on the water with an equal but opposite force). Cells that are flaccid or plasmolysed (right) have a pressure potential of zero.

Turgid and plasmolysed cells.

2. Osmotic potential (ψπ).

ψπ is the osmotic (chemical) component: pure water will flow from where it is in high concentration (i.e. dilute solutions) to where it is in low concentration (i.e. concentrated solutions).

Osmotic potential means that water will flow from dilute solutions (those that contain much water) to concentrated solutions (those that contain little water).

ψπ is also written as ψs and called the solute potential. These terms are synonymous.

Unfortunately, you will also come across a symbol in the literature, π, properly called the osmotic pressure, but frequently misnamed the osmotic potential. Don't use this!!! There's a whole world of reversed sign and inverted substraction related pain if you do.

ψπ = − C R T

Osmotic potential is minus CRT. Pure water contains no solutes, so ψπ = 0, as it should. Water will tend to flow from dilute solutions (or pure water) to concentrated solutions through a water-permeable membrane. ψπ of a solution is always negative, because all solutions contain less water than the same volume of pure water.

3. Gravitational potential (ψg).

ψg = ρ g h

Water at great height has a higher potential than water on the ground. There is a negligible difference between cells, but this factor is important for trees though!

Then we have two weird ones that are used in special circumstances, and are really just combinations of some of the above factors for specific circumstances.

4. Matric potential (ψm).

Matric (matrix) potential is due to the adsorption of water onto surfaces (surface tension and capillarity), making it less available for diffusion. Matric potential is therefore also always negative. It's really a complicated combination of osmotic and pressure potentials, but it is more convenient to pool these together for soils.

5. Vapour potential (ψv).

ψv = R T ⁄ Vw ln ( C ⁄ Csat )

The vapour potential measures the water potential of vapour in air. Vapour potential is always positive. It's really just the pressure potential for water vapour in air, but more convenient. Again, there is a negligible difference between cells, but is useful for describing water loss from leaves.

ψ = ψ0 + ψp + ψπ + ψg + ψm + ψv

The water potential in any part of the plant is the algebraic sum of the components and is normally negative. However, most of the component potentials are rather small in cells (and there can be little difference in gravitational potential between a cell and its environment), so to a good approximation, the water potential is just the sum of the pressure and osmotic components.

ψ = ψp + ψπ

OK. This is where the mess happens. That equation is often written in this way instead:

ψ = P − π

P is identical to ψp, (P = ψp), but π is the negative of the osmotic potential (π = −ψπ). π is a historical hangover, and is properly called the osmotic pressure. Unfortunately, it tends to be called the osmotic or solute potential too. This means you will get confused whether osmotic potential is positive or negative, because even scientists don't use the name consistently. Forget the name, and just remember, ψπ is negative for all solutions: pure water has ψπ of 0, and since pure water will flow to a lower potential, ψπ must be negative (lower than zero) for any water containing solutes. Since π = -ψπ, then π must be positive for all solutions. Just ignore what it's called, and make sure the sign if negative if you're using the ψπ equation, and positive if you're using the π equation.

In the same way that reactions with a negative free energy change are spontaneous, water will flow spontaneously from a high water potential to a low water potential (think of this as 'downhill' if you like. Water under pressure (high P) and containing few solutes (low π, therefore not very negative ψπ) has a high water potential (there's a 'lot' of water in a very pumped up (turgid) cell containing nearly pure water). Water at low pressure(low P), containing lots of solutes (high π, therefore very negative ψπ) will have a water potential close to zero (there's 'not much' water in a beaker full of sugar syrup). Hence water will flow from the cell to the beaker, from high (nearly zero) potential to low (very negative) potential. This flow will carry on until the water potentials of the cell and beaker are the same, i.e. to equilibrium, where ∆ψ = 0.

The measurement of water potential is somewhat difficult, and generally involves the use of an piece of equipment called a psychrometer. A psychrometer is a very sensitive thermometer connected to a drop of saline inside a sealed chamber. The test sample is placed into the chamber, and the water potential of the air is allowed to come to equilibrium with that of the sample. The saline drop is then exposed to the air.

If ψsaline > ψair, (as in the picture), water will evaporate and cool the psychrometer.

If ψsaline < ψair, water will condense and warm the psychrometer.

Psychrometer.

To determine the water potential of the cell, we test different concentrations of saline solution. The one that produces no temperature change has the same ψ as the cell (and this can be readily calculated using the −CRT equation.

The pressure potential component can be measured with very small pressure probes. Air in a capillary tube is compressed. Osmotic potential can be measured by the effect of solutes on freezing point.

Micromanometer.

Calculations using ψ

  1. If we take a flaccid plant cell containing 0.3 M solutes, it will have a ψpi; of −0.732 MPa (i.e. −C R T). Since the cell wall exerts no pressure on the contents of a flaccid cell, its pressure potential component will be zero. Its water potential ψ will equal ψπ + ψp = −0.732 + 0 = −0.723 MPa. Easy.
  2. If we then put this cell in a beaker of 0.1 M sucrose, what will happen? To work this out, we need to calculate the water potential of the sucrose solution. ψ = ψπ + ψp. 0.1 M sucrose has an osmotic potential of −0.244 MPa, and the pressure component will again be zero (since pressure potentials are measured relative to atmospheric pressure). Hence ψ = −0.244 MPa.
  3. Water flows from high to low potential, so it will flow from high (−0.244) to low (−0.732), i.e. from sucrose solution to cell. The cell will therefore swell and become turgid.
  4. As the cell swells with water, the cell wall will begin to exert a pressure on the water inside the cell, increasing its water potential, until eventually, we will reach equilibrium, and no further water will flow in. Since only a tiny amount of water will move into the cell, we know that the osmotic potential inside will stay very nearly the same. Likewise, because very little water will leave the beaker (and enter the cell), the osmotic potential outside the cell will stay very nearly the same. We can use these facts to calculate the pressure the cell wall exerts at equilibrium.
  5. At equilibrium, the water potentials of the sucrose and the cell contents will be the same. We know the water potential of the sucrose will not change much from −0.244 MPa, as atmospheric pressure and the 0.1 M sucrose concentration will remain nearly constant. We also know that the osmotic component of the cell will not change much either, because very little water will flow in, and the cell's contents will not be diluted appreciably. Hence, at equilibrium, the cell's water potential must also be −0.244 MPa, and:

    ψ = ψp + ψπ

    −0.244 = ψp + (−0.732)

  6. Hence, ψp must equal −0.244 − (−0.732) = 0.488 MPa. Make sure you understand what's going on with the plusses and minuses.
  7. If we dissolve the cell wall with cellulase, what will happen?
  8. Alternatively, if we then pick this turgid cell out of the 0.1 M sucrose, and drop it into a beaker of 0.3 M sucrose, where will the water go? Answers.

I hope this helps clear up this nasty subject, or at least point out where you're likely to get horribly confused:

I would like to apologise to all students who run up against this topic for the inconsistent whims of professional biologists . I am very, very sorry.

Xylem transport

Water moves from one part of the plant to another down a water potential gradient. Different components of ψ are important at different stages.

Different components of water potential are important at different points, although essentially a plant is just an exciting diversion in the flow of water from soil (-0.3 MPa) to air (-100 MPa).

Soil to roots: the water potential of the soil is negative: this is mainly due to its low matric potential. Water enters the root via its cell walls, which are very hydrophilic (they contain a lot of cellulose) and have even more negative matric potentials.

Root hair inveigling itself between soil crumbs.

Water can travel for some distance into the root along the cell walls (the so called apoplastic pathway); however, it cannot cross the suberised Casparian strip in the endodermis.

The Casparian strip forces water crossing the endodermis to travel into the symplast.
The cell wall of the endodermis (pink inner strip of cells) is waterproofed by the suberised Casparian strip, which forces water to enter the symplast before it can enter the root xylem.

It has to enter the root cells by osmosis, since their osmotic potential is more negative than the matric potential outside, due to their high content of solutes.

The Casparian strip prevents water entering the vascular bundle in the core of the root without first entering the symplast.

The apoplast is that part of the plant that water can move through without crossing a membrane. It consists of the cell walls and air spaces between the plant cells The symplast is the pathway water can take having crossed a membrane, i.e. through the cytoplasm and the plasmodesmata that connect between cells.

The apoplast consists of cell walls and air spaces (like those in xylem); the symplast of cytoplasm and plasmodesmata.

Roots to stems: once inside the symplast, water can pass from cell to cell via the plasmodesmata. It must cross another membrane before it can enter the xylem. It can do this in two ways, depending on the time of day.

At night, neighbouring cells actively secrete ions into the xylem and lower its osmotic potential. Water enters by osmosis and generates a positive pressure in the xylem. This is called root pressure, and evidence for it is that many plants guttate (leak sap) in the morning.

During the day, transpiration creates a negative pressure potential (tension) in the xylem. This passively sucks water out of the root cells against an osmotic gradient. Sometimes called this is called 'reverse osmosis' (although this is a poor choice of words).

Stems to cells: cells are full of solutes, whereas the xylem contents are mostly water, consequently, there is a large water potential difference, mostly due to the difference in osmotic potentials. Water tends to flow into cells from the xylem.

Cells to stomata: the energy for 'sucking' water into the xylem is provided by heat, which evaporates water from the leaf mesophyll. Air has a low water potential, as determined the % saturation of the air with water (relative humidity). Water evaporates from the cell walls of mesophyll cells and is lost from the stomata as vapour.

The sum total of these different pathways is called the transpiration stream:

The suction in xylem is transmitted downwards by the cohesion (hydrogen-bonding) of the water molecules in the xylem and their adhesion to the xylem walls (surface tension). Every so often a water column will snap (audibly if you have a small microphone) to leave a cavity filled with water vapour. This is called cavitation, and prevents water uptake.

Cavitation is the breakage of water columns in xylem.

The cavity is prevented from spreading to neighbouring vessels by the surface tension of water at pit membranes and perforation plates. Root pressure is able to refill some cavities and restore normal xylem function.

Bibliography

Peer Review.
This page has been peer reviewed by 2 people. Thanks to Tony Kloze for his proofreading.