Water Potential

Water Potential

Water potential is a measure of the differences in potential energy between a water sample with solutes and pure water. Water moves via osmosis from an area of higher water potential (more water molecules, less solute) to an area of lower water potential (less water, more solutes)

WATER POTENTIAL and PLANTS

Water potential is a measure of the potential energy in water, specifically, water movement between two systems. Water potential can be defined as the difference in potential energy between any given water sample and pure water (at atmospheric pressure and ambient temperature). Water potential is denoted by the Greek letter Ψ (psi) and is expressed in units of pressure (pressure is a form of energy) called megapascals (MPa). The potential of pure water (Ψpure H2O) is designated a value of zero (even though pure water contains plenty of potential energy, that energy is ignored). Water potential values for the water in a plant root, stem, or leaf are expressed relative to Ψpure H2O.

The water potential measurement combines the effects of solute concentration (s) and pressure (p):

Ψsystem = Ψs + Ψp

where Ψs = solute potential, and Ψp = pressure potential.  The Addition of more solutes will decrease the water potential, and the removal of solutes will increase the water potential. The addition of pressure will increase the water potential, and the removal of  pressure (creation of a vacuum) will decrease the water potential.

Water always moves from a region of high water potential to an area of low water potential, until it equilibrates the water potential of the system. At equilibrium, there is no difference in water potential on either side of the system (the difference in water potentials is zero). In order for water to move through the plant from the soil to the air (a process called transpiration), Ψsoil must be > Ψroot > Ψstem > Ψleaf > Ψatmosphere.

Let’s consider solute and pressure potential in the context of plant cells:

• Solute potential (Ψs), also called osmotic potential, is negative in a plant cell and zero in distilled water, because solutes reduce water potential to a negative Ψs. The internal water potential of a plant cell is more negative than pure water because of the cytoplasm’s high solute content. Because of this difference in water potential, water will move from the soil into a plant’s root cells via the process of osmosis. This is why solute potential is sometimes called osmotic potential. Plant cells can metabolically manipulate Ψs by adding or removing solute molecules.

• Pressure potential (Ψp), also called turgor potential, may be positive or negative. Positive pressure (compression) increases Ψp, and negative pressure (vacuum) decreases Ψp. Positive pressure inside cells is contained by the rigid cell wall, producing turgor pressure. Pressure potentials can reach as high as 1.5 MPa in a well-watered plant. A Ψp of 1.5 MPa equates to 210 pounds per square inch (psi); for a comparison, most automobile tires are kept at a pressure of 30-34 psi.  A plant can manipulate Ψp via its ability to manipulateΨs and by the process of osmosis. If a plant cell increases the cytoplasmic solute concentration, Ψs will decline, water will move into the cell by osmosis, and Ψp will increase. Ψp is also under indirect plant control via the opening and closing of stomata. Stomatal openings allow water to evaporate from the leaf, reducing Ψp and Ψtotal of the leaf and increasing the water potential difference between the water in the leaf and the petiole, thereby allowing water to flow from the petiole into the leaf.