Lab Lesson Water Potential in Plants APRIL 2023
Channel: ScenseMe
Date: 18/06/2023

Delivered On: 5 APRIL 2023

There are two major ways to move molecules: A. Bulk (or Mass) Flow- is the mass movement of molecules in response to a pressure gradient. The molecules move from high to low pressure, following a pressure gradient. B. Diffusion - the net, randomly movement of individual molecules from one area to another. The molecules move from high to low concentration, following a concentration gradient. Another way of stating this is that the molecules move from an area of high free energy (higher concentration) to one of low free energy (lower concentration). The net movement stops when a dynamic equilibrium is achieved. Osmosis is a specialized case of diffusion; it represents the diffusion of a solvent (typically water) across a membrane.

Water potential is a measure of the energy state of water. This is a particularly important concept in plant physiology because it determines the direction and movement of water. Water potentials in intact plant tissue are usually negative (because of the large quantities of dissolved solutes in cells). Water always moves from an area of higher water potential to an area of lower water potential. Water potential is affected by two factors: pressure and the amount of solute.

Equation for water potential (must account for the factors that influence the diffusion of

water): Ψw = Ψp + Ψs + Ψg

Where, Ψw = water potential
Ψp = pressure potential
Ψs = solute or osmotic potential
Ψg = gravity potential

Solute (or osmotic) potential (Ψs) is the contribution due to dissolved solutes. Pure water at atmospheric pressure has a solute potential of zero. As solute is added, the value for solute potential becomes more negative. This causes water potential to decrease also. In sum, as solute is added, the water potential of a solution drops, and water will tend to move into the solution. The solute potential of a solution can be calculated with the Van’t Hoff equation:

Ψs = - miRT

Where, m = molality (moles/1000 g)
i = ionization constant (often 1.0)
R = gas constant (0.0831 liter bar/mole K)
T = Temperature in degrees Kelvin (273 + °C of solution)

Pressure (or Pressure Potential; Ψp)- In a plant cell, pressure exerted by the rigid cell wall that limits the further water uptake. It is usually positive, although may be negative (tension) as in the xylem. Pressure can be measured with an osmometer. Matric potential is the contribution to water potential due to the force of attraction of water for colloidal, charged surfaces. It is negative because it reduces the ability of water to move. In large volumes of water, it is very small and usually ignored. However, it can be very important in the soil, especially when referring to the root/soil interface. Gravity (Ψg) is contributions due to gravity, which is usually ignored unless referring to the tops of tall trees.


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