WATER POTENTIAL: COMPONENTS, METHODS AND EXAMPLES - ENVIRONMENT - 2025

The water potential is the free energy or capable of doing work, which has a certain volume of water. Thus, the water at the top of a waterfall or waterfall has a high water potential that, for example, is capable of moving a turbine.

The symbol that is used to refer to the water potential is the capital Greek letter called psi, which is written Ψ. The water potential of any system is measured in reference to the water potential of pure water under conditions considered standard (pressure of 1 atmosphere and the same height and temperature of the system to be studied).

Osmotic potential. Source: Kade Kneeland / CC BY-SA (https://creativecommons.org/licenses/by-sa/4.0)

The factors that determine the water potential are gravity, temperature, pressure, hydration and the concentration of solutes present in the water. These factors determine the formation of water potential gradients and these gradients drive the diffusion of water.

In this way, the water moves from a site with high water potential to another with low water potential. The components of the water potential are the osmotic potential (concentration of solutes in the water), matric potential (adhesion of water to porous matrices), gravitational potential and the pressure potential.

Knowledge of the water potential is essential to understand the functioning of various hydrological and biological phenomena. These include the absorption of water and nutrients by plants and the flow of water in the soil.

Components of water potential

The water potential is made up of four components: osmotic potential, matric potential, gravitational potential and pressure potential. The action of these components determines the existence of hydric potential gradients.

Osmotic potential (Ψs)

Normally, water is not in its pure state, since it has dissolved solids in it (solutes), such as mineral salts. The osmotic potential is given by the concentration of solutes in the solution.

The higher the amount of dissolved solutes, there is less free energy of the water, that is, less water potential. Therefore, water tries to establish an equilibrium by flowing from solutions with a low concentration of solutes to solutions with a high concentration of solutes.

Matric or matrix potential (Ψm)

In this case, the determining factor is the presence of a hydratable material matrix or structure, that is, it has an affinity for water. This is due to the adhesion forces created between molecules, especially the hydrogen bonds formed between water molecules, oxygen atoms, and hydroxyl (OH) groups.

For example, adhesion of water to soil clays is a case of water potential based on matric potential. These matrices by attracting water generate a positive water potential, therefore the water outside the matrix flows towards it and tends to stay inside as it happens in a sponge.

Height or gravitational potential (Ψg)

The gravitational force of the Earth is in this case the one that establishes the potential gradient, since the water will tend to fall downwards. Water located at a certain height has a free energy determined by the attraction that the Earth exerts on its mass.

Movement of water by gravity. Source: Bilal ahmad / CC BY-SA (https://creativecommons.org/licenses/by-sa/4.0)

For example, water in a raised water tank falls freely down the pipe and travels with that kinetic (motion) energy until it reaches the tap.

Pressure potential (Ψp)

In this case, the water under pressure has greater free energy, that is, greater water potential. Therefore, this water will move from where it is under pressure to where it is not, and consequently there is less free energy (less water potential).

For example, when we dose drops using a dropper, by pressing the rubber knob we are applying a pressure that gives energy to the water. Due to this higher free energy, the water moves to the outside where the pressure is lower.

Methods to determine the water potential

There are a variety of methods for measuring water potential, some suitable for soil, others for tissues, for mechanical hydraulic systems and others. The water potential is equivalent to units of pressure and is measured in atmospheres, bars, pascals or psi (pounds per square inch in its acronym in English).

Here are some of these methods:

Scholander Pump or Pressure Chamber

If you want to measure the water potential of a plant leaf, you can use a pressure chamber or Scholander pump. This consists of an airtight chamber where the entire leaf (sheet with its petiole) is placed.

Measurement of water potential of a leaf with a pressure chamber. Source: Pressurebomb.svg: Aibdescalzoderivative work: Aibdescalzo / CC BY-SA (https://creativecommons.org/licenses/by-sa/3.0)

Then the pressure inside the chamber is increased by introducing a pressurized gas, measuring the pressure that is being reached by means of a manometer. The pressure of the gas on the leaf is increasing, to the point where the water contained in it gushes out through the vascular tissue of the petiole.

The pressure indicated by the manometer when the water leaves the leaf corresponds to the water potential of the leaf.

Pressure probes

There are several alternatives to measure the water potential using special instruments called pressure probes. They are designed to measure the water potential of the soil, based mainly on the matric potential.

For example, there are digital probes that work on the basis of introducing a porous ceramic matrix connected to a humidity sensor into the soil. This ceramic is hydrated with the water inside the soil until it reaches a balance between the water potential within the ceramic matrix and the water potential of the soil.

Subsequently, the sensor determines the moisture content of the ceramic and estimates the water potential of the soil.

Microcapillary with pressure probe

There are also probes capable of measuring the water potential in plant tissues, such as the stem of a plant. A model consists of a very thin, fine-tipped tube (micropillar tube) that is inserted into the tissue.

Upon penetrating living tissue, the solution contained in the cells follows a potential gradient defined by the pressure contained in the stem and is introduced into the micropyle. When the liquid from the stem enters the tube, it pushes an oil contained in it that activates a pressure probe or manometer that assigns a value corresponding to the water potential

Variations in weight or volume

To measure the water potential based on the osmotic potential, the weight variations of a tissue immersed in solutions at different concentrations of a solute can be determined. For this, a series of test tubes are prepared, each with a known increasing concentration of a solute, for example sucrose (sugar).

In other words, if there are 10 cc of water in each of 5 tubes, 1 mg of sucrose is added in the first tube, 2 mg in the second, and thus up to 5 mg in the last. So we have a rising battery of sucrose concentrations.

Then, 5 sections of equal and known weight are cut from the tissue whose water potential is to be determined (for example potato pieces). A section is then placed in each test tube and after 2 hours, the tissue sections are removed and weighed.

Expected results and interpretation

Some pieces are expected to lose weight from water loss, others will have gained weight because they absorbed water, and still others will maintain the weight.

Those that lost water were in a solution where the sucrose concentration was greater than the solute concentration within the tissue. Therefore, the water flowed according to the gradient of osmotic potential from the highest concentration to the lowest, and the tissue lost water and weight.

In contrast, the tissue that gained water and weight was in a solution with a lower concentration of sucrose than the concentration of solutes within the tissue. In this case, the osmotic potential gradient favored the entry of water into the tissue.

Finally, in that case in which the tissue maintained its original weight, it is inferred that the concentration in which it was found has the same concentration of solute. Therefore, this concentration will correspond to the water potential of the tissue studied.

Examples

Water absorption by plants

A 30 m tall tree needs to transport water from the ground to the last leaf, and this is done through its vascular system. This system is a specialized tissue made up of cells that are dead and look like very thin tubes.

Movement of water in plants. Source: Laurel Jules / CC BY-SA (https://creativecommons.org/licenses/by-sa/3.0)

Transport is possible thanks to the differences in water potential that are generated between the atmosphere and the leaf, which in turn is transmitted to the vascular system. The leaf loses water in the gaseous state due to the higher concentration of water vapor in it (higher water potential) compared to the environment (lower water potential).

The loss of steam generates a negative pressure or suction that forces the water from the vessels of the vascular system towards the leaf blade. This suction is transmitted from vessel to vessel until reaching the root, where the cells and intercellular spaces are imbibed with the water absorbed from the soil.

The water coming from the soil penetrates the root due to a difference in osmotic potential between the water in the epidermis cells of the root and that of the soil. This occurs because the root cells have solutes in higher concentrations than soil water.

Mucilages

Many plants in dry environments retain water by producing mucilage (viscous substance) that are stored in their vacuoles. These molecules retain water, reducing its free energy (low water potential), in this case the matric component of the water potential being decisive.

An elevated water tank

In the case of a water supply system based on an elevated tank, the same is filled with water due to the effect of the pressure potential. The company that provides the water service, puts pressure on it using hydraulic pumps and thus overcomes the force of gravity to reach the tank.

Once the tank is full, the water is distributed from it thanks to a potential difference between the water stored in the tank and the water outlets in the house. Opening a tap establishes a gravitational potential gradient between the water in the tap and that of the tank.

Therefore, the water in the tank has higher free energy (higher water potential) and falls mainly due to the force of gravity.

Diffusion of water in the soil

The main component of the soil's water potential is the matric potential, given the adhesion force that is established between the clays and the water. On the other hand, the potential of gravity affects the vertical displacement gradient of the water in the soil.

Many processes that occur in the soil depend on the free energy of the water contained in the soil, that is to say on its water potential. These processes include plant nutrition and transpiration, infiltration of rainwater, and evaporation of water from the soil.

In agriculture it is important to determine the water potential of the soil to properly apply irrigation and fertilization. If the matric potential of the soil is very high, the water will remain attached to the clays and will not be available for absorption by plants.

References

1. Busso, CA (2008). Use of the pressure chamber and thermocouple psychrometers in the determination of hydric relations in plant tissues. ΦYTON.
2. Quintal-Ortiz, WC, Pérez-Gutiérrez, A., Latournerie-Moreno, L., May-Lara, C., Ruiz-Sánchez, E. and Martínez-Chacón, AJ (2012). Water use, water potential and yield of habanero pepper (C apsicum chinense J acq.). Magazine Fitotecnia Mexicana.
3. Salisbury, FB and Ross, CW (1991). Plant Physiology. Wadsworth Publishing.
4. Scholander, P., Bradstreet, E., Hemmingsen, E. and Hammel, H. (1965). Sap Pressure in Vascular Plants: Negative hydrostatic pressure can be measured in plants. Science.
5. Squeo, FA (2007). Water and Hydric Potential. In: Squeo, FA and Cardemil, L. (Eds.). Plant Physiology. University of La Serena editions