Water Potential

Water potential has been shown to be useful in comprehending and computing the movement of water throughout plants, animals, and soil. The potential energy of water is commonly stated in terms of potential energy per unit volume and is frequently denoted by the Greek letter.

Water potential is a term that encompasses a variety of distinct potential drivers of water movement that may work in parallel or in opposite directions. Numerous potential elements may be active concurrently within complicated biological systems. If, for example, you add solutes, they lower the potential (negative vector), but if you increase the pressure, the potential rises (positive vector). If the flow isn’t blocked, water will move from a place where there is more water to a place where there is less water. A common example is salt-containing water, such as saltwater or the fluid inside a live cell. These solutions have a negative water potential in comparison to pure water. Without any limitation on flow, water will flow from a locus of greater potential (clean water) to a location of reduced potential (the solution); flow will continue until the potential difference is equalised or balanced by another water potential element, such as pressure or elevation.

Components of water potential

The term “water potential” refers to the amount of potential energy contained in water, or the difference in potential energy between a particular water sample and pure water (at atmospheric pressure and ambient temperature). The Greek symbol ψ (psi) denotes the potential of water, which is expressed in pressure units (pressure is a type of energy) called megapascals (MPa). The potential of pure water (Ψwpure H2O) is zero (even though pure water contains plenty of potential energy, that energy is ignored). Water potential values for water contained within a plant’s root, stem, or leaf are therefore stated in terms of Ψwpure H2O.

In plant solutions, the water potential is controlled by solute content, pressure, gravity, and matrix influences. The following equation can be used to decompose the potential of water into its constituents:

Ψsystem = Ψtotal = Ψs + Ψp’ + Ψg’ + Ψm

where,

• Ψs = solute potential

• Ψp, = pressure potential

• Ψg, = gravity potential

• Ψm = matric potential

Term “system” refers to the water potential of the root water (Ψstem), the soil water (Ψsoil), the leaf water (Ψleaf), the stem water (Ψstem), and the water in atmosphere (Ψatmosphere), depending on aquatic system in question. As different components of a system change, the total water potential of the system decreases or increases. When this occurs, water equilibrates by migrating from a system or compartment with a greater water potential to one with a lower water potential. This equalises the water potential difference between the two systems (Δ) to zero (Δ = 0). Thus, for water to pass through the plant from the soil to the air (a process known as transpiration), the following conditions must exist:

Ψsoil > Ψroot > Ψstem > Ψleaf > Ψatmosphere.

The Water only moves in response to Δ, not to the response to individual components. Although, because individual components influence the total Ψsystem, a plant can control the water movement by manipulating individual components (especially Ψs).

Pressure Potential

Pressure potential is a component of the total water potential within plant cells that is determined by mechanical pressure. As water enters a cell, the pressure potential increases. Water passing through the cell wall and membrane increases the total amount of water inside the cell, exerting an outward pressure that is resisted by the cell wall’s structural rigidity. By generating this pressure, the plant can maintain its turgor, which allows it to retain its stiffness. Without turgor, plants get deformed and wilt.

Generally, the pressure potential in a plant cell is positive. The Pressure potential is negligible in the plasmolyzed cells. When water is drawn through an open system, such as a plant xylem vessel, negative pressure potentials arise. Tolerating negative pressure potentials (often referred to as tension) is a critical adaptation of the xylem. This tension can be empirically determined using a pressure bomb.

Osmotic potential (solute potential)

In a plant cell, the solute potential (Ψs), also known as osmotic potential, is negative, while it is zero in pure water. Cytoplasmic pressures in cells are typically between –0.5 and –1.0 MPa. By using some of the available potential energy in the water, solutes diminish the water’s potential (resulting in a negative Ψs). Solute molecules dissolve in water because water molecules can form hydrogen bonds with them; a hydrophobic molecule such as oil, which cannot form hydrogen bonds with water, cannot dissolve in solution. Because the energy in hydrogen bonds between solute molecules and water is trapped in the bond, it is no longer available to perform work in the system. In other words, when solutes are added to an aqueous system, the amount of available potential energy decreases. Thus, Ψs decreases as the concentration of the solute increases.

Due to the fact that Ψs is one of the four components of the Ψsystem or Ψtotal, a drop in s results in a decrease in the Ψtotal. Water gravitates toward locations with a lower Ψs value (and consequently a lower Ψtotal). The semipermeable membrane that divides the two sides of the tube in the accompanying figure enables water to pass but not solutes. The solute has been added to the right side of the first tube. By adding solute to the right side of the tube, Ψs decreases, forcing water to flow to the right side. As a result, the water level on the right side is greater

Plant cells can alter Ψs (and, by extension, Ψtotal) metabolically by adding or removing solute molecules. Thus, by their ability to exert metabolic control over Ψs, plants exert control over Ψtotal.

Matrix potential (Matric potential)

When water comes into touch with solid particles (for example, clay or sand particles in soil), the adhesive intermolecular interactions between the water and the solid can be rather strong. Surface tension and the production of menisci inside the solid matrix are facilitated by the forces between water molecules and solid particles, in combination with attraction between water molecules. After then, force is required to dislodge these menisci. The amount of the matrix potential is dependent on the distances between solid particles—the menisci width (as well as capillary action and the difference in Pa at the capillary ends)—and the chemical composition of the solid matrix (meniscus, macroscopic motion due to ionic attraction).

The absolute value of matrix potential is frequently rather considerable in relation to the other components of water potential outlined previously. The matrix potential significantly decreases the energy state of water adjacent to particle surfaces. Although water transport is slowed by matrix potential, it is critical for water supply to plant roots and engineering uses. The matrix potential is always negative due to the fact that the water absorbed by the soil matrix has a lower energy state than pure water. Matrix potential exists only above the water table in unsaturated soil. When the matrix potential approaches 0, practically all soil pores are totally saturated and have their maximal retentive capacity. Matrix potential varies significantly between soils. When water flows into less wet soil zones with comparable porosity, the matrix potential typically ranges between 10 and 30 kPa.