The water potential, also known as the chemical potential of water, is a measurement of the free energy associated with water. The Greek letter (psi) represents water potential, which is defined in relation to the water potential of pure water, which is zero. As a result, psi is always negative. Mega Pascals are the units of water potential (MPa). At the same temperature, it is a relative quantity that is affected by concentration, pressure, and gravity. Water potential can be represented as the sum of component potentials.
Ψ= Ψs+ Ψm+ Ψp + Ψg
Where, Ψs = Solute osmotic potential (symbol π)
Ψm = Matric potential (symbol T)
Ψp = Pressure potential (symbol P)
Ψg = Gravitational potential (symbol G)
Osmotic potential :
The component created by the solute dissolved in the cell sap, mostly vacuolar sap, is the osmotic potential, Ψs (or π)
Matric Potential :
Water contained in micro capillaries or bonded on the surfaces of cell walls and other cell components is referred to as the matric potential Ψm (or T).
Pressure potential :
The turgor pressure created by water diffusion into protoplasts confined in walls that resist expansion is known as the pressure potential Ψp (or P). Because of root pressure, p is normally negative in transpiring plants’ xylem and positive in guttating plants’ xylem.
Gravitational Potential :
When comparing potentials in leaves at different heights on trees and in soils, the influence of gravity, Ψg (or G), is a term of little consequence within a root or a leaf, but becomes essential when comparing potentials in leaves at different heights on trees and in soils.
Water must resist a gravitational force of 0.01 Mpa/m to travel upward in a tree stem, while gravity causes water to drain downhill in soil. Because the volume of matric water is so little in comparison to the volume of vacuolar water in the parenchyma, prospective water makes only a small proportion of the total water; yet, matric potential can regulate the cell water potential. As a result, the matric potential and gravitational potential values for herbaceous plants and annual field crops with a low vertical height (less than 10 m) are tiny and sometimes disregarded.
Water changes from having a lower negative water potential to having a higher negative water potential.
Importance of water potential :
Water potential is a diagnostic technique that allows plant scientists to put a numerical value on the water condition of plant cells and tissues. The stronger a plant cell’s or tissue’s ability to absorb water, the lower its water potential. Conversely, the greater the tissue’s capacity to deliver water to other more dry cells and tissues, the higher its water potential.
Water potential is therefore utilized to assess water stress and shortage in plant cells and tissues. The leaves of most plants rooted in well-watered soils have water potentials ranging from -2 to -8 bars on average. Leaf water potential will become more negative than -8 bars when soil moisture supply decreases, and leaf growth rates will decrease. When water potential falls to roughly -15 bars, most plant tissues will stop growing (i.e., will not increase).
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