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Essay on Water Potential in Plants
Essay Contents:
- Essay on the Meaning of Water Potential in Plants
- Essay on Plant-Water Relations New and Old Terminology
- Essay on the Determination of Plant-Water Potential
- Essay on the Measurement of Plant-Water Potential
Essay # 1. Meaning of Water Potential in Plants:
The chemical free energy of water in its purest form is also called water potential (ΨW). Purest form means there are no other molecules in it. The chemical energy is maximum and its value is given as 0 bars. Addition of solutes to pure solvent decreases the chemical free energy of pure water, because certain amount of energy of a number of water molecules is used for binding to the surface of solutes.
So the total value of water potential of a solution is less than zero; it is always expressed in negative pressure values. Here, it is equal to DPD; if the water potential of pure water is zero and DPD is also zero. But the water potential of solution is less than zero expressed in negative value, but DPD of the solution is expressed in positive value.
These energy relations are governed by the said equations, understanding of it is very important:
Ψw= Ψs + Ψp + Ψg
where, Ψw = water
Ψs = Solutes – Solute potential or osmotic potential
Ψp = Pressure – hydrostatic pressure of the solution, it is often called turgour pressure, which can be negative or positive
Ψg = Gravity – will not be considered for normal calculations.
Pure water: Ψp = 0 MPa
Ψs = 0 MPa
Ψw = Ψp + Ψs = 0 MPa
DPD of pure water = 0 bars; DPD of a solution = (+) bars
Ψsw of pure water = 0 bars; Ψw of a solution = (-) bars
Water potential of pure water in an open container is zero because there is no solute and the pressure in the container is zero. Adding solute lowers the water potential. When a solution is enclosed by a rigid cell wall, the movement of water into the cell will exert pressure on the cell wall. This increase in pressure within the cell will raise the water potential (Fig 5.1).
Water potential (Ψ) = Pressure potential (ΨP) + Solute potential (Ψs)
Water potential is the measure of potential energy in water and drives the movement of water through plants.
Key Points:
1. Plants use water potential to transport water to the leaves.
2. Water potential is a measure of the potential energy in water as well as the difference between the potential in a given water sample and pure water.
3. Water potential is represented by the equation:
ΨW = Ψs + Ψp + Ψg + Ψm
4. Water always moves from the system with a higher water potential to the system with lower water potential.
5. Solute potential (Ψs) decreases with increasing solute concentration; a decrease in Ψs causes a decrease in the total water potential.
6. The internal water potential of a plant cell is more negative than pure water; this causes water to move from the soil into plant roots via osmosis.
Water potential is a measure of the potential energy in water or the difference in potential energy between a 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 (Ψw pure 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, therefore, expressed in relation to Ψw pure H2O.
Water potential in plant solutions is influenced by solute concentration, pressure, gravity and factors called matrix effects.
Water potential can be broken down into its individual components using the following equation:
Ψt = Ψs + Ψp + Ψg + Ψm
where, Ψs = Solute potential
Ψp = Pressure potential
Ψg = Gravity potential
Ψm = Matric potential.
As the individual components change, they raise or lower the total water potential of a system. When this happens, water moves to equilibrate, moving from the system or compartment with a higher water potential to the system or compartment with a lower water potential. This brings the difference in water potential between the two systems (Δ) back to zero (Δ = 0).
Therefore, for water to move through the plant from the soil to the air (transpiration), the conditions must exist as such:
Ψsoil > Ψroot > Ψstem > Ψleaf > Ψatmosphere.
Water only moves in response to Ä, not in response to the individual components. However, because the individual components influence the total Ψsystem, a plant can control water movement by manipulating the individual components (especially Ψs).
Essay # 2. Plant-Water Relations New and Old Terminology:
Water relations in plant cell can be described in old set of terminology: diffusion pressure deficit (DPD), osmotic pressure (OP) and turgor pressure (TP) or in terms of new terminology. Water potential (Ψ) concept is the new term for DPD. Both DPD and V are numerically equal but are opposite in algebraic sign.
Ψ = (-) DPD
Similarly osmotic potential (Ψs) is numerically equal to osmotic pressure.
Ψs = (-) OP
Turgor pressure is referred to as turgor potential (Ψp) which is not only numerically equal to TP but similar in algebric sign also.
Ψp = TP
Metric pressure (MP) is usually neglected but under new terminology (Ψm) it exists in the following relationship:
(Ψm) = (-) MP
When plasmolised cell is placed in pure water, it absorbs water and cell wall gradually increases as the vacuole swells in size and at a certain point plasmalemma exerts pressure on cell wall, the turgor pressure. Thus turgor pressure is a centrifugal pressure exerted by cytoplasm and cell membrane on cell wall.
Using old terminology, water status of the cell is described as DPD and its contributing pressure as follows:
DPD = OP – TP
DPD = OP, when TP = 0
The same situation is explicable in new terminology and water potential equation.
Ψ = (Ψs) + (Ψp) + (Ψm)
Ψ = Ψs + Ψp when Ψm is negligible
Ψ = Ψs when Ψp is zero.
It is important to keep notice of algebric signs while attempting data handling and numerical problems.
(-)Ψt = (-) (Ψs) + (+) (Ψp) + (-) (Ψm)
Essay # 3. Determination of Plant-Water Potential:
Energy status of plant cells is determined by three/four major factors:
1. Ψs = Solute potential
2. Ψp = Pressure/turgor potential
3. Ψm = Matric potential
4. Ψg = Potential due to gravity.
Total plant-water potential can be expressed, in terms of its component potentials, as indicated below:
Ψw = Ψs + Ψp + Ψm + Ψg
1. Ψs: Solute potential (-ve):
When sugar is added to a beaker, the solute potential or energy drops as water molecules ‘bond’ to the sugar. Thus to decrease in leaf relative to the root, sugars or other molecules accumulate in the leaf dropping it potential -1 to -2 MPa.
2. Ψp: Pressure/turgor potential (+ve or -ve):
As water is transpired from the leaves it creates a negative pressure or tension. Imagine sucking on a straw rapidly down to the ‘last drop’…The solution ‘hangs’ in the straw due to this negative pressure. This pressure, generally, is on the order of less than -2 MPa (Mega Pascals).
Pressure can be positive, however, as with guttation, when there are so many solutes in a root that water accumulates and pushes up. Ions continue to be pumped into the xylem even though transpiration has ceased at night. Increased concentration results in a lower osmotic potential in the xylem and a gradient is developed across the root. Water moves into the xylem in response to the gradient.
3. Ψm: Matric potential (-ve):
As water adheres to cell walls it lowers it potential. The walls of xylem are made up of carbohydrates which attract water molecules. In a sense, matric potential is equivalent to solute, but occurs when the molecules are bound rather than in solution.
4. Ψg: Potential due to gravity:
This force in not critical at a cellular level but may be important in tall trees.
Plant-water potential increases with added pressure (turgor) and decreases with increase in osmotic pressure. In the past, the condition of water in plants used to be expressed in terms of its diffusion pressure deficit (DPD) or equivalent term such as suction force.
DPD = TP + IP + OP
where, DPD = diffusion pressure deficit (atm or bars)
TP = turgor pressure
IP = imbibitional pressure
OP = osmotic or solute pressure.
Values of OP are considered to be always positive, values of TP and IP are also considered positive and DPD of pure water at the reference is taken as zero: thus DPD values in plant systems are always positive and water will tend to move from a point of low DPD to one of high DPD.
Turgor pressure potential = + 10 bars
Imbibitional potential = – 1 bar
Solute potential = – 18 bars
Total water potential = – 9 bars
If the cells are placed in pure water (ψ = 0), water will move into the cells (high to low potential). Similarly, water moves from cell where ψ = – 9 bars to one where ψ = -12 bars. The magnitude of water potentials in SPAC is given in Table 5.1.
In any system, if the condition of water is considered in terms of its energy state (potential), it is relatively easy to predict its direction of movement or its effect on some process. Water potentials are normally expressed in terms of energy units per unit mass or per unit volume. Typical units are joules kg-1, ergs g-1, ergs cm-3 or ergs mole-1. Water potentials in soils and plants are now expressed in pressure units of atmospheres or bars.
Essay # 4. Measurement of Plant-Water Potential:
Several methods are available for measuring plant-water potential:
1. Pressure Chamber
2. Psychrometer
3. Dew Point Hygrometer
4. Osmometer
5. Leaf Temperature Measurement
6. Canopy Temperature Measurement
7. Canopy Air Temperature Differentials
8. Diffusive Resistance
9. Transpiration Rate Measurement, etc.
Commonly used plant-water stress indicators are plant-water potential and relative water content.
Plant-water stress can be measured using any of the above methods. Only two methods of measuring plant-water potential, pressure chamber and psychrometer methods are discussed.
1. Pressure Chamber:
Pressure chamber measures the tension on the water within the water conducting tissue (xylem). This method involves cutting a leaf and its attached petiole from the plant, protecting it from any water loss and sealing the leaf inside a pressurisation chamber with a small amount of the cut end of the petiole exposed outside. Pressure within the chamber is increased, until water is observed at the cut end of the petiole (Fig 5.2).
Tension within the xylem is believed to be equivalent to the pressure in the chamber at the first appearance of water and is further believed to be a measure of the physiological “dryness” of the plant, called the plant-water potential.
The units of water potential are negative pressures, with the most commonly used unit being the bar, which is equivalent to about 14.5 pounds per square inch (psi). In the scientific literature, the Megapascal (MPa) unit, which is simply 10 bars, is preferred.
2. Psychrometer:
Psychrometers (Fig 5.3) measure the water vapor pressure of a solution or plant sample, on the basis of the principle that evaporation of water from a surface cools the surface.
Measurement is by placing a piece of tissue sealed inside a small chamber that contains a temperature sensor (thermocouple) in contact with a small droplet of a standard solution of known solute concentration (known Ψs and thus known Ψw).
If the tissue has a lower water potential than that of the droplet, water evaporates from the droplet, diffuses through the air and is absorbed by the tissue. This slight evaporation of water cools the drop. The larger the difference in water potential between the tissue and the droplet, the higher the rate of water transfer and hence the cooler the droplet.
If the standard solution has a lower water potential than that of the sample to be measured, water will diffuse from the tissue to the droplet, causing warming of the droplet. Measuring the change in temperature of the droplet for several solutions of known Ψw makes it possible to calculate the water potential of a solution for which the net movement of water between the droplet and the tissue would be zero, signifying that the droplet and the tissue have the same water potential (Fig 5.4).
Cryoscopic osmometer (Ψs measurement) and pressure probe (Ψp measurement) are also available for measuring plant-water potential.