In this article we will discuss about:- 1. Subject-Matter of Water Potential 2. Measurement of Water Potential 3. Methods 4. Components 5. Water Potential in Cells 6. Movement of Water from Cell to Cell.
Subject-Matter of Water Potential:
In recent years the term chemical potential of water is replaced by water potential. This is designated by the Greek letter psi (Ψ). Water potential is measured in bars. The latter is a pressure unit. When the water potential in a plant cell or tissue is low the latter is capable of absorbing water.
On the other hand, if the water potential of the cell tissue is high it indicates their ability to make available water to the desiccating surrounding cells. Clearly water potential is used as a measure to determine whether the tissue is under water stress or water deficit.
It needs mentioning that it is the difference between the water potential in a system under study and that in a reference state which is taken as the water potential value.
The reference state is pure water at the temperature and atmospheric pressure comparable to that of the system being investigated. As will be clear from Fig. 6-2, the water potential in the reference state is arbitrarily taken a value of 0 bar. The same figure also shows range of Ψ in the different tissues. As will be observed herbaceous leaves of mesophytes have water potentials ranging from —2 to —8 bars.
When the water decreases in the soil the water potential tends to become more negative than —8 bars. It may be added that if the water potential falls beyond —15 bars, most plant tissues stop growing.
The response of herbaceous and desert-growing plant leaves vary when the water potential falls below —20 to —30 bars. Similarly seeds and pollen or spores are having very low water potentials and the values may be as low as —60 to —100 bars.
Measurement of Water Potential:
In studies concerning plant water relations, information on water potential in plant cells and tissues is very vital. Several methods are used to measure water potential but none of them is infallible.
Methods of Water Potential:
Some of the methods are given below:
i. Vapour Equilibrium Method:
Here the pressure of water vapour in equilibrium with the water in a tissue sample enclosed in a small chamber is measured.
The water vapour pressure is measured with the help of thermocouple psychrometer. This is an accurate method to measure tissue water potential.
Some of these psychrometers can measure the water potential of attached leaves up to ± 1 bar.
ii. Vapour Immersion Method:
This method is based on the fact that when a plant tissue is placed in an atmosphere in which water vapour is maintained at constant vapour pressure, there will be a net transfer of water between the tissues and the surrounding atmosphere till an equilibrium is reached.
The difference in the water potentials of the tissue and the environment will determine the quantity of water transferred.
iii. Liquid Immersion Method:
Usually two methods are employed and these are the liquid immersion and dye methods. The former is similar to the vapour immersion method. In general, dye method has several advantages.
iv. Pressure Chamber Method:
Using pressure chamber water potential can be measured within minutes. Further compared to other methods, no precise temperature controls are needed.
The apparatus is also relatively less expensive. This method is especially suited for field studies.
Components of Water Potential:
Keeping in view that a typical plant cell is made up of a vacuole, a cell wall and the cytoplasm between the two, usually three major sets of internal factors are visualized which contribute towards water potential (Fig. 6-3).
These are shown below:
Ψ or Ψw = ΨM + Ψs + Ψp; Ψs = Ψπ
From the given equation it may be inferred that water potential in a plant cell is equal to the sum of the matric potential (ΨM) which is due to the binding of water molecules to protoplasmic and cell wall contents, the solute potential (Ψs; Ψπ) due to the dissolved solutes in the vacuoles and lastly the pressure potential (Ψp) which is due to the pressure developed within cells and tissues. These potentials like the water potential are expressed in terms of bars.
In the followings brief accounts of the three components of water potential are given:
i. Matric Potential:
Matric refers to the matrix. It is the force of adsorption with which some water is held over the surface of collodial particles in the cell wall and cytoplasm.
It is also written in negative values. In the young cells, seeds and cells of xerophytes its value is appreciable. In the cells of mesophytic plants this is nearly —0.1 atm.
In such instances matric potential is often ignored since it does not contribute significantly to the total water potential.
Accordingly sometimes the equation is modified as below:
Ψ or Ψw = Ψs + Ψp
ii. Solute Potential:
This refers to the potential developed by the solute particles in a solution. It is equal to the osmotic potential. Solute potential depends upon the number of particles. In fact, solute potential has replaced the old term osmotic pressure.
The difference is that while the former is expressed in bars with a negative, the latter is written as positive bars. Accordingly when the solute potential decreases it attains more negative value. Several methods are used to measure solute potential in an extracted cell sap. One of these is through the usage of thermocouple psychrometer. Solute potential values vary in plant cells from different species.
iii. Pressure Potential:
This is the hydrostatic pressure which develops in a plant cell due to the inward flow of water: (Ψp). It is also referred to the turgor pressure. Environmental conditions greatly influence the volume, water content, water potential and pressure potential of a cell. In a plasmolysed plant cell, the turgor pressure is zero.
Thus water potential equals the solute pressure or negative osmotic pressure. Or the other hand, in the fully turgid state, the water potential of the cell is zero. At this moment, pressure potential or turgor pressure is equal to solute pressure. Currently very few methods are available to measure pressure potential.
Figure 6-3. A summary diagram showing relationship of different potentials in a cell having elastic walls.
Water Potential in Cells:
The concepts developed on the basis of artificial systems using sugar solution can be successfully transferred to a cell (Fig. 6-4).
Cell is enclosed by a semipermeable membrane and osmosis takes place across this membrane. If a cell is immersed in a solution having high Ψπ (e.g. pure water or a dilute solution), water will diffuse in the cell and the latter will become turgid.
The external solution is referred to as hypotonic solution. In a situation where cell is immersed in a solution having Ψπ equal to its cell sap, no net water diffusion would occur and the cell will remain flaccid or lacks turgor. This solution is called isotonic solution. If the concentration of external solution is more than the cell sap, its Ψπ will be lower than that of the cell sap. If a cell is immersed in such a solution (hypertonic), water will diffuse out and the protoplast will pull inside and become plasmolyzed [Fig. 6-4 (C)].
If such a plasmolyzed cell is placed again in a hypotonic solution, it will again become turgid.
Water potential of a cell has two components (e.g., osmotic and pressure potentials) as follows:
Ψ = Ψπ+ Ψp
When a cell is immersed in water or a solution and comes in equilibrium the water poential of cell (Ψinside) is equal to the water potential outside (Ψ outside):
Ψπ (inside) + Ψp (inside) = Ψ (inside) = Ψ (outside)
Ψ (outside) is also the total of Ψπ (outside) and Ψp (outside). At atmospheric pressure Ψp = 0, therefore Ψ (outside) = Ψπ (outside).
Thus at equilibrium
Ψπ (inside) + Ψp (inside) =Ψπ (outside)
This may also be mentioned as Ψπ (inside) = Ψπ (outside) -Ψp (inside) and this osmotic potential of the cell sap can thus be measured.
Movement of Water from Cell to Cell:
Differences in water potential (∆Ψ) are important for the water movement in and out of the cell. These differences are relevant as compared with the environments. Likewise water moves from cell to cell by diffusing down the water potential gradient between the two cells.
The direction of water movement and the force of movement are linked with water potential in each cell and also on the difference between the water potential of the two cells (Fig. 6-5).
In the instance mentioned below we observe that:
Obviously water will flow from cell B to cell A i.e., towards the lower or more negative water potential.
The value of ∆Ψ is very vital since it is directly proportional to the rate of movement of water from one cell to another.
The rate and amount of water movement is dependent upon the difference in water potential on either side of the membranes.
Osmotic potential of a solution:
The osmotic potential of a solution can be measured with the help of an osmometer (Fig. 6-6). It is a simple apparatus consisting of a cylinder having water tight sliding piston and a differentially permeable membrane at one end.
When it is immersed in pure water (Ψπ = 0) P, the pressure developed in the osmometer is equal and opposite to the osmotic potential at equilibrium.
Ψπ = RT/V
V = volume of the solution containing a given amount of the solutes
T = temperature as expressed in degree absolute
R = gas constant (solute molecules freely diffuse as if they were a gas, the constants K1 and K2 can be replaced by it).
Water movement as explained on the basis of old approach to osmosis:
For a long time osmosis was explained on the basis of water diffusion from a zone of high concentration to the lower concentration (diffusion pressure deficit: DPD).
However, this is not correct since some of the solutions occupy a volume smaller than the same weight of pure water.
It was also believed that a solution in a cell was as if sucking water into the cell by a force regarded as a negative pressure.
Several terms w ere used to explain these concepts. In recent years several of these terms have been discarded and more acceptable explanations based on thermodynamic concepts have been advanced. Terms used currently and their old equivalent corresponding terms are given in Table 6-1.