- The water potential term was coined by Slayer and Taylor (1960). It is a modern term that is used in place of DPD, The movement of water in plants cannot be accurately explained in teams of difference in concentration or in another linear expression.
- The best way to express the spontaneous movement of water from one region to another its in terms of the difference of free energy of water between two regions (from higher free energy level to lower free energy levels).
- According to principles of thermodynamics, every component of the system is having a definite amount of free energy which is a measure of potential work that the system can do. Water Potential is the difference in the free energy or chemical potential per unit molar volume of water in the system and that of pure water at the same temperature and pressure.
- It is represented by Greek letter or the value of is measured in bars, pascals, or atmospheres. Water always moves from the area of high water potential to the area of low water potential. The water potential of pure water at normal temperature and pressure is zero. This value is considered to be the highest. The presence of solid particles reduces the free energy of water and decreases the water potential. Therefore, the water potential of a solution is always less than zero or it has negative value.
Components of Water Potential:
- A typical plant cell consists of a çell wall, a vacuole filled with an aqueous solution, and a laver of cytoplasm between the vacuole and cell wall. When such a cell is subjected to the movement of water then many factors begin to operate which ultimately determine the water potential of cell sap.
- For solution such as contents of cells, water potential is determined by 3 major sets of internal factors:
(a) Matrix potential
(b) Solute potential or osmotic potential
(c) Pressure potential
- Water potential in a plant cell or tissue can be written as the sum of matrix potential (due to Minding of water to cell and cytoplasm) the solute potential (due to concentration of dissolve solutes which by its effect on the entropy components reduces the water potential) and pressure potential (due to hydrogenation pressure, which by its effect on energy components increases tile water potential).
- In the case of the plant cell, m is usually disregarded and it is not significant in osmosis. Hence, the above-given equation is written as follows.
- It is defined as the amount by which the water potential is reduced as the result of the presence of the solute, s are always in negative values and it is expressed in bars with a negative sign.
- The plant cell wall is plastic and it exerts a pressure on the cellular contents. As a result of the inward wall pressure, hydrostatic pressure is developed in the vacuole it is termed as torpor pressure. The pressure potential is usually positive and operates in plant cells as wall pressure and turgid pressure, Its magnitude varies between +5 bars (during the day) and +15 bars (during the night).
Important Aspects of Water Potential:
(1) Pure water has maximum water potential. by definition is zero. (2) Water always moves from a region of higher to one lower.
(2) All solutions have lower w than pure water.
(4) Osmosis in terms of water potential occurs region of higher water potential to a region of lower water potential through a semi-permeable membrane.
Osmotic Relations of Cells According to Water Potential:
- In case of fully turgid cell:
- The net movement of water into the cell is stopped. The cell is in equilibrium with the water outside. Consequently, the water potential in this case becomes zero. Water potential is equal to osmotic potential + pressure potential.
- In the case of flaccid cell: The turgid becomes zero. A cell at zero turgid has an osmotic potential equal to its water potential.
- In the case of polysemous cell:
- When the vacuolated houseparents cells are placed in a solution of sufficient strength, the protoplast decreases in volume to such an extent that they shrink away from the cell wall and the cells are polysemous. Such cells are the negative value of pressure potential (negative torpor pressure).
- Suppose there are two cells A and B, cell A has osmotic potential -16 bars, pressure potential – 6 bars, and cell B as osmotic potential –10 bars and pressure potential- 2 bars, What is the direction of movement of water?
- Water potential of cell A v, +y,”- 16 + 6–10 bars y of cell B=-10 +2-8 bars.
- As the movement of water is from higher water potential (lower DPD) to lower water potent is (higher DPD), hence the movement of water is from cell B to cell A.
- If the osmotic potential of a cell is – 14 bars and its pressure potential is 7 bars. What would be its water potential?
- We know w. v,+ v,
- Given, osmotic potential (y.) is-14 bars.
- Pressure potentials (y) is 7 bars
- Therefore, Water potential = (-14) + 5= -9 bars.