Here is an experiment to demonstrate osmotic pressure in plant tissues.
The phenomenon ‘Osmosis’ refers to the movement of water from a solution of higher water potential to one of lower water potential, across a differentially permeable membrane which separates the two solutions.
The magnitude of osmotic forces in plant cells and tissues can be estimated in terms of solute potential (Ψ S), which was formerly termed as ‘Osmotic Pressure’. The solute potential is expressed in bars with a negative sign.
There are several methods available at present for the measurement of solute potential in plant systems — using thermocouple psychomotor or by membrane osmometer, or by cryoscopic method, or by plasmolytic method.
In the plasmolytic method — which is based on the phenomenon of plasmolysis — a solution is identified which will cause only slight —just barely visible — separation of the protoplast from the cell wall.
This condition is known as ‘incipient plasmolysis’. At incipient plasmolysis, the cell wall exerts no pressure on the cell contents (i.e. pressure potential is zero). Therefore, the water potential in the cell is equal to the solute potential.
Water potential (Ψ w) = Solute potential (Ψ s) + Pressure potential (Ψ p).
The procedure involves immersing different pieces of the same tissue in a series of graded solutions of known water potentials. Usually, sucrose solutions are used because plant cells absorb sucrose from an external solution at very low rates.
After the tissue samples have been immersed in sucrose solutions for about 15-30 min, the cells in each sample are examined under a microscope. None of the cells in the pieces immersed in dilute solutions will be plasmolysed, whereas all the cells in the pieces immersed in concentrated solutions will be plasmolysed.
Somewhere in the series of graded solutions will be one in which half of the cells will be slightly plasmolysed. This solution is considered as the one which induces incipient plasmolysis. The water potential in this solution is equal to the solute potential in the cells. The values of solute potential in plant cells have been found to lie between -10 to -20 bars.
Materials and Equipments Required:
1. 12 beakers (50 ml) containing 20 ml of one of the following: water, 0.05, 0.10, 0.15, 0.20, 0.25, 0.30, 0.35, 0.40, 0.45, 0.50 and 0.60 molar sucrose solutions.
2. Lower epidermal pealing’s of Rhoeo leaves.
4. Slide, Cover-glass, Forceps, Scalpel, Graph papers, Blotting paper etc.
1. Take several thin pealing’s of lower epidermal layer of Rhoeo leaves and then dip in water.
2. Examine the uni-formness of the pealing’s before placing them in different test solutions separately.
3. Dip the pealing’s in test solutions for 20-25 minutes.
4. Examine the pealing’s under microscope to determine the number of cells plasmolysed against each test solution. Finally calculate the % of plasmolysis in each test solution and graphically plot the data.
5. Record the molar concentration from the graph (Fig. 3.4) at which 50% plasmolysis is possible.
6. Solute potential in them determined by the following formula:
where, m = Molarity of solution
i = Ionization constant, numerical value of 1 for sucrose
R = Gas constant (0.083 liter bars/mol. degree)
T = Absolute temperature (= °C + 273)
Measurement of the solute potential of plant tissues by plasmolytic method: