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Here is an experiment to demonstrate water potential of plant tissues.
Theory:
The chemical potential of water is referred to as water potential Ψ (psi) and is a property of great importance to an understanding of water movement in the plant-soil-air system. Water potential (Ψ) is usually expressed in terms of pressure (e.g. bars). Absolute values of chemical potential of water potential (Ψ) are not easily measured, but differences in y can be measured with comparative ease.
The fundamental cell water potential Ψ is:
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Ψ cell = Ψ π + Ψ p + Ψm1
where Ψ cell = Water potential of a cell
Ψ π = Osmotic potential
Ψ p = Pressure potential (turgor pressure)
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Ψ m = Metric potential
The water potential (Ψ) of pure water at normal atmospheric pressure is equal to zero; hence the y of water in cells and solution is typically less than zero or negative.
According to one common method of measuring water potential in plant tissues, uniform sample pieces of tissues are placed in a series of solutions of a non-electrolyte like sucrose or mannitol. The object is to find that solution in which the weight and volume of the tissue does not change indicating neither a net loss nor a net gain in water.
Such a situation would mean that the tissue and the solution are in osmotic equilibrium to begin with, and so the Ψ of the tissue must equal the Ψ of the external solution. Thus, if one can calculate the Ψ of the external solution in which no change in weight or volume of the tissue occurs, one can calculate the Ψ of the tissue.
Materials and Equipments Required:
1. 12 beakers (250 ml) containing 100 ml of one of the following: distilled 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. Potato slice freshly prepared from tubers by cork borer (approx. 1 cm in diameter).
3. Analytical balance.
4. Graph papers, blotting paper etc.
Procedure:
1. Using a cork-borer of approx. 1 cm diameter obtain from a single potato tuber 12 cylinders, each at least 3 cm — preferably 4 cm — long.
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2. Cut all the 12 cylinders into measured and uniform length with a razor blade, bearing a clean transverse cut at the end of each cylinder.
3. Place the cylinders between the folds of a moist paper towel, on which the positions of the cylinders are denoted by the series of concentrations of sucrose to be used.
4. Using an analytical balance, weigh each cylinder to the nearest milligram.
5. Immediately after each cylinder is weighed, cut it into uniform slices, approximately 2 mm thick, and place all the slices obtained from one cylinder in one of the test solutions.
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6. Do this for each cylinder, being sure that the initial weight of the cylinder placed in each test solution is accurately recorded.
7. After 1.5-2.0 hrs. of incubation, remove all the slices from one test solution, blot gently on paper towels and weigh.
8. Repeat this procedure until all the samples have been weighed in the chronological order, in which they were initially placed.
9. Present the data in a tabular form showing initial weight, final weight, change in weight and percentage change in weight
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where percentage change in weight = Final weight – Initial weight/Initial weight
10. Then construct a graph (Fig. 3.3) plotting changes in weight or % change in weight (on ordinate) versus sucrose concentration (in molality, m) and osmotic potential (in bars) (on abscissa).
11. Calibrate the osmotic potential axis after first calculating the osmotic potential (Ψπ) for each sucrose solution.
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Use the following formula:
Ψπ = mi RT
where, m = Molality of the solution
i = ionization constant; numerical value of ‘1’ for sucrose
R = gas constant (0.083 liter bars/mole degree)
T = absolute temperature (= °C + 273)
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12. Determine by interpolation from the graph the sucrose concentration in which no net change in weight has occurred. Calculate the Ψπ for this solution; this value then equals the water potential (Ψ) of the tissue.
