Aim: To determine the water potential of a potato tuber by tissue volume and weight method

Tashi Wangmo, Yonten Choki, Sonam Yonten, Tshering Rabten

I.          THEORY

Water potential is the ability of water molecules to move freely in solution, or the difference in potential energy between a given water sample and pure water. It is denoted by the Greek letter psi (Ψ)[1] (Bland, 2014). The potential of pure water is zero. Movement of water in and out of the cell in both plant and animal cell is governed by the relative solute concentration, when the cells are separated by semi permeable membranes. If the solute concentration outside the cell tends to be higher than inside, water moves out of the cells leading to cell shrinkage and vice versa. The condition where cells shrink in size is called plasmolyzed state. The relation between tissue and water plays an important role in regulating the solute transport within the plant (Oparka et al., 1988). According to Boyer, 1968, the water potential of the tissue affects the growth rate because of their role of turgor in cell enlargement. The direction of the water movement can be detected in two ways: a) by measuring the change in tissue such as length, weight or volume and b) measuring change in the solution concentration(Bethke et al., 2009). water content of tubers fluctuates and depends on rate of transpiration and water availability in the soil (Geigenberger et al., 2021). This experiment basically deals with the change in the weight of the potato tissue in varying sucrose concentration. Knowing the water potential has a great scope in agriculture as it can determine better growth and productivity of individual plants.

Figure 1:  Some of the materials used; a) Sucrose, b) Potatoes, c) Conical flasks, d) Cork borer and glass rod

MATERIALS REQUIRED

Materials	Chemicals
Potato tuber (Specimen), Cork borer, measuring cylinder, Conical flask, Analytical balance, Dropper, Petri plate, Blotting paper, Beaker, Spatula, Forceps, Blade, Test tubes, Glass rod and Test tube rack.	Sucrose Distilled water

III.          PROCEDURE

1. A stock solution was prepared by dissolving 17.1 g of sucrose in 50 ml of distilled water (Figure 2a).
2. Different concentrations of sucrose from the stock solution were prepared (0.05M, 0.15M,0. 20M, 0.25M, 0.30M, 0.35M, 0.40M and 0.45M) by diluting from stock solution making the final volume to 50ml in 8 different test tubes (Figure 2b).
3. Took out a cylindrical section of potato with the help of Cork borer (Figure 2c).
4. Cut each cylindrical potato in the length of 2-3 cm with the help of a blade. A total of 18 potato cylinders were taken each with 2cm in length (Figure 2d). Each potato tuber was weighed before immersing in the sucrose solution in the test tube with the help of analytical balance (Figure 2e). Note the Initial reading of two potatoes tuber and immersed in the various concentrations of sucrose solution.
5. Incubated the potato for about 1 to 1.5 hours in the solution (Figure 2f).
6. After the completion of incubation period, the potato tuners were again weighted. Marked the final readings
7. The samples were measured with analytical balance in sequence starting from lowest concentration to highest concentration.

FIGURE 2 A) STOCK SOLUTION PREPARATION, B) USAGE OF CORK BORER, C) MEASUREMENT OF A POTATO TUBER BY ANALYTICAL BALANCE, D) DIFFERENT SUCROSE CONCENTRATION, E) 2CM LENGTH OF POTATOES TUBER, F) POTATO IN SUCROSE SOLUTION. DATA AND FIGURES

IV.    Result     

• Two readings are tabulated to get the exact mean value. The percent (%) change in weight for initial and final weight values are also calculated. The formula used is:

Table 1: Observation table showing change in potato weight by tissue and weight method

Sucrose solution (M)	Initial weight (g)	Initial mean weight (g)	Final weight (g)	Final mean weight (g)	Change in weight (g)	% Change in weight
Control (distilled water)	1.06	1.15	1.22	1.15	0	0
1.17	1.08
0.05	1.91	1.49	1.98	1.59	0.1	6.71
1.08	1.21
0.15	1.1	1.09	1.18	1.16	0.07	6.42
1.12	1.15
0.2	1.1	1.09	1.14	1.15	0.06	5.5
1.09	1.17
0.25	1.11	1.11	1.16	1.16	0.05	4.5
1.17	1.07
0.3	1.08	1.14	1.13	1.1	-0.04	-3.5
1.22	1.07
0.35	1.11	1.09	1.07	1.05	-0.04	-3.66
1.22	1.04
0.4	1.09	1.16	1.06	1.11	-0.05	-4.31
1.1	1.16
0.45	1.11	1.11	1.1	1.05	-0.06	-5.4
1.17	1.05

• DESCRIPTION OF TECHNIQUES AND FIGURES

Stock solution is prepared by measuring 17.1g of sucrose with analytical balance and dissolved in 50ml of distilled water. Various concentrations ranging from 0.05M, 0.15M, 0.2M, 0.25M, 0.3M, 0.35M and 0.4M are prepared. With the help of a cork borer, a cylindrical tube of potato tuber is prepared. Two potato tubers, each with 2cm are taken in each test tube with various concentrations so that the chances of errors in reading gets minimized.  Before the potatoes are dipped in solution, their initial weight is recorded. It was kept in the solution for 1 and ½ hour. A control was also prepared alongside to compare the results. After 1 and ½ hours the potato tubers were measured again by analytical balance and noted down their final readings and calculated the change in weight and % change in weight (Table 1)

VI. DISCUSSION

Graph 1. % of change in weight against the sucrose concentration (M).

Measuring of the potato tubers weight after 1 and ½ hour, there was a change in their weight. A control was set up with two potato tubers in distilled water. Since the water potential of a pure water is 0, there was no change in the weight of potato tubers placed inside distilled water. However, those placed in various concentrations of sucrose solution have led to change in potato 5weight. The sucrose solution with 0.05M which is least concentrated, prepared by dissolving 2.5 ml of sucrose from the stock solution and 47.5 ml of distilled water, the % change in weight was found to be 6.71g. As the concentration of sucrose solution increases the water potential decreases. As the water potential decreases the weight of the potato tuber also decreases respectively (Graph 1). The weight of potato decreases as the concentration increases, this is because due the presence of solutes, the water from potato moves out through the exo-osmosis process. The water potential values can further reach to the lowest negative value if the concentration of solute keeps on increasing.

From the above graph we can make out the solute potential of the potato tuber was 0, at the sucrose concentration of 0.28M.

Where, I= Ions it breaks into (Sucrose and Glucose always have ionization constant of 1) C= Molar concentration (0.28M) R= Pressure Constant (0.0831-liter bar/ mole k) T= Temperature in Kelvin (273+ °C)

If the pressure potential is 0, then the water potential will be equal to the solute potential. The solute potential can calculate using the formula;

 Graph 1. Relation between water potential and sucrose concentration.

Figure 3. Relation between tissue water potential and concentrations of Mannitol and Ethylene glycol (Oparka et al., 1988).

The water potential of a tissue always decreases as the concentrations of various solution increases. To ensure that our results are true, it was compared with the findings of other published paper with regard to tissue water potential. Mannitol and ethylene glycol was used to make solution. A potato tuber was incubated for 90 minute (Figure 3) and progressive decrease was observed in their weight[2] (Oparka et al., 1988).

V. CONCLUSION AND PRECAUTIONS

Determining the water potential of a plant with the tissue volume and weight method can help in survival and growth of a plant. The water potential will be different for each plant though they belong to the same species. This is because it depends on the nature and quality of nutrients that they are grown with. Some of the precautions to be taken into consideration while carrying the experiment are;

• The analytical balance must be tare to zero before measuring the weight of the next potato tuber.
• The length of each potato tuber must be uniform.
• Excessive sucrose solution and water must be blotted if required, before measuring the weight to remove the errors.
• The test tubes must be marked properly with respective concentrations.

VI.          REFERENCES

Bethke, P. C., Sabba, R., & Bussan, A. J. (2009). Tuber Water and Pressure Potentials Decrease and Sucrose Contents Increase in Response to Moderate Drought and Heat Stress. 519–532. https://doi.org/10.1007/s12230-009-9109-8

Bland, W. L. (2014). Short C o m m u n i c a t i o n. November 1986. https://doi.org/10.1007/BF02852927

Geigenberger, P., Reimholz, R., Geiger, M., Merlo, L., Stitt, M., Geigenberger, P., Reimholz, R., Geiger, M., Merlo, L., Canale, V., & Stitt, M. (2021). water deficit Linked references are available on JSTOR for this article : Planta in response to short-term water deficit. 201(4), 502–518.

Oparka, K. J., Wright, K. M., Oparka, K. J., & Wright, K. M. (1988). Planta ( 1988 ) 175 : 520-526 Planta Influence of cell turgor on sucrose partitioning. 175(4), 520–526.

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[1] Psi Ψ is expressed in the unit of MPa- Mega Pascal.

[2] As the tissue weight decreases, water potential also decreases.