Measuring water potential in potato tubers
Objective
The objective of this lab is to measure the water potential of potato tuber tissues. We will monitor water uptake or loss by tissues in different sucrose solutions to identify an isotonic solution with water potential equal to that of the tissue.
Introduction
Unlike animals, plants do not have a circulatory system driven by a mechanical pump. Instead, plants rely on physical forces to move water in and around their tissues, both short distances from cell to cell and long distances through the xylem. Water potential () is a measure of the driving force that governs the movement of water from the soil into plants and finally into the atmosphere. Water potential is the amount of energy per unit volume (or pressure) contained in a system (like a plant cell, tissue, or soil) and is expressed in units of megapascals (Mpa). For reference, pure water in a free standing solution has a water potential of zero, while most plant cells have a negative water potential. The atmosphere typically has a very negative water potential, much more negative than a typical plant tissue, while the water potential of the soil solution is typically less negative than a typical plant tissue. Water will always move from a tissue or area of higher water potential to a tissue or area of lower (more negative) water potential. Thus, it is energetically favorable under normal conditions for water to move into a plant from the soil, through the plant, and out to the atmosphere down a water potential gradient. However, the rate of movement varies considerably depending upon such external factors as soil moisture availability and relative humidity of the atmosphere.
Water potential of a plant cell is made up of two important components, and the relationship among these components is expressed mathematically as:
(Eq. 1) = s + p
is the overall water potential of a cell.
s is the solute or osmotic potential and represents the contribution made by dissolved solutes to . Adding dissolved solutes to a system always decreases water potential, thus this component is always negative. In a plant cell, important contributors to solute potential include mineral ions, sucrose, starch, amino acids, proteins, and anything else that can accumulate to high levels in the cytosol or vacuoles.
p is the pressure potential and represents the contribution made by pressure to . Fully turgid cells whose plasma membranes are pressing against the cell wall have a positive p. Cells at incipient plasmolysis (the point at which the membrane is just barely touching the cell wall) have a p of zero. Cells under tension, like those in the xylem during active evapotranspiration, have a negative p.
The objective of this experiment is to measure the of potato tubers. When placed in a free standing sucrose solution, water will move into or out of a plant tissue depending upon its water potential relative to the solution. Gain or loss of water can be detected by weighing the plant tissue before and after immersion in the solution. By incubating tissues in a series of sucrose solutions of different concentrations, the solution that causes no change in tissue weight can be identified. The water potential of this isotonic solution is assumed to equal the water potential of the tissue.
PROCEDURE
1) Create the following sucrose solutions (MW = 342.3 g/mol): Distilled water, 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7 molal (mol/kg solution) sucrose. Add 75 mL of each solution to the appropriate labeled beaker using a separate graduated cylinder for each solution.
2) Cut 16 cylinders from a potato and trim each cylinder to 4 cm in length with a knife, being careful to remove the peel. Work quickly to prevent the tissues from drying out as you cut them. What problems would tissue drying cause in the experiment?
3) Quickly blot the cylinders on paper towels to remove any excess moisture and weigh the cylinders in sets of two to the nearest 0.01g. Record the weights in Table 1. After weighing, quickly transfer one set of two cylinders to each beaker and ensure that they are fully immersed in the solution.
4) After 45 minutes, remove the cylinders, blot excess moisture with paper towels, and reweigh them in sets of two, exactly as in step 3. Record the weights in Table 1.
Calculations
First, subtract the initial tissue weights from the final weights. Second, divide the difference by the initial weight and multiply by 100 to get the percent weight change. Record your calculations in Table 1. Next, plot the percent change in weight (ordinate) vs. sucrose concentration (abscissa) using Excel. Using this figure (and the regression line determined from it), determine the exact concentration of sucrose that would cause no change in weight in the potato tubers. The water potential of this solution will equal the water potential of the potato tissue.
In an open solution where there is no turgor pressure, the p is equal to zero. Thus, the of such a solution is equal to the s of a solution. Calculate the s of the solution causing no change in weight of the potato tissues using the following formula:
(Eq. 2) s = miRT
m = molarity
i = ionization constant = 1 for sucrose
R = gas constant = 8.31 J K ^{1} mol ^{1}
T = room temperature in K (ºC + 273 = K)
First, convert molality of the appropriate sucrose solution to mol m^{3 }(Note that 1 molal = 1 x 10^{3} mol m^{3}), and then use Eq. 2 to calculate the s of your solution. The results of this calculation will be in units of J m^{3} (energy per unit volume), which is equal to a unit of pressure in Pa. Convert your answer to Mpa by dividing by 10^{6}. Show your calculations in the space provided and include them in your report.
Table 1. Weight change in potato tissues in sucrose solutions of different concentration.
Molal
[Sucrose]

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

Initial weight









Final weight









weight









% weight









Do your calculations of here:
Due next week in lab:
1. Excel graph with regression line.
2. Table 1.
3. Value of s with calculations shown.
4. Answers to the following questions:

Why did we use sucrose as the solute in our solutions? How might using another solute influence the results?

Tissues in which treatments have a water potential equal to that of their solution after the incubation period? How can you tell?

What influence would increasing temperature have on our calculation of water potential?

The results of the calculations of water potential usually vary among years, among lab sections, and even among groups within one lab. Why might this be?

What deficiencies were present in our experimental design? Disregarding human error, how could even better data be obtained?
