# Measuring water potential in potato tubers

MEASURING WATER POTENTIAL IN POTATO TUBERS
OBJECTIVE
The objective of this lab is to measure the water potential of potato tuber tissues.
We will measure water uptake or loss by tissues in different sucrose solutions to
identify an isotonic solution with water potential equal to that of the potato tissue.
INTRODUCTION
The water potential is a measure of the force that drives the movement of water
across the cell membrane by osmosis. Pure water has a water potential of zero,
while plant cells, normally, have a negative water potential. Water will always move
from an area of higher water potential to an area of lower (more negative) water
potential. Thus, under normal conditions, water will move from pure water into a
plant cell.
The objective of this experiment is to measure the water potential of potato tubers.
When placed in a free-standing sucrose solution, water will move into or out of plant
tissue depending upon its water potential relative to the sucrose 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.
METHOD
1) Create 1500 ml of one of the following five sucrose solutions:
0.00 M (distilled water), 0.15 M, 0.30 M, 0.45 M, 0.60 M.
Explanation of molarity:
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M is a unit of molarity, which indicates the number of dissolved moles per
liter: 𝑚𝑚𝑚𝑚𝑚𝑚 𝑙𝑙 −1.
One mole of sucrose is equivalent to 342 grams of sucrose.
So if you want to make 1500 ml of 0.15 M you need to dissolve (0.15 x
342) x 1.5= 76.95 grams in 1500 ml.
2) Cut 5 rectangular potato strips from a potato. Make them three cm long and
1 cm thick (square).
3) Blot the strips on paper towels to remove any excess moisture and weigh
each strip to the nearest 0.1g. Record the weights, you can use the table
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below as an example to record data. After weighing, put a marked toothpick
into each potato strip (for identification)
5) After 45 minutes, remove the strips, blot excess moisture with paper towels,
and reweigh them. Record the weights again.
Calculations
Calculate the average percentage change for each batch, as well as the standard
deviation. Plot the average percent change in weight (y-axis) versus the sucrose
concentration (x-axis).
Using this graph (and the trend line determined from it), determine the exact
concentration of sucrose that would cause no change in weight in the potato tubers
(0%). The water potential of this solution will equal the water potential of the
potato tissue.
Calculate the water potential of the solution causing no change in weight of the
potato tissues using the following formula:
Water potential = −𝒎𝒎 ∙ 𝑻𝑻 ∙ 𝟖𝟖. 𝟑𝟑𝟑𝟑
Where:
m = molarity x 1000
T = room temperature in Kelvin (oC + 273 = K)
Use the equation to calculate the water potential of your solution. The results of this
calculation will be in units of pressure: Pascal (P). Convert your answer to
MegaPascal (MPa) by dividing by 106. Show your calculations and include them in
your report. Can you find online information of the water potential of potato tissue
to verify if your calculation is (roughly) correct?
Write a full lab report on this experiment. The lab report will be assessed against
the following criteria: analysis, evaluation, communication. The rubrics are given
on the pages below. READ THEM BEFORE HANDING IN!
Here is possible setup for a table for the RAW data (you’ll have to add title,
headers, units, uncertainties yourself!). In a next table you need to present the
PROCESSED data (averages and standard deviations). The processed data is the
basis for your SCATTER GRAPH (with error bars/trendline).
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Sucrose Concentration / M &plusmn; 0.05 Trial Initial mass / g &plusmn; 0.05 Final mass / g &plusmn; 0.05 Percentage Change
0
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0.15
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0.3
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5
0.45
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0.6
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