7.1 Plant Hormones Fruit Ripening Experiment
Objective
The purpose of this experiment is to measure fruit ripening
caused by the plant hormone ethylene, by using an iodine
indicator to detect the conversion of plant starch to sugar.
Hypothesis :
The ripening of an unripe fruit will be unaffected by storing it with
a banana.
Introduction
You've heard that 'one bad apple spoils the whole bushel', right? It's true. Bruised, damaged, or overripe
fruit gives off a hormone that accelerates the ripening of the other fruit.
Plant tissues communicate by means of hormones. Hormones are chemicals that are produced in one
location that have an effect on cells in a different location. Most plant hormones are transported through
the plant vascular system, but some, like ethylene, are released into the gaseous phase, or air.
Ethylene is produced and released by rapidly-growing plant tissues. It is released by the growing tips of
roots, flowers, damaged tissue, and ripening fruit. The hormone has multiple effects on plants. One is
fruit ripening. When fruit ripens, the starch in the fleshy part of the fruit is converted to sugars. The
sweeter fruit is more attractive to animals, so they will eat it and disperse the seeds. Ethylene initiates
the reaction in which the starch is converted into sugar.
Iodine solution binds to starch, but not to sugar, forming a dark-coloured complex. You can estimate how
ripe a fruit is by whether or not it is darkened after painting it with an iodine solution. Unripe fruit is
starchy, so it will be dark. The more ripe the fruit is, the more starch will have been converted to sugar.
Less iodine complex will be formed, so the stained fruit will be lighter.
Materials
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8 sealable plastic bags, large enough to contain a whole apple/pear and banana
4 ripe bananas
8 unripe pears or 8 unripe apples (pears usually are sold unripe, so they may be a better choice than
apples)
potassium iodide (KI)
iodine (I)
distilled water
graduated cylinders
large brown glass or plastic bottle (not metal)
shallow glass or plastic tray or dish (not metal)
knife for cutting fruit
Safety Information
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Do not use metal utensils or containers for preparing or storing the iodine solutions. Iodine is
corrosive to metals.
The iodine solutions will stain skin and clothing.
Read the safety information for chemicals used in the lab and follow safety precautions.
After the experiment is completed, the stain may be washed down the drain.
Prepare the Test & Control Groups
1. If you are not sure your pears or apples are unripe, test one using the staining procedure outlined
below before continuing.
2. Label the bags, numbers 1-8. Bags 1-4 will be the control group. Bags 5-8 will be the test group.
3. Place one unripe pear or apple in each of the control bags. Seal each bag.
4. Place one unripe pear or apple and one banana in each of the test bags. Seal each bag.
5. Place the bags together. Record your observations of the initial appearance of the fruit.
6. Observe and record the changes to the appearance of the fruit each day.
7. After 2-3 days, test the pears or apples for starch by staining them with the iodine stain.
Make the Iodine Stain Solution
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Dissolve 10 g potassium iodide (KI) in 10 ml of water
Stir in 2.5 g iodine (I)
Dilute the solution with water to make 1.1 liters
Store the iodine stain solution in a brown or blue glass or plastic bottle. It should last for several
days.
Stain the Fruit
1. Pour the iodine stain into the bottom of the shallow tray, so that it fills the tray about half a
centimeter deep.
2. Cut the pear or apple in half (cross section) and set the fruit into the tray, with the cut surface in
the stain.
3. Allow the fruit to absorb the stain for one minute.
4. Remove the fruit and rinse the face with water (under a tap is fine). Record the data for the fruit,
then repeat the procedure for the other apples/pears.
5. Add more stain to the tray, as needed. You can use a (non-metal) funnel to pour unused stain back
into its container, if you wish, since it will remain 'good' for this experiment for several days.
Examine the stained fruit. You may wish to take photographs. The best way to compare the data is to set
up some sort of scoring. Compare the levels of staining for unripe versus ripe fruit. The unripe fruit
should be heavily stained, while fully ripe or rotting fruit should be unstained. How many levels of
staining can you distinguish between the ripe and unripe fruit?
You may wish to make a scoring chart, showing staining levels for unripe, ripe, and several intermediate
levels. At a minimum, score your fruit as unripe (0), somewhat ripe (1), and fully ripe (2). This way, you
are assigning a quantitative value to the data so that you can average the value for ripeness of the
control and test groups and can present the results in a bar graph.
Test Your Hypothesis
If the ripening of the fruit was unaffected by storing it with a banana, then both the control and test
groups should be the same level of ripeness. Were they? Was the hypothesis accepted or rejected? What
is the significance of this result?
You can take your experiment further with variations, such as these:
Fruit produces ethylene in response to bruising or wounding, too. Will the pears or apples in the
experiment ripen more quickly if the ethylene concentration is higher, from using bruised bananas rather
than undamaged bananas?
If you have more bananas, you will have more ethylene. Does using more bananas cause the fruit to
ripen faster?
Temperature affects the ripening of fruit, too. Not all fruits are affected the same way. Apples and pears
ripen more slowly when refrigerated. Bananas blacken when they are refrigerated. You could place a
second set of Controls and Test Bags in the refrigerator to explore the effect temperature on ripening.
Fruit ripening is affected by whether or not the fruit remains attached to the parent plant. Ethylene is
produced in response to removing the fruit from its parent. You can design an experiment to determine
whether fruit ripens more quickly on or off the plant. Consider using a smaller fruit, such as tomatoes,
which you can find on/off the vine in supermarkets.
Writing up your practical report
Write the report in the 3 rd person, past tense.
eg :
“ We placed a banana…..” should be written “ A banana was placed…”
“I or we found that….” should be written “It was found that….”
Title: make it descriptive
Introduction
-Background
Include the discussion of the following questions:
- What are plant hormones?
- How does a plant use them?
- What are the 5 main hormones?
- What are tropisms?
- What does ethylene do to fruit?
- How can you test for the effect ethylene?
- What are the chemical and physical changes that occur as fruit ripens?
- How can an iodine stain be used to distinguish between ripe and unripe fruit?
-Aim
- 1 short paragraph outlining the aim of this investigation
-Hypotheses
- What are your hypotheses? ( H1 and H2 etc……..)
Materials and Methods
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Include all the materials used and a step by step guide to the experiment.
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Include the following table in your results. Draw a graph illustrating these results.
Include a brief description of the results.
Results
Temp
0
C
Control or Experimental
group
Fridge
Control 1
Pear only
250
Control 2
Pear only
Fridge
Experimental 1
Pear and ripe banana
250
Experimental 2
Pear and ripe banana
250
Experimental 3
Pear and 2 ripe bananas
Photo
before
Photo
after
Observations
+ve or –ve
starch test
Overall
ripening
H, M, L
Discussion
Include a discussion of the following questions. Refer to Table 1 or Graph 1 when necessary.
If the ripening of the fruit was unaffected by storing it with a banana, then both the control and
test groups should be the same level of ripeness. Were they? Was the hypothesis accepted or
rejected? What is the significance of this result?
What other factors were varied and did they have any impact on the degree of ripening?
What are some of the triggers for ethylene production by plants?
How does the presence of ethylene affect fruit ripening?
Any improvements to the experimental design you can suggest
What is a future experiment that can be completed to further understand the effect of ethylene?
Conclusion
A clear statement of whether or not you findings have supported your hypotheses
References
List all references , websites etc you have consulted
Other things to remember
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Type up your practical report
Include a list of all references
Make sure you use your own words
Due : ( DEADLINE)- Friday August 1
email to Pete hamilton.peter.p1@edumail.vic.gov.au