A Science and Plants for Schools – Student Project Starter
Respiration . . . and how it changes during ripening
and storage of fruits and vegetables
Have you thought about respiratory activity in fruits and vegetables AFTER they have
been harvested . . . an apple when picked off a tree, or a box of strawberries, bag of
potatoes or bundle of carrots in a supermarket?
In plants, respiration consumes a significant fraction of the carbon fixed each day in
photosynthesis, and half of this respiration is for maintenance rather than growth.
Some studies with corn (Zea mays) and perennial ryegrass (Lolium perenne) have
indicated that increases in respiratory rate lead directly to reduced crop yields.
The greatest respiratory rates are found in the more metabolically active tissues, such
as those in the growing and developing regions of the plant. In general terms, the
more mature plant tissues have lower respiratory rates. A spectacular exception to this
rule is the spadix of the Arum maculatum (Cuckoo pint) plant, which has a very high
respiratory rate for just one night, to drive off volatile chemicals as insect attractants.
The other major exception is climacteric fruits, in which a marked rise in respiratory
rate accompanies ripening. This can lead to some interesting investigations, for
example, using different fruits or vegetables and measuring respiratory rate during the
ripening process.
You will find that there are several ways you can study the metabolic rate of plant
tissues such as fruits and vegetables. Here are some suggestions that might be
interesting to investigate:

Does the rate of respiration vary during the ripening process?

Does the temperature of storage affect the rate of respiration?

Does the skin affect the exchange of gases between a fruit or vegetable and the
atmosphere?

Does an infected fruit or vegetable have a higher respiratory rate than a healthy
one?
Comparing differences in respiratory activity in tissue slices using
TTC
Dehydrogenase enzymes have an important role in aerobic respiration. The technique
described below depends upon the fact that these dehydrogenase enzymes can donate
the hydrogen ions they remove from a respiratory substrate, to a colourless
compound, causing it to change colour.
An investigation from Science and Plants for Schools, www.saps.org.uk/students
When the colourless chemical tetrazolium chloride (TTC) diffuses into actively
respiring tissues it accepts electrons from the mitochondrial electron transport chain,
reducing it to a pink compound, known as formazan. The accumulation of this pink
compound stains the tissues red, and the intensity of the red colour is proportional to
the rate of respiration in those tissues.
This test can be used with slices of fruits or vegetables, or imbibed seeds. The
formation of the pink colour in the tissues allows the active and less active areas of
the specimen to be distinguished.

Cut the material (with a razor or scalpel) and place face down in a watch glass
or Petri dish, or other shallow container, containing some 1% TTC solution (1).
In a warm room, the most actively respiring tissues of the cut surface becomes
coloured pink within 10 to 20 minutes, but longer periods of incubation may be
needed if the respiratory rate is slow.
The staining is permanent and such slices can be kept in a refrigerator until the next
lesson, or even dried out.
(1) Tetrazolium chloride is available as a powdered sodium salt from Philip Harris
Ltd.
Measuring the production of carbon dioxide (from respiration) with
baryta water
Respiration in plant tissues gives rise to carbon dioxide as a waste product. This gas is
absorbed by barium hydroxide solution, an alkaline solution known as baryta water.
This is then titrated with an acid to find out how much carbon dioxide has
accumulated in it. This test allows an estimation of the carbon dioxide output of the
plant material to be made.

Obtain two air-tight containers (such as a tupperware box, or jam-jar with lid)
of an appropriate size for the plant material. Make sure that there is plenty of
room for some air as well - the plant material requires oxygen for respiration!

Place the solution of baryta water (barium hydroxide 0.025 M) in an open dish
of some sort, near the bottom of each container. Remember that the carbon
dioxide is heavier than air and will settle downwards.

Place the plant material in one of the containers, and leave the other as a
control. The baryta water must not be in direct contact with the plant material.

Leave the containers for a few hours - or overnight. Then remove the dishes of
baryta water from each container, and titrate them with 0.01 M hydrochloric
acid, using phenolphtalein as an indicator:
An investigation from Science and Plants for Schools, www.saps.org.uk/students

Carbon dioxide reacts with the barium hydroxide as follows:
Ba(OH)2 + CO2 ------> BaCO3 + H2O

Remove any crust that forms on the baryta water and continue with the clear
solution from underneath. Add a few drops of phenolphthalein indicator to a
sample of baryta water which should become pink. Then add the acid carefully,
and note the volume required to just turn the pink to colourless. Repeat this
procedure with the second sample.

The difference in the results of the titration between the control container, and
the container with plant material, as determined by the volumes of acid needed,
gives a measure of the carbon dioxide produced by the plant material.
Given that 1 cm3 of 0.01 M acid is equivalent to 0.22 mg of CO2, it should be possible
to calculate the mass of CO2 produced by the plant material.
An investigation from Science and Plants for Schools, www.saps.org.uk/students