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.

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:
  • 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,