7.9 - Investigating transport in plants

Question 1

Describe ringing experiments

Answer 1

• At the start of a ringing experiment, a section of the outer layers (protective layer and phloem) is removed around the complete circumference of a woody stem while it is still attached to the rest of the plant.
• After a period of time, the region of the stem immediately above the missing ring of tissue is seen to swell.
• Samples of the liquid that has accumulated in this swollen region are found to be rich in sugars and other dissolved organic substances.
• Some non-photosynthetic tissues in the region below the ring (towards the roots) are found to
  woody stem before wither and die, while those above the ring continue to grow.
• These observations suggest that removing the phloem around the stem a has led to
  1) the sugars of the phloem accumulating above the ring, leading to swelling in this region
  2) the interruption of flow of sugars to the region below the ring and the death of tissues in this region.
  —> showing evidence that the phloem is responsible for

Question 2

Describe isotope experiments

Answer 2

• Radioactive isotopes are useful for tracing the movement of substances in plants.
• For example the isotope 14^C can be used to make radioactively labelled carbon dioxide (14^CO2).
• If a plant is then grown in an atmosphere containing 14^CO2, the 14^C isotope will be incorporated into the sugars produced during photosynthesis.
• These radioactive sugars can then be traced as they move within the plant using autoradiography. In our example, this involves taking thin cross-sections of the plant stem and placing them on a piece of X-ray film.
• The film becomes blackened where it has been exposed to the radiation produced by the 14^C in the sugars.
• The blackened regions are found to correspond to where phloem tissue is in the stem.
• As the other tissues do not blacken the film, it follows that they do not carry sugars and that phloem alone is responsible for their translocation.

Question 3

What is some more evidence that translocation of organic molecules occurs in the phloem

Answer 3

• When phloem is cut, a solution of organic molecules flow out.
• Plants provided with radioactive carbon dioxide can be shown to have radioactively labelled carbon in phloem after a short time.
• Aphids are a type of insect that feed on plants. They have needlelike mouthparts which penetrate the phloem. They can therefore be used to extract the contents of the sieve tubes. These contents show daily variations in the sucrose content of leaves that are mirrored a little later by identical changes in the sucrose content of the phloem.
• The removal of a ring of phloem from around the whole circumference of a stem leads to the accumulation of sugars above the ring and their disappearance from below it