3.6.8 Tracer & Ringing Experiments

• Tracer and ringing experiments have been used to investigate mass transport in plants
• It is described as a ringing experiment because it involves the removal of a ring of surface tissues from the stem of the plant while leaving the stem core intact

The distribution of xylem and phloem tissue in the roots, stem and leaves in a herbaceous dicotyledonous plant

• As the phloem is located towards the outside of the stem and the xylem towards the centre, the ring removes the phloem only with the xylem remaining intact
  • The site of ringing is often shown on diagrams as a small black bar dividing the stem

• After the ringing has been done the plant can then be exposed to a radioactive tracer so that the direction and rate of translocation can be investigated
• ¹⁴CO₂ is commonly used for these experiments as it is readily absorbed by the leaves and used in photosynthesis to produce sucrose
• The sucrose formed will be radioactive so its subsequent movement around the plant via translocation can be traced
• The amounts of radioactive carbon present in different parts of the plant can be detected
• If the mass flow hypothesis is correct then the bulk flow of phloem sap should be in one direction (from source to sink) and occur at the same rate in any sieve tube at the same time
  • As leaves are the site of photosynthesis they are the source tissue, sink tissues can be above or below the leaves

• If the xylem is damaged during the ringing process the plant will not have an adequate supply of water and will wilt

Results from a ringing experiment with radioactive carbon dioxide

• An experiment was carried out in which different plants were ringed at different locations on the stem
• The plants were then supplied with radioactive carbon dioxide (¹⁴CO₂)
• After a period of time the levels of radioactive carbon in the different parts of the plant were measured
• There is a control plant used in this experiment to illustrate where sucrose is translocated when a plant is intact

Image showing the setup of a tracer and ringing experiment. The leaves have been enclosed to ensure the uptake of radioactive carbon dioxide.

• The results from the experiment show that:
  • The phloem is involved in the transport of sucrose
    • There is no radioactive sucrose detected past the ringing point on the stems (where the phloem has been removed)

  • In the phloem the transport of sucrose occurs both upwards and downwards
    • Sucrose is translocated from the source tissues in the leaves to the sink tissues above and below