Tracer & Ringing Experiments (AQA A Level Biology): Revision Note

Tracer & ringing experiments

• Evidence to support the mass flow hypothesis has come from experiments involving tracers and stem ringing

• Tracers are chemicals that can be traced as they move through an organism; a common example of a tracer used in studies on plants is radioactive carbon dioxide, ¹⁴CO₂

  • It is readily absorbed by the leaves and used in photosynthesis to produce sucrose

  • The sucrose formed will be radioactive so its movement around the plant via translocation can be traced

• Tracers can be used in ringing experiments that involve removing a ring of tissue from the outside of a plant stem

  • 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

A ringing experiment with radioactive carbon dioxide

• An example of a ringing experiment is described below:

  1. A series of plants were ringed at different locations on the stem

     • A control plant did not have a ring of tissue removed

  2. The plants were then supplied with radioactive carbon dioxide (¹⁴CO₂)

  3. After a period of time the levels of radioactive carbon in the different parts of the plant were measured

• The results from the experiment show that:

  • the phloem is involved in the transport of sucrose, and not the xylem

    • There is no radioactive sucrose detected past the ringing point on the stems, due to the break in the phloem at this point

  • 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