3.6.8 Tracer & Ringing Experiments

Tracer Experiments

Tracer experiments are used to study the transport of sucrose in plants.

1. Plants are grown in an environment containing radioactively labelled carbon dioxide (14CO2).
2. The radioactive carbon is incorporated into sugars during photosynthesis, labelling them with 14C.
3. The movement of the labelled sugars is traced using autoradiography:

• The plant is placed on photographic film that darkens when exposed to radiation.
• Areas that appear black on the film correspond to regions where radioactive sugars are present.

4. These dark areas highlight the phloem, showing that sucrose is transported in this tissue.

Ringing Experiments

Ringing experiments investigate the role of the phloem in mass transport:

5. A ring of bark (including the phloem) is removed from the stem, leaving the xylem intact.
6. Over time:

• Above the ring: Swelling occurs as sucrose accumulates in the phloem.
• Below the ring: Tissues die as they are starved of sucrose and nutrients.

7. This demonstrates that sucrose transport occurs in the phloem, not the xylem.

Key Insights:

• Tracer experiments confirm the phloem's role in sucrose transport by tracking radioactive sugars.
• Ringing experiments show that removing the phloem prevents nutrient transport, leading to tissue death.

Key Terms:

• Tracer Experiment: A method using radioactive isotopes to track the movement of substances in plants.
• Autoradiography: A technique to detect radioactive materials, producing a photographic image of their location.
• Ringing Experiment: A practical method to show that sucrose transport occurs in the phloem.