Ringing experiments can be used to test whether the phloem transports sucrose.
1. A ring of outer bark tissue should be removed.
2. This removes the phloem also.
3. The plant should be left for a time to photosynthesis before the contents of the phloem are analysed.
4. Above the ring, there is a high concentration of sucrose, revealing that sucrose is transported in the phloem.
5. Below the ring, there was no sucrose, suggesting that it had all been used up by the plant tissues and not been replaced due to the removal of the phloem tubes above.
Plants can photosynthesise in the presence of radioactive tracers such as carbon-14 in carbon dioxide. A section of the stem is then placed on photographic film and exposed to the radioactive source, producing an autoradiograph.
When this was carried out, the areas of exposure coincided with the position of the phloem in the steam, therefore revealing that sucrose is translocated in the phloem.
Aphids have mouthpieces called stylets which they can insert into a sieve tube and draw up the contents of the phloem. Therefore, this aphid can be removed, leaving its stylet behind. Due to the pressure in the phloem, the sap is forced out the stylet, and so can be collected and analysed.
When analysed, the presence of sucrose as been confirmed.
Aphids and Radioactive Tracers
The aphid experiment can be extended and the plant can photosynthesise in CO2, with the carbon being the radioactive isotope carbon-14. This revealed that the radioactivity, and therefore the sucrose, translocated at a speed between 0.5 and 1 metre/hour. This is much quicker than the rate of diffusion alone and so confirms that some additional mechanism must be occurring.