Translocation is the transport of manufactured food substances from the leaves to all parts of the plant. The movement is usually described as downwards from the leaves to the roots but it is also known to occur on the reverse direction especially in the non-growing season.
While the downward movement will take food and mineral salts to growing branches, root tips and storage organs such as roots and stem tubers, corms, bulbs and rhizomes, upwards and side ways movements of manufactured food substances are necessary to serve the growing shoot tips, flowers and storage organs such as seeds and fruits. Photosynthesis takes place in the leaves and the manufactured food substances such as sucrose is traced to the sieve tubes of the phloem tissue in the veins of the leaves and stems of plants.
Substances translocated in plants
Substances translocated in plants are sugar, fatty acids, amino acids, oil, hormones (auxins) and alkaloids (steroids).
Experiment to show that translocation takes place through the phloem
Ringing or girdling experiment
Aim: To demonstrate evidence of translocation of manufactured food substances from the leaves to other parts of the plant through the phloem.
Method: A medium sized, healthy growing young plant such as Delonix regia, mango, orange, etc. is selected on the school compound or garden. Rings of bark including phloem tissues are removed from the stem at a point far above the ground level and below the first branches and leaves. A control experiment in which the ringing allows some leaves below is made.
Result: After two weeks, the portion of the stem above the ringed region becomes swollen. On testing the plant juice above and below the ringed areas for sucrose (using dilute hydrochloric acid and later Fehlings solutions A and B), it is found that the juice above the ringed area contains sucrose while the juice below ringed area contains no sucrose.
In the control in which some leaves are left below the ring, sucrose is found above and below the ringed areas.
Conclusion: Manufactured food substances in the leaves are translocated through the phloem to all parts of the plant.