4.3 - Translocation

Definition and Necessity of Translocation

• Translocation is the process of transporting organic substances such as sucrose, amino acids and hormones in the phloem from the leaves to other parts of the plant such as the roots and stem.
• Translocation transports photosynthetic products from leaves to parts that need them for growth and respiration, including roots, fruits, shoot tips and developing flowers.
• It also transports excess photosynthetic products to rhizomes, tubers and bulbs.
• Phloem sap contains sucrose and amino acids, showing that translocation occurs in the phloem.

Pathways of Translocation in Plants

• Transpiration pulls water along the xylem vessel against gravity.

From the Leaf Cell into the Sieve Tube

1. Sucrose is actively transported from the leaf cell through the companion cell into the sieve tube.
2. Sucrose entering the sieve tube reduces its water potential.
3. Water moves from the xylem into the sieve tube by osmosis.
4. The movement of water increases the hydrostatic pressure in the sieve tube.

Along the Sieve Tube to Other Plant Organs

5. Increased hydrostatic pressure pushes the phloem sap along the sieve tube to other plant organs.
6. Sucrose is transported from the sieve tube to stems, roots, shoots, fruits and tubers by active transport.
7. The increased water potential in the phloem causes water to return to the xylem by osmosis.