Translocation in the Phloem

Translocation

the process by which plants move organic compounds (primarily sugars produced during photosynthesis) from the leaves to other parts of the plant through the phloem tissue

Translocation is bidirectional

Phloem sap can move UP or DOWN the plant from the source tissue.

1. In source tissue, glucose is made in photosynthesis and converted to sucrose for transport in the source cells

2. Sucrose is actively transported into companion cells using a sucrose - H+ cotransport protein.

3. Sucrose moves from the companion cells to the sieve tube cells through plasmodesmata.  Called “phloem loading”.

4. Phloem loading leads to a high concentration of sugars in the phloem, making the phloem HYPERTONIC

5. In response, water enters sieve tube cells from the xylem via osmosis

6. The addition of water increases the pressure of the liquid in the phloem.  Increases HYDROSTATIC PRESSURE.

7. Phloem unloading (active transport) leads to lower sugar concentration (as a result, the phloem cells become hypotonic)

8. Water leaves the phloem and enters xylem via osmosis.  This water rejoins the transpiration stream.

9. The removal of water decreases pressure in the phloem.

10. Sucrose is moved into sink cells where it is converted to starch for longer-term energy storage, or converted back to glucose to fuel metabolic reactions of the sink cell.

  

Sucrose-H+ cotransport protein

Materials translocated in phloem

Translocated solutes are mainly carbohydrates

Sucrose is the most common translocated sugar

Phloem also contains: amino acids, proteins, inorganic ions, and plant hormones

Rate of translocation

Movement in the phloem is rapid, well in excess of rates of diffusion (~1m/h)

The pressure-flow model of phloem translocation

At source end of pathway:

• Active transport of sugars into sieve cells

• Water flows into sieve cells and turgor pressure increases

At sink end of pathway:

• Unloading (active transport again) of sugars

• Water flows out of sieve cells and turgor pressure decreases

Flow is driven by a gradient of pressure.

Energy is required to establish the pressure gradient, but energy is not required by cells of the pathway itself.

Sink vs Source

Stem Girdling

A method used to study phloem transport by removing a ring of bark from a tree, which disrupts the flow of nutrients and causes a buildup of sugars above the girdle → Without the ability to store the sugar in the roots, the plants will store the sugar in fruit → produce fruits bigger than usual