Translocation concept (phloem)

Translocation is the movement of manufactured food substances, primarily sugars (sucrose), from the leaves (where they are made during photosynthesis) to all other parts of the plant where they are needed for growth, respiration, or storage.

• Source: The part of the plant where sugars are produced or stored in excess (e.g., mature leaves, storage organs like tubers or bulbs).
• Sink: The part of the plant where sugars are used or stored (e.g., growing points like root tips, shoot tips, developing fruits, flowers, storage organs).

The phloem is the vascular tissue responsible for translocation. It is composed of several cell types:

• Sieve tube elements: These are living cells that form long tubes. They have perforated end walls called sieve plates which allow sap to flow through. They lack a nucleus and most other organelles at maturity to allow for efficient transport.
• Companion cells: These are adjacent to sieve tube elements and are metabolically very active. They have a nucleus, ribosomes, and many mitochondria, and are responsible for loading and unloading sugars into and out of the sieve tube elements, often via active transport.
• Phloem parenchyma: Storage cells.
• Phloem fibres: Provide structural support.

Key characteristics of phloem for efficient transport:

• Sieve tubes form continuous pathways.
• Sieve plates allow for flow but can be blocked if damaged.
• Companion cells provide metabolic support for active transport.

The most widely accepted explanation for translocation in the phloem is the mass flow hypothesis (also known as the pressure flow hypothesis). It describes how a pressure gradient drives the movement of phloem sap.

1. Loading at the Source:

   • Sucrose, produced in the mesophyll cells of leaves, is actively transported into the companion cells and then into the sieve tube elements. This requires energy (ATP).
   • This active loading of sucrose into the sieve tube elements at the source creates a high solute concentration within the sieve tube.
   • Consequently, the water potential inside the sieve tube decreases.
   • Water then moves from the adjacent xylem (where water potential is higher) into the sieve tube elements by osmosis.
   • This influx of water increases the hydrostatic pressure within the sieve tube at the source end.

2. Movement through the Phloem:

   • The high hydrostatic pressure at the source end and lower pressure at the sink end creates a pressure gradient.
   • This pressure gradient drives the bulk flow (mass flow) of the phloem sap (water and dissolved sugars) along the sieve tubes from the source to the sink.

3. Unloading at the Sink:

   • At the sink, sucrose is actively transported out of the sieve tube elements and into the surrounding sink cells (e.g., root cells, developing fruits) where it is used for respiration, growth, or converted into starch for storage.
   • The removal of sucrose from the sieve tube elements increases the water potential inside the sieve tube.
   • Water then moves out of the sieve tube elements by osmosis and into the surrounding cells or back into the xylem.
   • This outflow of water reduces the hydrostatic pressure within the sieve tube at the sink end, maintaining the pressure gradient.

Summary of Mass Flow:

While the mass flow hypothesis is widely accepted, there are some pieces of evidence that support and some that question its completeness.

Evidence for Mass Flow:

• Ringing experiments: Removing a ring of bark (which includes the phloem) from a tree stem leads to swelling above the ring and starvation below, demonstrating that sugars are transported downwards in the phloem.
• Aphid stylets: Aphids feed by inserting their stylets (mouthparts) directly into sieve tube elements. Phloem sap exudes under pressure from the cut stylets, indicating a positive pressure within the sieve tubes.
• Radioactive tracers: Using radioactive carbon dioxide ($^{14}$CO$_2$) shows that sugars produced during photosynthesis (containing $^{14}$C) are transported rapidly in the phloem to other parts of the plant.
• High pressure in sieve tubes: Measurements confirm a higher hydrostatic pressure in sieve tubes at the source compared to the sink.

Evidence against (or limitations of) Mass Flow:

• Bidirectional flow: While the overall direction is source to sink, different sieve tubes can transport in different directions simultaneously, which is difficult to explain with a simple mass flow model within a single tube.
• Speed of flow: The observed speed of translocation can sometimes be faster than predicted by the pressure gradient alone.
• Energy requirement: Active loading and unloading require significant energy, which is consistent with the presence of companion cells, but the extent of active transport throughout the entire pathway is debated.