Evidence for Mechanism of Translocation

• In Phloem Sieve Tubes:

  • Ringing tree
  • Remove phloem tissue
  • Causes sucrose accumulation above ring
  • Cannot flow down
  • Reduces water potential so water diffuses into cells
  • Tree trunk swells partially
  • Damage triggers cell division to store starch produced from excess sugar; also causes swelling
  • Sugar movement is faster than diffusion alone
  • Radioactive CO_2 used in photosynthesis
  • Radioactive sucrose found in phloem sieve tubes
  • Aphids
  • Insert mouthparts into sieve tubes to feed on sugary phloem sap
  • Used to collect sap for analysis
  • Anaesthetised
  • Detached, leaving mouthpart in position
• Requirement for ATP for Active Transport:

  • Companion cells have many mitochondria to make ATP for active transport
  • Sucrose loading is against concentration gradient
  • Flow rate in phloem is greater than just diffusion
  • Can be stopped by inhibiting aerobic respiration’s ATP production with a poison
  • Also temperature (enzyme activity) and oxygen-dependent (respiration)
• Evidence

  For Loading Mechanism:

  • pH of companion cells (and phloem sap) is higher than surrounding cells
  • As they actively transport protons out cells
  • Also sucrose concentration is higher at source than sink

  Against Mass Flow:

  • Sieve plates role is unclear
  • Not all molecules in phloem sap move at same rate
  • Sucrose moves to all sinks same rate
  • No faster to sinks with lower sucrose concentration