Translocation

• Mass flow hypothesis: (at the source leaf)

  1. sucrose is loaded into the sieve tube by facilitated diffusion via the companion cells and plasmodesmata, this reduces water potential in the sieve tube
  2. Water enters sieve tube by osmosis from surrounding tissue down a water potential gradient
  3. This increases the hydrostatic pressure in the sieve tube
• Mass flow hypothesis: (at the sink roots, fruit, meristem)

  4. The sucrose unloads from the sieve tube at the sink via diffusion or active transport. This increases the water potential in the sieve tube

  5. Water molecules move out of the sieve tube. The sucrose is being used at the sink. It may be converted into starch or used for respiration

  6. The pressure gradient means that the phloem sap (sucrose) moves from source to sink
• Problems with mass flow hypothesis:

  • the flow of sucrose solution can be bidirectional within one sieve tube
  • different molecules have been observed moving at different rates in the same tube
• Alternate hypothesis: cytoplasmic streaming
• Evidence of translocation:

  Ringing - shows solute moves through phloem not xylem.

  Radioactive labelling of CO2 - CO2 is converted to sucrose, so it shows rate of transport

  Aphids - have stylus that penetrate phloem and can act as micropipettes where the sap drips out of. Anaesthetise aphids before removing stylis. Can compare phloem contents at different times of day.