Evidence for Mechanism of Translocation

• Ringing a tree to remove phloem tissue causes sucrose accumulation above the ring as it cannot flow down, this reduces water potential so water diffuses into the cells; the tree trunk swells partially use to the cells turgidity - damage to the trunk triggers increased cell division above cut to produce more cells to store starch produced from excess sugar; this increased cell number also causes swelling
• Movement rate of sugars in the phloem is much faster than diffusion alone, also when radioactive CO_2 is used in photosynthesis, radioactive sucrose is found in phloem sieve tubes
• Aphids insert mouthparts into phloem sieve tubes to feed on the sugary phloem sap so are used to collect phloem sap for analysis; anaesthetised and body detached, leaving mouthpart in position (phloem sap can now be collected)
• Companion cells have many mitochondria to make ATP for active transport as sucrose loading is against a concentration gradient; flow rate in phloem is greater than having just diffusion
• Can be stopped by inhibiting aerobic respiration’s ATP production with a poison, also it is temperature dependent (involves enzyme activity) and oxygen dependent (for respiration)
• Evidence for the Loading Mechanism: pH of companion cells (and phloem sap) is higher than surrounding cells, as they actively transport protons out of cells, also sucrose concentration is higher at source than sink
• Evidence Against Mass Flow: Sieve plates role is unclear, not all molecules in phloem sap move at same rate, sucrose moves to all sinks at same rate; no faster to sinks with lower sucrose concentration