What is translocation?
The movement of assimilates to where they are needed. It is a process that occurs in the phloem and requires energy.
Moves assimilates from source to sink. Source is at a high concentration, sink at low concentration.
Enzymes maintain the concentration gradient by changing the dissolved substances at the sink.
The mass flow hypothesis
Active transport actively loads assimilates into the sieve tubes of the phloem at the source. This lowers the water potential inside the sieve tubes, so water enters the tubes by osmosis. This creates a higher pressure.
Assimilates are removed from the phloem to be used up. This increases the water potential inside the sieve tubes, so water leaves the tubes by osmosis. This lowers the pressure
The result is a pressure gradient which pushes assimilates along the sieve tubes to where they are needed.
Mass flow evidence
- If you remove a ring of bark (phloem…