Question

could you explain the process of The Pressure Flow Hypothesis of Food ?

Answer 1

Tissue from which phloem translocation starts is called SOURCE and end point is called SINK
Glucose produced in photosynthesis is converted temporarily to starch and later converted to sucrose.This is the main organic product transported through phloem.
this sucrose is loaded to sieve tube elements through transfer cells using metabolic energy. This is called the phloem loading which is an active process
This increases the solute potential or the solute concentration of sieve tube cells and reduces the water potential in sieve tubes. so water enters the sieve tubes by osmosis from neighbouring xylem vessels. and creates a hydrostatic pressure with in sieve tubes.
Due to this pressure sucrose is transported to the storage tissues or growing regions as a mass flow passively. In this manner water,sucrose,dissolved substances like amino acids,vitamins are transported.

at the sink (storage tissue or growing region)sucrose is actively sent out of the sieve tubes in to storage cells through transfer cells.This is called phloem unloading.When sucrose is removed out of sieve tubes by ex osmosis it creates a pressure potential gradient from source to sink (as the solute concentration decreases and water potential increases due to that)