Explain the mechanism of the pressure flow hypothesis.
When glucose is prepared at the source by photosynthesis, it is converted into sucrose.
- Sucrose is then transported into the companion cells and then into the phloem sieve tube cells by active transport. This is called loading of the source.
- Loading creates a hypertonic condition in the phloem due to which water from the adjacent xylem cells moves into the phloem by osmosis.
- As the osmotic or hydrostatic pressure builds up inside the phloem, the sap moves to the surrounding areas of the lower osmotic pressure, i.e. the sink.
- As the sucrose moves into the sink, the water potential in the phloem increases which again moves water into the xylem.