The theory behind Mass flow hypothesis which is also called as pressure flow hypothesis describes the movement of sap via phloem, proposed by the German physiologist Ernst Munch in 1930. A highly concentrated organic sugar especially sugar in the cells of phloem from a source like a leaf forms a diffusion gradient which draws water in the cells from adjacent xylem. This develops turgor pressure in the phloem which is also called as hydrostatic pressure.

Phloem movement occurs by mass flow from sources of sugar to sugar sinks. The phloem movement is bidirectional but unidirectional in xylem cells. Due to this multidirectional flow, it is not uncommon for sap in the sieve-tubes besides to move in opposite direction based on the fact that sap cannot travel easily between adjacent sieve tubes.

Mechanism

When the movement of minerals and water via xylem is driven mostly by negative pressure and movement via phloem is driven by hydrostatic pressure. This process is called as translocation and accompanied by a process known as phloem loading and unloading. Cells in sugar sources load a sieve tube by osmosis developing pressure that pushes the sap low. The cells deliver solutes out of the elements of sieve-tube and produce opposite effects. The sugar gradient from source creates pressure flow via sieve tube towards the sink.

• Glucose is formed by photosynthesis in the cells of mesophyll and some glucose is utilized in the cells during respiration. The leftover glucose is transformed into non-reducing sugar.
• Sucrose is delivered to the neighbor cells of minute veins of the leaves.
• Sucrose diffuses from neighbor cells to the elements of sieve tube via plasmodesmata. Hence, the amount of sucrose rises in the elements of sieve tube.
• Water travels from the close xylem to the leaf vein by osmosis and rises the hydrostatic pressure of the elements of sieve tube.
• The Hydrostatic pressure shifts the sucrose along with other substances via the cell of sieve tube towards the sink.
• In storage sinks, sucrose is eliminated into apoplast before entering the sink’s symplast.
• The water travels out of the cells via osmosis and lowers the hydrostatic pressure in them. Hence, a gradient of pressure is developed as a result of the entry of sugar at source and elimination of sucrose at the sink.
• The phloem sugar is eradicated by the cortex of root and stem and utilized by cellular respiration. The starch is insoluble and does not exert any osmotic effect. Ultimately, pure water is left and drawn into xylem vessels by transpiration pull.

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