Plant Biology: Munch Pressure Flow Hypothesis

Munch Pressure Flow Hypothesis

• The Munch pressure flow system is the only understandable mechanism for translocation of assimilates in plants.
• It is driven by metabolic processes in source and sink tissues, involving phloem loading and unloading.
• Phloem loading in source tissues exerts a push, while unloading in sink tissues exerts a pull, driving mass flow in the phloem.

Phloem Loading

• Assimilates from mesophyll cells of the source are actively loaded into the phloem.
• This decreases the water potential of the phloem sap, drawing water from xylem tissues into the sieve cells.
• The resulting increase in hydrostatic pressure pushes assimilates to flow from one sieve tube to another through open sieve pores.

Phloem Unloading

• In the physiological sink, assimilates are removed from the sieve cells, increasing the water potential.
• Water moves out of the sieve cells into the xylem, reducing the hydrostatic pressure.

Translocation and Transpiration

• The circulation of water under pressure flow from the source to the sink (translocation) occurs in the phloem, carrying assimilates.
• In contrast, water from the sink cells to the source cells (transpiration) occurs in the xylem.

Sieve Plate Pores

• About 70% of sieve plate pores are freely kept open in many angiosperm species.
• On wounding, callose, a carbohydrate, is synthesized to plug sieve pores and protect against major loss of photosynthates.