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Questions and Answers
What is the primary mechanism described by the Munch pressure flow hypothesis for the translocation of assimilates in plants?
What is the primary mechanism described by the Munch pressure flow hypothesis for the translocation of assimilates in plants?
What role does callose play in the phloem transport system?
What role does callose play in the phloem transport system?
Which process in the source tissue initiates the movement of water into the sieve cells?
Which process in the source tissue initiates the movement of water into the sieve cells?
How does water circulate in the plant under the pressure flow mechanism?
How does water circulate in the plant under the pressure flow mechanism?
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What happens to the water potential in the sink tissue as assimilates are removed from the sieve cells?
What happens to the water potential in the sink tissue as assimilates are removed from the sieve cells?
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Study Notes
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.
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Description
Understand the Munch pressure flow system, a mechanism for translocation of assimilates in plants, driven by metabolic processes in sources and sink tissues.
