Question
Explain the pressure flow hypothesis (mass flow hypothesis) for translocation of food through the phloem. Who proposed it, and what is the evidence for and against it?
Solution — Step by Step
Phloem is the vascular tissue in plants that transports organic compounds — primarily sucrose — from leaves (source) to other parts of the plant (sink). This movement is called translocation.
Unlike xylem (where water moves passively due to transpiration pull), phloem transport is active, bidirectional, and requires energy. The pressure flow hypothesis (also called mass flow hypothesis) proposed by Ernst Munch in 1930 is the most widely accepted explanation.
- Source: Any organ that produces or releases sugars into the phloem. Primarily photosynthesizing leaves (during growing season). Can also be storage organs (roots, tubers releasing starch as sugars).
- Sink: Any organ that uses or stores sugars. Includes growing regions (root tips, shoot tips, developing seeds), storage organs (roots, fruits), and non-photosynthetic tissue.
The direction of phloem transport is always from source to sink — a key fact. This means flow can go upward (leaves to shoot tips) or downward (leaves to roots) depending on where the sink is.
The hypothesis proposes that food moves through phloem by a bulk flow driven by a pressure gradient between source and sink. Here’s how the pressure gradient is created:
At the source (high pressure):
- Photosynthesis produces glucose, which is converted to sucrose
- Sucrose is actively loaded into the phloem sieve tubes (using ATP — this is why phloem transport requires energy)
- The increased sucrose concentration creates a high osmotic concentration inside the sieve tube
- Water from the adjacent xylem moves into the sieve tube by osmosis (down its water potential gradient)
- This water entry creates a high turgor pressure (hydrostatic pressure) at the source end
At the sink (low pressure):
- Cells at the sink actively unload sucrose from the sieve tube (for use in metabolism or storage)
- This reduces the osmotic concentration in the sieve tube at the sink end
- Water leaves the sieve tube by osmosis (into sink cells or back to xylem)
- This creates a low turgor pressure at the sink end
The result: A pressure gradient from source (high pressure) to sink (low pressure) drives the bulk flow of phloem sap (sucrose + water) from source to sink.
Evidence supporting the hypothesis:
- When a phloem sieve tube is punctured, sap oozes out under pressure (positive pressure confirmed)
- Ringing experiments: if phloem is removed in a ring around a stem, food accumulates above the ring
- Aphid stylet experiments: aphids feed by inserting their stylets into phloem; when the stylet is cut, sap continues to flow out under pressure for hours — confirmed mass flow
Limitations/criticism:
- The sieve plates (perforated cross-walls of sieve tubes) create resistance to flow — calculations suggest the pressure gradient alone may be insufficient for observed flow rates
- Temperature experiments show phloem transport is reduced by cold (suggesting an active, energy-dependent process) — but if it were purely pressure-driven, temperature should have less effect
- Bidirectional simultaneous transport in the same sieve tube (observed in some studies) is difficult to explain by simple mass flow
Despite these limitations, the pressure flow hypothesis remains the best available explanation.
Why This Works
The elegance of the pressure flow hypothesis is that it links two osmosis-driven processes (loading at source, unloading at sink) to create a pressure gradient that does the transport work. The plant doesn’t need pumps or motors — osmosis and the energy spent on active loading/unloading are sufficient to move sugars over meters of distance.
The xylem and phloem work in close coordination: water entering phloem at the source comes from xylem; water leaving phloem at the sink can return to xylem. This interconnection is a beautifully efficient design.
Alternative Method
A simple analogy: imagine a pipe connecting a high-pressure pump (source) to a low-pressure drain (sink). If you continuously pump fluid in at one end and remove it at the other, fluid flows continuously from pump to drain. The leaf’s active sucrose loading is the “pump” and the sink’s unloading is the “drain.” The phloem sieve tubes are the pipe.
For NEET and CBSE Class 11, the pressure flow hypothesis is a 3-5 mark question. The complete answer should include: (1) name and proposer (Munch), (2) the concept of source and sink, (3) the osmotic mechanism at both ends — active loading creates high pressure at source, unloading creates low pressure at sink, (4) bulk flow from high to low pressure. Draw a diagram showing a source leaf, phloem tube, sink organ, with arrows showing sucrose movement and water movement (from xylem into phloem at source, from phloem back to xylem at sink).
Common Mistake
Students often confuse the direction of water movement in this process. At the source, water moves from xylem INTO phloem (osmosis into the high-sucrose phloem). At the sink, water moves from phloem OUT to the sink cells (or back to xylem). Students sometimes reverse these directions. The key logic: water always moves by osmosis from a region of lower solute concentration (higher water potential) to higher solute concentration (lower water potential). At source, phloem has high sucrose → water enters phloem from xylem.