Pressure Flow Mechanism

The most widely accepted hypothesis, explaining phloem transport is called pressure flow mechanism, first proposed by Munch in 1927. Two regions can be recognized in the plants, source and sink.

Sugar is actively loaded into the sieve tube element at the source. As a result of differences in water potential, water moves osmotically into the sieve tube element. At the sink sugar is actively unloaded and water leaves the sieve tube element by osmosis. The gradient of sugar from source to sink causes pressure flow through the sieve tube toward the sink.

An area where sugar is made is called source e.g. green leaves and stem. Any area where sugar is stored or used is called sink e.g. young leaves, fruits, seeds and roots. According to pressure flow mechanism water containing sugar in solution flows under pressure through the phloem. It involves the following.

a) Glucose is produced by photosynthesis in the mesophyll cells of the green leaves. Some glucose is used within the cells during respiration. The rest of glucose is converted into non-reducing sugar i.e. sucrose.

b) It has been shown that the sucrose concentration in sieve tubes in leaves is commonly between 10 to 30 percent whereas it forms only 0.5% solution in the photosynthesis cells.

c) The sucrose is actively transported to the companion cells of the smallest vein in a leaf.

d) The sucrose diffuses through the plasmodesmata to sieve tube elements. As a result, concentration of sucrose increases in the sieve tube cells or elements. The sucrose is actively transported to the sieve elements.

e) Water moves by osmosis from the nearby xylem in the leaf vein. This increases the hydrostatic pressure of the sieve tube elements.

f) Hydrostatic pressure moves the sucrose and other substances in the sieve tube cells, and then moves to sink. In the storage sinks, such as sugar beet root and sugar cane stem, sucrose is removed into apoplast prior to entering symplast of the sink.

g) Water moves out of sieve tube cells by osmosis, lowering hydrostatic pressure. Thus the pressure gradient is established as a consequence of entry of sugars in sieve elements at the source and removal of sugar i.e. sucrose at the sink.

h) The presence of sieve plates greatly increases the resistance along the pathway and results in the generation and maintenance of substantial pressure gradient in the sieve elements between source and sink.

The sieve elements contents are physically pushed along the transportation pathway by bulk flow. The pressure flow theory accounts for the mass flow of molecules within phloem. As the sap is pushed down the phloem sugar is removed by the cortex of both stem and root and is consumed or converted into starch. Starch is insoluble and exerts no osmotic effect. Consequently the osmotic pressure of the contents of phloem decreases. Finally relatively pure water is left in the phloem and this is thought to leave by osmosis or be drawn back into nearby xylem vessels by suction of the transpiration pull.

The pressure flow mechanism depends upon: 1) Turgor pressure 2) Difference of osmotic pressure gradient along the direction of flow between the source and the sink.

The objection leveled against the pressure flow mechanism is that it does not explain the phenomenon of bidirectional movement i.e. movement of different substances in opponent directions at the same time. The phenomenon of bidirectional movement can be demonstrated by applying two different substances at the same time to two different points of phloem of a stem and following their longitudinal movement along the stem. The bidirectional movement occurs in a single sieve tube or not. If the mechanism of translocation operates according to pressure flow hypothesis, bidirectional movement in a single sieve tube is not possible. Experiments to demonstrate bidirectional movement in a single sieve tube are technically very difficult to perform. Some experiments indicate that bidirectional movement may occur in a single sieve tube, whereas others do not.