Introduction

The pressure-flow hypothesis often referred to as the mass flow hypothesis, is the best-supported explanation for sap transportation through the phloem. It was proposed by Ernst Münch, a German plant physiologist, in 1930. A diffusion gradient (osmotic gradient) is created when a high concentration of organic compounds, particularly sugar, inside phloem cells at a source, such as a leaf, attracts water into the cells from the neighbouring xylem.

 In the phloem, this causes turgor pressure, also known as hydrostatic pressure. Bulk flow (mass flow) transports phloem sap from a sugar source to sugar sinks. The movement of phloem cells is bidirectional; however, the movement of xylem cells is unidirectional (upward). 

For this multi-directional flow, as well as the fact that sap cannot easily pass between nearby sieve tubes, sap in adjacent sieve tubes frequently flows in different directions.

Sources and Sinks

• Any portion of the plant that makes or releases sugar is referred to as a sugar source
• Storage organs like the roots are sugar sources during the plant’s growing season, which is normally in the spring, while the plant’s numerous growing areas are sugar sinks
• The leaves are sources after the growth period when the meristems are dormant, and the storage organs are sinks. Seed-bearing organs in development are always sunk

Mechanism

While most of the time, negative pressures (tension) drive water and mineral flow via the xylem, positive hydrostatic pressure drives movement through the phloem. This is known as translocation, and it is carried out by a mechanism known as phloem loading or unloading. 

A sieve-tube element is “loaded” by cells in a sugar source by actively transferring solute molecules into it. By osmosis, water enters the sieve-tube element, creating pressure that forces the sap down the tube. 

Cells aggressively transport solutes out from sieve-tube elements in sugar sinks, causing the exact opposite effect. The pressure-flow through the sieve tube toward the sink is caused by the gradient of sugar from source to sink.

The following are the mechanisms:

• Photosynthesis inside the mesophyll cells of green leaves produces glucose. During breathing, some glucose is utilised by the cells. The remaining glucose is transformed into sucrose, a non-reducing sugar. Sucrose concentrations in sieve tubes in leaves are typically between 10 and 30 percent, but it only forms a 0.5 percent solution in photosynthetic cells
• Sucrose is actively delivered to the partner cells of the leaves’ tiniest veins
• The sucrose diffuses from the partner cells to the sieve tube elements via the plasmodesmata. As a result, the sucrose concentration in the sieve tube elements rises
• Water travels via the same leaf vein via osmosis from the neighbouring xylem. The fluid pressure of the sieve tube sections rises as a result
• Sucrose and other chemicals are pushed through the sieve tube cells and into a sink by hydrostatic pressure
• Sucrose is taken from the apoplast before entering the symplast of storage sinks such as sugar beetroot and sugar cane stem
• Osmosis causes water to flow out of the sieve tube cells, lowering the hydrostatic pressure between them. As a result of sugars entering sieve elements at the source and sucrose being removed at the sink, a pressure gradient is created
• The presence of sieve plates significantly increases resistance throughout the channel, resulting in the creation and maintenance of significant pressure gradients within sieve components between the source and the sink
• The cortex of both the stem and root removes the phloem sugar, which is either eaten by cellular respiration or transformed into starch. Because starch is insoluble, it has no osmotic impact. As a result, the osmotic pressure of the phloem contents falls
• Finally, relatively clean water remains in the phloem, which is supposed to pass by osmosis or be dragged back into adjoining xylem vessels by the transpiration pull’s suction

Evidence

The notion is supported by many pieces of evidence. First, when the stem is cut or penetrated by an aphid’s Stylet, a classic experiment demonstrating the translocation function of phloem, there is an exudation of solution from the phloem. 

Second, organic solute concentration gradients between the sink as well as the source have been demonstrated. 

Third, viruses or growth compounds are translocated downward to the roots when they are introduced to a well-illuminated leaf.

 However, when the chemicals are applied to shaded leaves, no downward translocation occurs, indicating that diffusion is not a plausible method involved in translocation.

Criticisms

The hypothesis is frequently met with opposition or criticism. Some suggest that mass flow is a passive activity, whereas companion cells support sieve tube conduits.

 As a result, the idea ignores the phloem’s living aspect. Furthermore, amino acids and sugars (examples of organic solutes) are discovered to be translocated at varying speeds, which contradicts the hypothesis’ premise that all materials being transported would travel at the same pace. 

The idea has two flaws: bidirectional solute movement in the translocation process and the fact that translocation is significantly influenced by changes in ambient circumstances like temperature and metabolic inhibitors.

The pressure-flow mechanism is criticised for failing to explain the phenomena of bidirectional movement, which occurs when separate substances move in opposite directions at the same time. By simultaneously administering two distinct compounds to the phloem of a stem at two separate locations and tracking their longitudinal passage up the stem, the phenomena of bidirectional movement may be illustrated. 

Bidirectional movement in a single sieve tube is not feasible if the translocation mechanism acts according to the pressure-flow concept. Experiments demonstrating bidirectional motion in a single sieve tube are physically complex. The bidirectional movement has been observed in a single sieve tube in certain studies, but not in others.

Conclusion

In 1930, German plant scientist Ernst Munch introduced this theory. The mass flow hypothesis states that a never-ending flow of water plus dissolved nutrients between the source (where sugars are created) and sink causes the translocation of glucose and other sugars inside phloem (where sugars are utilized).

 The high concentration of sugar, as well as other organic compounds in the phloem source cells, causes a diffusion gradient of an osmotic gradient. Water is taken out of the nearby xylem, resulting in hydrostatic pressure, which pushes the sap.