Introduction  

The Mass Flow Hypothesis can be called the pressure flow hypothesis. This theory was proposed by a German plant physicist Ernst Münch. At a source like a leaf, inside phloem cells, a high concentration of organic substances, primary sugar, create a diffusion gradient that draws water from the adjacent xylem into the cells. Hydrostatic pressure is hence created in the phloem. Mass flow of phloem sap occurs from sugar sources to sugar sinks. Unidirectional movements happen in the xylem cells and bidirectional movements in the phloem due to the fact that the sap cannot move with ease between adjacent sieve tubes. Because of their multi-directional flow, the sap in the sieve-tubes does not move in the opposite direction.

Sugar Sources and Sugar Sinks

In a plant, sugar is the part that produces or releases sugar. Mostly during the spring season, the roots act like a sugar source, and the other growing parts of the same plant behave as sugar sinks. After the growth, meristems become dormant, leaves behave like sugar sources, and storage organs behave like sugar sinks. Seed-bearing organs like fruits, that are still developing, always act like sugar sinks.

Mechanism of Mass flow Hypothesis

Negative pressure drives the movement of water and minerals through the xylem and Hydrostatic pressure drives the movement of movement through the phloem. This process is called translocation which is performed by a process called phloem loading and unloading. Loading of sieve-tube elements is done by cells in the sugar source by actively transporting solute molecules into it. The water moves into the sieve-tube elements by the process of osmosis due to this. Pressure is created that pushes the sap down the tube. Cells actively transport solutes out of the sieve-tube elements in the sugar sinks which produce an absolutely opposite effect. Pressure is created due to the gradient from the sugar source to sink through in sieve tubes in the direction of the sink.

The mechanism of mass flow hypothesis:-

  1. The process of photosynthesis in the mesophyll cells of the green leaves helps in the production of glucose. Some of it is used during respiration, and the rest is converted into sucrose, a non-reducing sugar. The concentration of sucrose in sieve tubes lies between 10-30% and forms 0.5% in the solution of photosynthesis cells.
  1. Transport of sucrose is actively made into the companion cells of the smallest veins in the leaves.
  2. Diffusion of sucrose occurs from the companion cells into the sieve tube elements via plasmodesmata. This results in the increase of concentration of sucrose in the sieve-tubes elements.
  1. The water movement is supported by osmosis from the nearby xylem in the same leaf vein, which increases the hydrostatic pressure of the sieve-tube elements.
  2. Due to the entry of sugars in the sieve tubes and removal of sucrose at the sink. The turgor pressure  along the pathways is increased by the sieve plates. This results in the generation and maintenance of the substantial pressure gradients in the elements between the source and sink.
  1. The cortex of both stem and root removes the phloem sugar. It is then converted into starch or consumed by cellular respiration. Starch is insoluble and exerts no osmotic effect. Consequently, there’s a decrease in the osmotic pressure of the contents. Finally, pure water remains in the phloem and, by the suction of transpiration pull, it is drawn back into the nearby xylem vessels.

Criticisms of the Mass Flow Hypothesis

  1. It is believed by the critics that the mechanism of translocation of plants occurs due to the presence of metabolic processes and not hydrostatic pressure. According to them, mass flow is a passive process, and sieve-tube cells are supported by the companion cells, which indicates that phloem’s living nature is negated by the mass flow hypothesis.
  2. The fact stated by the hypothesis that the rate of transport of material is uniform has been disproved. Critics believe that the translocation of amino acids and sugars happens at different rates throughout.
  3. Munch Mass flow Hypothesis talks only about unidirectional movement of pressure flow and doesn’t account for that move in opposite directions at the same time. Critics disapproved of this since its approval would mean that bidirectional movement is not possible, which is untrue.

Conclusion 

Mass flow Hypothesis is also known as a pressure-flow hypothesis, and it proves to be the best-supported theory to explain the movement of sap through the phloem.

A turgor or hydrostatic pressure is created in the phloem, which helps in the movement of materials through the phloem. This movement occurs by bulk flow or mass flow from sugar sources to sugar sinks. Water enters the phloem by osmosis as the sugar is accumulated inside the phloem. A fixed mechanism is followed for the movement of sap and production of food in the leaves, which is discussed above in detail.