The mass flow hypothesis, also called the Pressure-flow hypothesis, provides an explanation for the transport mechanism of sap through the phloem. It was devised by a German plant physiologist, Ernst Munch in 1930. It is based on the difference in osmotic pressure inside the phloem cells. Phloem is one of the living tissues in plants responsible for maintaining its nutrient transport system along with the xylem.
According to the mass flow hypothesis, an osmotic or diffusion gradient causes the sap to move from the source to the sink along with the water.
The source is any part of the plant that produces or releases sugar or inorganic nutrients (mainly amino acids).
The sink is any part of the plant that receives the nutrients and uses them for its growth.
For example, roots are the source during the growth period while leaves act as sources after the growth when the meristems remain dormant.
Leaves, branches, or any other growing plant part becomes sink during the growth period while storage organs are the sinks during the dormant stage.
A gradient refers to the difference in concentrations leading to the difference in pressure. This difference in pressure provides the driving force for the transport of nutrients along with the water movement.
In plants, a high concentration of nutrients builds up inside phloem’s sap of a source like roots. This causes pressure to build up known as Turgor or hydrostatic pressure.
Under this pressure, water from the adjacent xylem travels toward the phloem cells while mass flow allows the transport of phloem sap to the sink from the original source.
The movement of nutrients through phloem under positive hydrostatic pressure is known as translocation and happens via loading and unloading. Cells from a nutrient source load solute molecules into a sieve tube of phloem.
Through the process of osmosis, the water from the adjacent xylem enters the sieve element. Due to the pressure gradient created, the sap containing the nutrients is forced down the tube. At similar times, cells transport out the solutes from the sieve elements of the sugar sinks.
These are the following steps in the mechanism of the mass flow hypothesis −
During photosynthesis, glucose is produced by the mesophyll cells of the leaves. The leftover glucose after breathing is stored as sucrose, a nonreducing sugar.
The sucrose is continuously transported to the neighboring cells of the smallest veins of a leaf.
Through plasmodesmata, the sucrose (and other nutrients) accumulated in the adjacent cells diffuse and enter the sieve tube contributing to their increased concentration.
At a similar frequency and in the neighbouring xylem, water travels to the source cell through the process of osmosis. This results in an increase in the hydrostatic pressure in the sieve elements.
Under the influence of the turgor pressure, the nutrients are pushed down the elements of the sieve tube towards the sink.
With the outward movement of water, the gradient developed eases out gradually.
Once at the sink, the stem and root cortex consume the phloem sugar through cellular respiration or conversion to starch.
Any water, if left, is transported back to the xylem and released as water vapor through transpiration.
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Although the mass flow hypothesis is the best-explained theory for the transport of nutrients from the phloem’s sap, there are certain objections to it. Scientists are unable to reach a consensus as some experiments have validated the mass flow hypothesis but some have failed.
The major objections to the theory are -
Mass flow is considered to be a passive process as the sieve elements gain support from the companion cells. It, therefore, contradicts the living nature of phloem.
The mass flow hypothesis assumes the transport of all the nutrients like sugars and amino acids at a similar pace along the mass flow. However, sugars and amino acids are known to translocate at different rates owing to differences in their physical properties.
The hypothesis fails to explain the bidirectional movement of different substances in parallel directions at similar frequencies. It has been postulated that the concept of bidirectional movement is applicable to two different substances at two different points of the phloem at the same time but not with one sieve tube. Scientists have been able to track the longitudinal passage of the nutrients through the stem at different points but not at the same points at the same time.
The hypothesis does not account for the change in translocation rate due to changing ambient conditions like temperature, humidity, metabolic inhibitors, etc.
The mass flow hypothesis aims to explain the phenomenon of continuous and simultaneous transport of sugar and other nutrients from the source to the sink in the phloem’s sap. The process is facilitated by the formation of a gradient pressure arising due to the concentration difference, similar to that of osmosis. The source and the sinks are dependent upon the growth stage of the plant. Though it is the only hypothesis to explain the translocation of nutrients, there are objections to its validity.
Q1. What do you understand by the term sieve element?
Ans: Sieve elements are lengthy and narrowed cells connected by porous sieve plates at the transverse ends to form sieve tubes. They help in the translocation of solutes between the adjacent cells in the phloem. They do not contain nuclei and have a reduced number of organelles to support maximum translocation. They need the support of companion cells to function effectively.
Q2. Explain the role of companion cells in the transport mechanism of a plant?
Ans: They provide the metabolic support to sieve elements during the loading and unloading of nutrients at both, source and sink. They contain:
A flexible plasma membrane that extends the volume of nutrient exchange
Mitochondria to provide the energy needed for active transport of the nutrients and
Various transport proteins to facilitate the actual transport.
Q3. What are the typical sucrose concentrations in sieve tubes?
Ans: The concentration of sucrose in sieve tubes of green leaves varies between 10-30 percent. However, it makes up only 0.5 percent of the total solution in a photosynthetic cell.
Q4. What do you understand by the term plasmodesmata?
Ans: Plasmodesmata are the intercellular bridges between neighboring algal and plant cells. Their function is to provide cytoplasmic and membrane continuity to nutrients as well as communication and signals. Thus, they are instrumental in plants’ growth and defense.
Q5. Do all plants follow the mass transfer hypothesis?
Ans: It has been established that not all plants follow the active translocation of nutrients, particularly in primitive plants of tropical regions. In a contradictory theory of polymer trap mechanism by Robert Turgeon, sugar molecules are polymerized to raffinose and other oligosaccharides during the transportation through plasmodesmata.
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