Phloem Transport Flow from Source to Sink

The green parts of a plant are responsible for the production of food. The non-green parts of the plant rely on the photosynthetic cells for their nutritional needs. The vascular tissue phloem transports the food in the form of sucrose to the rest of the body. Consider the phloem transport system in greater detail. Travelling from source to sink takes place in the opposite direction of travel. Translocation of organic solvents is the term used to describe the transport of organic solutes from one part of the plant to another through phloem sieve tubes in the plant.

The bidirectional flow -source and sink

The photosynthetic portion of the plant serves as the source, and the portion that stores the food serves as the sink portion. However, in the early spring, when the leaves have been shed, the sugar stored in the roots mobilises the organic material towards the developing buds. We can see that the direction of the source and sink has been reversed in this example. We can conclude that bidirectional food flow occurs in the phloem as a result of this fact.

Phloem translocation 

Translocation in the phloem is the term used to describe the movement of various molecules, such as sucrose, amino acids, and other compounds, through the phloem of a plant. Gravity does not affect the translocation of nutrients in the phloem system. As opposed to this, sucrose is translocated from the point of supply (the leaf) to the point of metabolism or storage, which is referred to as a sink. Depending on the requirements of the plant, the source and sink points can be reversed.

When sugar is transported from the root to the growing buds in early spring and from photosynthesizing leaves to roots in summer, it demonstrates the bidirectional movement of sap in the phloem, as seen in deciduous trees. The movement in the xylem, on the other hand, is unidirectional, i.e., it always moves upwards. As a result, the food contained within phloem sap can be transported in any direction necessary depending on the situation.

Phloem Sap is composed of a variety of compounds

Phloem sap is the term used to describe the organic matter that is transported through the phloem. It is composed of sucrose and water, as well as hormones (auxin, gibberellins, cytokinins, and abscisic acid), amino acids, and other sugars, among other things.

Pathways of Translocation 

1. Firstly, sugars and other organic materials travel through the phloem cells of the plant thanks to the action of sieve elements.

2. The term “sieve element” refers to both the highly differentiated sieve cells of gymnosperms and the relatively unspecialized sieve cells of angiosperms, which are both types of sieve cells.

3. Moreover, the phloem tissue contains companion cells as well as parenchymal cells in addition to the sieve elements mentioned above.

4. Additionally, phloem tissue contains fibres and sclereids (which serve to protect and strengthen the tissue) as well as laticifers (which contain latex-producing cells). Only sieve cells, on the other hand, are directly involved in translocation.

Mechanisms of Translocation in the Phloem

The mass flow hypothesis, also known as the pressure-flow hypothesis, is the best explanation for the process of sugar translocation in plants. It was proposed by German physiologist Ernst Munch in 1930 and refined by Grafts. This hypothesis is supported by the following evidence:

1. The movement of organic matter (sucrose) in solution from source to sink is caused by the development of an osmotic pressure gradient between them.

2. The production of glucose by photosynthesis in the mesophyll cells of the leaves has resulted in the conversion of glucose into sucrose (disaccharide sugar) for transportation purposes.

3. Sucrose is transported from the mesophyll cell to the sieve-tube companion cells through the active transport mechanism. Phloem loading is the term used to describe this process.

4. The loading of sucrose into the phloem results in hypertonic conditions and negative osmotic potential in the water column of the plant. Because of osmosis, water from the adjacent xylem is drawn into the phloem, resulting in the generation of a high-pressure potential.

5. In addition to moving towards the region of low osmotic pressure, which is maintained at the sink region, the phloem sap also moves towards the region of high osmotic pressure.

6. The sucrose is removed from the phloem sap through an active process at the sink region. Due to the removal of sucrose, osmotic pressure decreases and water escapes from the phloem, causing the sieve cells to become flaccid. Phloem unloading is the term used to describe this process.

Indications that translocation occurs through the phloem

1. Experiment with Ringing or Girdling

To maintain a healthy potted plant, all of the tissue outside of the xylem, such as the woody stem’s bark, cortex, and phloem, is removed from a small section of the woody stem (girdling). The xylem is the only thing that connects the upper and lower parts of the plant anymore.

The accumulation of food material just above the girdling is observed after a few days of observation. Additionally, the roots of the girdled plant die first. This could occur as a result of the food not being transported to the roots. By conducting this experiment, it was demonstrated that the phloem is responsible for the translocation of organic material.

2. Phloem Transport and Its Mechanism of Action

Sieve tubes in the phloem form long columns with holes in the end walls, which are referred to as sieve tubes. A continuous channel is formed when cytoplasmic strands pass through the holes in the membrane.

3. The Hypothesis of Pressure Flow or Mass Flow

This theory was proposed by Munch and developed further by Mars and others in the following years. Higher plants use this mechanism to transport sugars, and it is widely accepted. 

Conclusion

At the source, glucose is produced by photosynthesis and converted to sucrose (sugar), after which it is transported to the various parts of the plant depending on their requirements for glucose. Active transport is then used to transport the sucrose into the sieve tube cells. Phloem hypertonic conditions are created as a result of this treatment. Osmosis is the process by which water is transferred from the adjacent xylem to the phloem.

As a result of high osmotic (turgor) pressure, phloem sap is forced to move to areas with lower turgor pressure. It is at the sink that the osmotic pressure decreases. When sugar enters the plant, phloem is responsible for actively transporting it out. When the sugar is removed from the solution, the osmotic pressure drops, causing water to flow out of the phloem and into the xylem instead.