Introductory Crop Physiology (1+1)

Mechanism of Translocation of Solutes

Mechanism of Translocation of Solute

Basically, there are three theories to explain the mechanism of translocation of solutes. They are:

1. Munch's mass flow or pressure flow hypothesis
2. Diffusion hypothesis
3. Protoplasmic streaming hypothesis
1. Munch's Mass flow Hypothesis (or) Pressure Flow Hypothesis

Animation showing pressure-flow model of phloem transport
Explanation:

• Sugar and then water enter sieve tubes at a source, in this case the leaves.
• This creates a positive pressure, which causes phloem contents to flow.
• Sieve tubes (composed of sieve tube elements) form a continious pipeline from a source to a sink, in this case it is the root (sink), where sugar and then water exit sieve tube
 


Courtesy: http://www.desktopclass.com/wp-content/uploads/2011/02/pressureflow.gif



• According to this theory translocation is a kind of blood circulation within the plant body and the mesophyll cells of the leaves acting as a heart.
• The pumping force is provided by the Osmotic concentration of the solutes to be translocated in phloem which is supported by the hydrostatic pressure developed due to entry of water from xylem.
• The carbohydrates like sucrose is produced in the mesophyll cells of leaves during photosynthesis which causes an increase in the osmotic concentration of these cells.
• Hence, these cells absorb water from the neighbouring xylem cells which in turn brings about an increased hydrostatic (turgor) pressure in these cells.
• This pressure forces the solution of mesophyll cells into the sieve tubes of phloem tissue through the sieve tubes which form a continuous net work within the plant body, food materials are transported from the leaf to root through the stem.
• In the leaves, food materials are continuously produced by photosynthesis and thus the concentration of food materials is always kept high.
• At the stem and roots, the food materials are continuously utilized for various metabolic activities and thus the concentration of solutes in stem and root are always kept at low level. Thus a gradient of hydrostatic pressure is established between the leaf and the root.
• Because of the gradients of hydrostatic pressure there will be a bulk or mass flow of solution and dissolved solutes from the leaves to the stem through the phloem.
• Hence, a mass flow of solutes occurs continuously from the leaf to the root through the stem. The tissue concerned in translocation is phloem tissue of root, stem and leaf.
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Such a mechanism may be illustrated by means of a simple physical model:

• Two cells A and B have membranes which are permeable only to water.
• They are connected by a glass tube C.
• Cell A contains a solution of high osmotic pressure such as sucrose solution. C
• ell B contains only water.
• When these two cells are placed in water in a vessel D
• water will enter into cell A from the vessel D as a result of endosmosis.
• This will create a hydrostatic or turgor pressure in cell A.
• This causes the sucrose solution to move out of cell A along the tube C.
• This in turn will force water out of the cell B.
• This process will continue until the concentration of sucrose in both cells is equal at which time the flow will cease.
• Cell A may be regarded as a source and cell B as a sink.
• If the sucrose solution in B is continuously removed, then the flow will continue from cell to cell B through the tube C.
• This theory is comparatively better in explaining the translocation of solutes and the model also gives positive results.
However, there are some serious objections against this most accepted theory also. They are:

1. This theory explains the unidirectional flow of solutes, whereas in plants, the flow of food materials is bidirectional at the same time.
2. This theory explains the translocation as a non physiologi¬cal process.
3. It calls for excessive turgor pressure to account for the flow through the pores of the sieve plates.
4. The cross walls present in between the sieve tubes offer a considerable resistance and thus prevent a rapid mass flow of food materials with in the phloem tissue.

Even though there are some demerits, mass flow hypothesis is the most widely supported theory.

2. Diffusion Hypothesis

• Diffusion is a simple process by which substances will move from its region of higher concentration to the region of lower concentration.
• Diffusion hypothesis believes that translocation is fundamentally simple diffusion and translocation will take place only if there is a concentration gradient between the supply end and consumption end. If there is no concentration on gradient there is no translocation.
• The rate of translocation will be greater when the concentration of the solutes in the supply end is also greater.
• The only merit of the theory is that it explains the simul¬taneous flow of organic compounds in opposite directions.
• But this theory is also not accepted because; translocation of solute is a rapid process whereas diffusion is a very slow process. So it cannot account for the rapid movement of food materials.
Activated Diffusion Hypothesis

• In 1937, Mason and Phillis proposed that the transport of food materials takes place through stationary cytoplasm by ac¬tivated diffusion.
• According to them, diffusion is hastened by activating the diffusing molecules or by decreasing the resistance to diffusion through the protoplasmic medium.
• This theory is not accepted because no such activation has been demonstrated experimentally.

3. Protoplasmic Streaming Hypothesis

• This theory was proposed first by de Vries in 1885 and elaborated by Curtis and her associates in 1935.
• According to them, moving protoplasm carries the solutes within the sieve elements and the protoplasmic fluid moves from cell to cell through large protoplasmic connections across the sieve plates.
• A circular movement or Cyclosis of living protoplasm has been observed in many different plant cells. (e.g.) Chara, Nitella, Staminal hairs of Tradescantia.
The evidences in support of this theory are:

1. Streaming of granular substances is observed in all living cells.
2. Streaming will allow a rapid transport than that of diffusion.
3. The solutes will be easily carried from one place to another by the streaming protoplasm
4. Protoplasmic connections are observed between adjacent sieve tubes through the pores present in the sieve plate
5. A recent evidence in favour of this theory comes from the works of Thaine (1967).

Transcellular cytoplasmic strands are existing between phloem cells and food particles moving within transcellular strands. Thaine (1967) suggests that the force or energy for this movement might be provided in the form of Adenosine Tri Phosphate (ATP) by mitochondria.
The major objections against this theory are:

i) The cytoplasmic streaming has never been observed in mature sieve elements
ii) The rate-of streaming cannot account for the rapid trans¬location of solutes.

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