Mechanism of Translocation

CHAPTER: 12(B)

Mechanism of Translocation

Mechanism of Translocation:

Ø  It is evident that the mechanism of the transport of solutes through phloem must be entirely different from the mechanism of translocation through the xylem.

Ø  A number of theories have been given to explain the mechanism, but none of them seems to be entirely satisfactory.

Ø  The theories are:

1. Diffusion Hypothesis
2. Interfacial Flow Hypothesis
3. Streaming of Protoplasm Theory
4. Contractile protein Hypothesis
5. Munch Mass Flow (Pressure Flow) Hypothesis

1. Diffusion Hypothesis:

•      According to this hypothesis the translocation of foods occurs from the place of high concentration to the place of lower concentration.

•      The hypothesis is not acceptable because the rate of translocation of foods in the phloem is much higher than can be achieved by the diffusion hypothesis.

•      Mason and Maskel’s (1936) gave activated diffusion hypothesis. According to this hypothesis respiratory energy is used in the process.

2. Interfacial Flow Hypothesis:

•      The spreading of oleate in an ether- water interface led Van den Honert (1932) to think that transported molecules might move along interfaces of membranes such as tonoplast. The solute molecules are believed to be absorbed and dispersed at the interface due to the reduction of surface tention.

•      The hypothesis, however does not have any experimental evidences to show the existence of such a phenomenon in plants.

3. Streaming of Protoplasm Theory:

•      de Vries suggested the streaming of protoplasm theory for the movement of solutes from one end of sieve tube to other end.

•      The particles from other end pass to adjacent sieve tube by simple diffusion.

•      The hypothesis was supported by Curtis

•      The streaming protoplasm acts as a conveyer belt.

•      Different substances move in different direction at the same time in the same sieve tube.

Objections:

1. The observed rates of protoplasmic streaming are much slower than the actual rate of translocation.
2. Protoplasmic streaming has not been observed in mature sieve tube of most of the plants.

4. Contractile protein Hypothesis:

•      The protoplasm has been found to be rich in fibrillar protein called P-protein.

•      The fibrils of P-protein have been observed to be often present in the sieve pores.

•      These fibrils have microtubules which consist of contractile proteins.

•      Due to this contractile proteins, the movement of solutes is accomplished.

5. Munch Mass Flow (Pressure Flow) Hypothesis :

•      The hypothesis of Muncs (1930) assumes that the protoplasm of a row of sieve tubes is connected with each other by means of plasmodesmata forming a continuous system, the symplast.

•      It is impermeable on the outer surface but is highly permeable throughout its mass.

•      The permeability enables the sap (including organic solutes) to flow en mass, ender certain condition.

•      According to the hypothesis when food is manufactured in the leaf, the osmotic pressure of the mesophyll cell is very much increased.

•      This causes absorption of water from the xylem elements of the leaf, resulting in an increase in their turgor pressure.

•      This succeeds in forcing some of the cell solution into the sieve tubes because of which the osmotic pressure of the phloem increases.

•      On the other hand, in the cells of the root or the storage organs, the food is either consumed or is converted into insoluble forms resulting in decrease in their osmotic pressure as well as their turgor pressure.

•      Under the conditions mentioned above, a turgor pressure gradient is established between the supply end in the leaf and the consumption end in the root and therefore, a mass flow of water (containing dissolved solutes) takes place in the phloem from the upper end to the lower end of the plant.

•      The numerous sieve plates help in maintaining the turgor pressure gradient throughout the length of the sieve tubes. At the consumption end, water diffuses out into the xylem elements of the root due to the lowering of the osmotic pressure. This water along with the absorbed water is translocated to the leaf through the xylem elements. A sort of cyclic circulatory system is formed.

Experiment illustrating Mass Flow Hypothesis:

Ø  The principle of mass flow can be illustrated with the help of a simple experiment.

Ø  Two chamber with semipermeable walls A and B are connected by a tube P containing water to form a closed system.

Ø  Chamber A contains a highly concentrated sugar solution while chamber B contain water.

Ø  The system is dipped in a water filled vessel .

Ø  Chamber A corresponds to the supply end in the leaf and the chamber B corresponds to the consumption end in the root.

Ø  The tube P corresponds to the longitudinal sieve tube system.

Ø  The outer vessel marked X corresponds to the xylem elements.

Ø  The concentrated sugar solution in the chamber A causes rapid endosmosis of water into it, resulting in the development of a very high turgor pressure.

Ø  The solution, therefore , flows in mass from A to B under the turgor pressure gradient till the solutions in both the chambers attain the same concentration.

Ø  If, however, sugar is continuously added to the chamber A and is removed from chamber B, a continuous mass flow of solution will take place from A to B through the tube P.

Ø  On reaching the chamber B water will diffuse out into the vessel and can again pass through it to  chamber A.

Ø  An analogous situation is found in plants.

Evidence of Mass Flow Hypothesis:

1. This theory has the merits of being based on sound physical principles.
2. The solutions in the phloem are in such a state that a slight injury would cause their gushing flow out of the tissue.
3. A strong evidence in support of the theory is provided by the observed movement of viruses or growth substances in illuminated and shaded leaves.

 Objection of Mass Flow Hypothesis:

1. Explains only translocation of downward direction, but fails the bidirectional movement.
2. This theory holds that the cells at supplying end have higher turgor pressure than the cells at the receiving end. But, in sprouting potatoes, the cells of the young supports are very turgid while the cells of the mother tuber are flaccid.
3. It is not a purely physical process, phloem transportation is an active process and requires metabolic energy.
4. Since content and other fibrills of the sieve tube reduces the speed of flow of solute eveen under high pressure.