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:
- Diffusion Hypothesis
- Interfacial Flow Hypothesis
- Streaming of Protoplasm Theory
- Contractile protein Hypothesis
- Munch Mass Flow (Pressure Flow) Hypothesis
- 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.
- 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.
- 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:
- The observed rates of protoplasmic streaming
are much slower than the actual rate of translocation.
- Protoplasmic streaming has not been observed in
mature sieve tube of most of the plants.
- 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.
- 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:
- This theory has the merits of being based on
sound physical principles.
- The solutions in the phloem are in such a state
that a slight injury would cause their gushing flow out of the tissue.
- 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:
- Explains only translocation of downward
direction, but fails the bidirectional movement.
- 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.
- It is not a purely physical process, phloem
transportation is an active process and requires metabolic energy.
- Since content and other fibrills of the sieve
tube reduces the speed of flow of solute eveen under high pressure.
