Translocation is the transport of organic solutes like sugars from leaves (the source) to other plant parts (the sinks) through the phloem. Several experiments provide evidence that phloem is the pathway for translocation, including the structure of phloem, ringing experiments, chemical analysis showing high sugar concentrations in phloem sap, and the use of radioactive tracers. The pressure flow hypothesis proposes that solutes move passively through the pressurized phloem network in response to pressure gradients from sources to sinks.

1. Overview of Translocation
(Phloem transport)
Dr. Anil V Dusane
Sir Parashurambhau College
Pune, India
anildusane@gmail.com
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2. Introduction
• Translocation is the transport of sugar through sieve tubes.
It is also called as phloem transport as it mainly takes place
through phloem.
• Definition: The movement of organic solutes (sugars and
other organic substances) from one part of a plant to the
other through phloem (sieve tubes) is called as
translocation.
• In simpler words, translocation is the transport of sugar
from source (site of production) to the sink (site of
utilization).
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3. Path of translocation
• Many physiologists consider that vascular tissues (xylem and
phloem) are involved translocation.
• Following are the evidences in favour of phloem being the path
of translocation.
1. Structure of phloem (anatomical studies)
2. Ringing experiment
3. Chemical analysis of cell sap
4. Use of radiotracer technique
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4. Evidences for Phloem transport
Evidences from structure of phloem
In order to understand the mechanism of phloem transport, it is essential to
know the detail structure of phloem. The Phloem is made up of four
components.
i. Sieve elements
ii. Companion (albuminous) cells
iii. Phloem parenchyma
iv. Phloem sclerenchyma/fibre cells
i) Sieve elements:
• These are elongated cells usually 100-150 μm long and connected end to
end.
• Actual transport takes place through sieve tubes.
• Continuity of sieve elements is established by plasmodesmata that runs from
cell to cell through sieve pores.
• It has been observed that sieve tubes have high Turgour Pressure (T.P.) than
that of surrounding tissues.
• Sieve elements are mainly involved in the transport of organic solutes.
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Sieve elements

5. Evidences for Phloem transport
ii) Companion cells (in angiosperms) or Albuminous cells (in gymnosperms):
These are the living cells which are closely associated with sieve elements.
Sieve elements are connected to companion cells through plasmodesmata.
Function: It provides energy to the sieve tubes.
iii) Phloem parenchyma: These are thin walled and similar to other
parenchyma
Function: Storage and lateral transport of water and solutes.
iv)Phloem sclerenchyma: It is thick-walled.
Function: Provides strength to phloem tissues.
• Thus the sieve tubes form continuous system through out the plant and
help in conducting the solutes. Long sized sieve tube in large fruit trees is
an additional evidence for phloem being involved in translocation. 5

6. Evidences for translocation
Evidences from Ringing experiment:
• This experiment involves selective removal of stem tissues in form of a ring
in A and B plants.
• In Plant A (only xylem removed) and Plant B(only phloem removed).
• Plant A shows elongation of stem and thus shows that the movement of
solutes takes place through phloem while Plant B does not show elongation
due to stoppage of movement of solutes due to removal phloem.
Evidences from chemical analysis of phloem cell sap:
• Several plants have shown that sieve tubes are having higher concentrations
of carbohydrates (sucrose) and organic nitrogen compounds than that of
vessels (xylem).
• Concentration of sugar in sieve elements is very high compared to what is
necessary for the maintenance of normal osmotic relationship with adjacent
cells.
• This clearly indicates that phloem tissues are involved in translocation. 6

7. Evidences for Phloem transport
Evidences from radiotracer technique:
• The use of radioisotopes viz. carbon (14C) and phosphorus (32P) has provided direct
evidence for transport of carbohydrates through phloem.
• Burr and others (1945) allowed bean leaf to photosynthesize in an atmosphere of 14C.
They observed that labeled sugar moved through phloem.
Experiment of Biddulph and Markle (1944):
• They studied translocation of radioactive isotopes of phosphorus (32P) after it had been
introduced into the leaves of cotton plants.
• They made a slit in bark below leaf to which they applied 32P.
• They separated xylem and phloem tissues at a particular place by introducing a wax
paper so as to prevent lateral conduction.
• After a short period, they found the presence of greater amount of 32P in phloem tissue.
• These results clearly indicate that the downward movement of P 32P labelled organic
compounds occurred in the phloem.
• This had supported that the translocation takes through the phloem.
• Radioautography technique has conclusively proved that,path of translocation is phloem.7

8. Rate of Phloem transport
• Photosynthates are translocated from the site of
production (source) to the site of storage or utilization
(sink).
• Flow rate is proportional to the gradient of pressure in the
phloem. Different substances are translocated at different
rates.
• Phloem sap concentration from source to sink is the
important factor for phloem transport.
• The quantity of dry matter transport per unit time per
cm2 of phloem is termed as specific mass transfer.
• Specific mass = transfer of dry matter (wt) per unit
time/Cross sectional area of phloem. 8

9. Direction of phloem transport
• Several experiments have shown that the direction of phloem sap is
governed by following rules.
• Mature leaves always act as source and never behave as sink (immature
leaves act as sink).
• Upper leaves translocate solutes towards stem apex, lower leaves
translocate solutes down wards (to the root) while the leaves in middle
portion translocate solutes to both directions i.e. up and downward
through bi-directional movement.
• Active sinks are fed by nearest source.
• Solutes travel in straight line along the phloem, with very little side-ways
spread i.e. scanty lateral movement.
• The pattern of movement can be changed by removal of source or sink.9

10. Types of translocations
Based on direction of sap flow there are three types.
Unidirectional translocation:
1. In this type, solute travels in one direction, from source to sink.
2. Example- leaf to stem.
Bi-directional translocation:
i. In this organic solutes move in both directions i.e. upward and
downward.
ii. There is possibility that some sieve tubes are involved in upward
direction while others in downward direction.
iii. The possibility of movement of solutes in both directions
simultaneously in the same sieve tube can not be ignored
completely.
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11. Types of translocations
Bidirectional translocation:
iv) A leaf in the middle portion transports the food in both directions.
v) Experiments carried on Vicia faba using 14C have supported bi-directional
transport.
vi) Some physiologist claim that bi-directional flow occurs as a result of
maturing of leaves that changes from sink to a source as the leaves
matures.
Lateral translocation:
i. In general, solutes travel in a straight line, with very little lateral
movement.
ii. Radiotracer techniques have supported this type of translocation
movement. 11

12. Mechanism of translocation
Diffusion theory, Activated diffusion theory, electro-osmosis theory and
Pressure flow or mass flow theory has been put forth to explain the
mechanism of translocation.
1. Diffusion Theory: Many physiologists are of opinion that the solute
moves from source to sink due to the simple physical process of diffusion.
Objections:
• The rate of flow of solute in sieve tube is at least 40,000 times higher than
what is possible by diffusion.
• Respiratory inhibitory chemicals affect the rate of flow of solutes. This
clearly indicates that translocation is a physiological process.
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13. Mechanism of translocation
2. Activated Diffusion Theory:
i. Manson and Phillis (1936) proposed this theory.
ii. According to this theory, solute transport in sieve tube is an activated
process and normal rate of diffusion is greatly enhanced due to
protoplasmic properties of sieve tube.
Objections:
i. Activation requires energy but sieve tube elements do not possess any
metabolic machinery.
ii. The known rate of translocation is very high and even activated diffusion
is not enough for transport of solutes at that rate.
iii. Experimental evidence does not support this theory.
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14. Mechanism of translocation
3. Mass flow/pressure flow/Munch hypothesis:
i. It is proposed by German scientist Munch (1930).
ii. According to this hypothesis translocation is a physical phenomenon.
Assumptions of hypothesis:
i. According to this hypothesis, turgor pressure (TP) gradient exist in between
the source and sink, and the translocation takes place passively in response to
turgor pressure gradient.
ii. This hypothesis assumes that, sieve tubes are connected to one another by
means of cytoplasmic connections forming a continuous system called
‘Symplast’.
iii. At the utilization end (sink) water diffuses into xylem of leaf and continuous
system is established.
iv. There is unidirectional flow of solute in phloem.
v. Water and solutes moves together in same direction and at same rate.
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15. Mechanism of translocation
Mechanism of Mass flow/pressure flow/Munch hypothesis:
The principle of mass flow can be illustrated with the help of
simple model.
i. Two chambers A and B with selective permeability are
connected by tube c.
ii. Here A (osmometer) is filled with highly concentrated sugar
solution and this corresponds to the source (Mesophyll cells).
B (osmometer) is filled with less concentrated solution (of
sugar) than A but still higher than it’s surrounding. It
corresponds to sink (root cells). ‘c’ corresponds to
longitudinal system of sieve tubes, connecting leaf and root
cells. ‘T’ represents xylem vessels, water from B returns to A
through this tube.
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‘
c
’
c
T
Mesophyll cell Root cell

16. Mechanism of translocation
iv) ‘A’ contains higher concentration of sugar. It absorbs larger quantity
of water and develops higher Turgor Pressure. The solution will flow
(mass flow) from A to B via c under pressure gradient.
v) The flow will be continuous until the concentration of both osmometers
is equalized.
vi) If a continuous addition of sugar at A and it’s removal at B is arranged
then there will be a continuous mass flow from A to B and a continuous
passage of water from B into outer vessel (pure water) would be
established.
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17. Mechanism of translocation
Evidences to support Mass flow/pressure flow/Munch hypothesis:
• This hypothesis assumes that there should be a pressure gradient (from
source to sink) along the sieve tubes. This was supported by experiments
of H.T. Hammel (1968). He took direct measurements of sieve tubes TP in
Oak (Quercus rubrum) with a special microneedle. The values (Pressure
gradient, 0.03-0.05 Mpa/mt) he obtained at two different points are
similar to the values that Munch model predicts.
• Active loading and unloading of sucrose favours pressure flow hypothesis.
• This hypothesis assumes that water and solutes should move together in
same direction at same rate along sieve tubes. Though (tritium) 3H-water
moves fast than 14C-sugar. But it may be due to more permeability of H2O
to sieve tubes than that of sugar. 17

18. Mechanism of translocation
• Evidences to support Mass flow/pressure flow/Munch hypothesis:
• This hypothesis assumes that solute is unidirectional in sieve tubes.
Though some physiologists claimed bidirectional flow, but this flow
appears due to a result of maturing leaves that change itself from sink to
source.
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19. Mechanism of translocation
Objections to Mass flow/pressure flow/Munch hypothesis:
• Munch hypothesis believes that translocation is a purely physical process
but rate of flow of solute is affected by ‘translocation inhibitory chemicals’.
This clearly indicates that translocation is a physiological process.
• Bidirectional flow can not be explained by this theory.
• Mass flow is possible only when sieve pores are not plugged but
sometimes mass flow takes place though the sieve tubes are plugged.
• According to this hypothesis there should always be a ‘+ve gradient’
between source and sink. However, it has been observed that a ‘+ve
gradient’ always does not exist but still mass flow takes place.
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20. Questions
• Q1. Discuss the various hypotheses put forward to explain the translocation of
organic solutes in plants.
• Q2. Define translocation. Explain any one type of the mechanism of
translocation.
• Q3. Describe in brief the use of tracer techniques in establishing the pathway of
translocation.
• Q4. Enumerate various theories so explain the mechanism of translocation
in detail (any two).
• Q5. Critically evaluate the various hypotheses regarding translocation of organic
solutes.
• Q6. Short notes:
i) Ringing experiments
ii) Much hypothesis
iii) Bidirectional translocation of phloem transport
iv) Phloem transport.
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21. Thanks
Dr. Anil V Dusane
Sir Parashurambhau College, Pune, India
anildusane@gmail.com
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