CHAPTER: 12(A)
Translocation
in Plants
(Xylem & Phloem)
(Xylem & Phloem)
Translocation
of Food:
• Translocation
is the movement of materials from leaves to other tissues throughout the plant.
• Plants
produce carbohydrates (sugars) in their leaves by photosynthesis, but
nonphotosynthetic parts of the plant also require carbohydrates and other organic and
nonorganic materials. For this reason, nutrients are translocated from sources
(regions of excess carbohydrates, primarily mature leaves) to sinks (regions
where the carbohydrate is needed).
• Some
important sinks are roots, flowers, fruits, stems, and developing leaves.
• Leaves
are particularly interesting in this regard because they are sinks when they
are young and become sources later, when they are about half grown.
• The
process of translocation of foods occurs mostly through the phloem.
While phloem lies alongside the xylem in veins in leaves, vascular bundles in
stems, and the vascular cylinder of roots, it is a completely different tissue
conducting in different directions and by different mechanisms.
Direction
of Translocation:
• The
translocation of organic solutes is predominantly in a downward or upward
direction.
• Radial
translocation is also quite common in plants.
• In
most cases, the organic food, synthesized by the leaves of a plant is
translocated to the lower parts of the plant for the purpose of storage or
consumption .
• The
translocation is, however , in a reverse direction, in the case of germinating
seeds and underground perennating structures.
• During
seed and fruit formation the traslocation of food is normally in a radial
direction.
• On
the whole the translocation of food material is always from a point of higher
concentration to points of lower concentration.
- Downward translocation: leaves to stems, roots
- Upward translocation: From seed bulb to buds, leaves flowers and fruits.
- Radial translocation: From the cell of pith to cortex
Path
of Translocation:
- Downward Translocation of Organic
Solutes:
• The
view that downward translocation of food material takes place through phloem is
supported by a large no. of evidences.
• They are as follows:
- Elimination of other Tissues:
• It
is well known that the xylem sap is conducted in an upward direction.
• It
is unlikely, therefore, that xylem can account for downward translocation of
organic substance .
• The
cells of the ground tissue are also thought to be incapable of doing so, since
they have not been found to contain soluble, transportable form of food
material.
• Thus
phloem remains the only tissue which can convincingly carry on the function.
- Structure and Distribution Phloem:
• The
structure of the phloem strongly suggests that it is well adapted for the
purpose of conduction.
• The
elongated sieve tubes, like the tracheids and vessels of the xylem, are
arranged end-to-end to form a continuous system throughout the plant.
• The
conspicuous perforations in their end walls, with strands of protoplasm
(plasmodesmata) connecting the adjoining sieve tubes help in increasing the
continuity of the conducting system.
- Effect of Blocking the phloem:
• The
blocking of sieve tubes by the deposition of callose (Complex Polysaccharides)
results in the stoppage of translocation process.
- Chemical analysis of Phloem Sap and
Xylem Sap:
• Chemical
analysis of sieve tubes sap proves that concentrated solution of sucrose is
translocated from the place of synthesis to other parts of the plant body.
• Glucose
and Fructose are sometimes found in traces only.
• The
amount of sucrose is more in phloem sap during the day and less in night.
• In
xylem the amount of sucrose is traces and also there is no diurnal fluctuation.
- Ringing or Girdling Experiment:
• A
very conclusive evidence in favour of phloem comes from ringing experiments.
• Complete
ringing experiment or girdling to remove the bark, which includes phloem from a
woody stem causes food materials to accumulate immediately above the ring.
• Whereas
the tissues below it gradually lose all the stored food and dry up.
• If
ringing is such that the phloem is not removed , the translocation of food is
largely unaffected showing thereby that phloem conducts food in a downward
direction.
- Upward Translocation of Organic
Solutes:
• According
to Dixon the Upward conduction of food takes place through the xylem.
• However,
scientist are not in agreement with him.
• According
to Curtis upward conduction of foods also takes place through phloem.
• This
view is based upon ringing experiments.
• He
took three woody plants:
- In plant A: ringing was done as described in
ringing experiment.
- In plant B: Xylem was injured in a ring but
phloem was left intact.
- In plant C: Xylem and Phloem both were in
normal Position.
Ø In
plant A & B all leaves above the ring were removed
Ø In
A there was no growth above the ring also the dry weight of this part was less.
Ø This
proves that upward conduction of food takes place through phloem.
Structure
of Phloem Tissues:
• They
posses, tubules structure for transport of food.
• They
are composed of living cells and have no mechanical functions.
• Phloem
tissues consists of:
- Sieve Tube 2.
Companion Cells 3. Phloem
Parenchyma 4. Phloem
Fibres
- Sieve
Tube:
• They
are long tube like structure involved in the transport of sucrose or glucose
throughout the plants.
• These
are formed by end to end fusion of cells called sieve tube elements or sieve
elements.
• The
wall of sieve tube elements are made up of cellulose and pectic substances, but
their nuclei are degenerated and lost as they mature. The cytoplasm is confined
to a thin peripheral layer.
• Two
adjoining end walls of neighboring sieve elements from a sieve plate.
• In
fact originally , plasmodesmata passed through the walls but later on these
pores are enlarged so that the walls look-like a sieve allowing the flow of
solution from one element to next.
- Companion Cells:
• A
thin wall elongated cells called companion cells associated with each sieve
tube.
• Both
are connected by simple pits.
• Each
companion cell is living containing dense protoplasm and a large elongated
nucleus seed.
• The
sieve tube elements depends on the adjacent companion cell , therefore they
remain living.
• Companion
cells control the passage of materials and provide energy to sieve tubes.
• They
help in conduction of food along with sieve tube.
• Found
only in Angiosperm not in Pteridophytes and Gymnosperm
- Phloem Parenchyma:
• Living
and often cylindrical in shape.
• Mostly
absent in monocots.
• These
mainly store food materials in form of starch and fats.
- Phloem Fibres:
• Sclerenchymatous
fibre cells present in phloem.
• They
provide mechanical supports and rigidity to plant organ.
• There
is no living components
Phloem
Loading & Phloem Unloading:
- Phloem Loading:
• Phloem
loading can be performed by the following four steps:
- Diffusion of triose phosphates from the matrix
of the chloroplast into the cytosol where it is finally converted into
sucrose.
- Sucrose travels a few cells from the mesophyll
to near the sieve elements (short-distance transport).
- Sugars enter the sieve elements-Companion cell
complex
- Sugars are then transported to the sink
(long-distance transport) through the vascular system (export).
Ø It
is believed that while the mass flow of sugars is a passive process the phloem
loading and unloading are energy consuming active process.
Ø The
sieve element-companion cell complex has a high concentration of sucrose due to
which the sugars enter these cells against the concentration gradient and at
the cost of energy.
Ø The
apoplastic transport of sucrose into sieve elements through ordinary companion
cells and transfer cells involves sucrose- H+ Symporter which
utilizes the energy of the proton pump when the protons diffuse back into the
cell.
Ø Symplastic
loading of sucrose, raffnose is predominantly through intermediary cells.
Ø There
are several evidences to prove that transport of sugars through the mesophyll
cells is mostly symplastic.
Ø It
is believed that the sugars may enter their cells walls (apoplast) from where
there is an active transport of sugars into the sieve elements and companion
cells.
- Phloem Unloading:
• The
transport of sugars from the sieve elements into the cells of the roots, tubers
etc where they are either stored or metabolized is generally symplastic and to
some extent apoplastic.
• Symplastic
unloading is along the concentration gradient and is, therefore, passive.
• The
apoplastic transport may be an active energy consuming process.
• The
phloem unloading (import) into storage tissues of underground rhizomes, corns,
tubers , roots etc. occur in the following steps:
- Transport of sugar out of the sieve elements.
- The sugars diffuse into the storage tissues
(Sink).
- The sugars are usually hydrolyzed and converted
into starch for the purpose of storage.
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