Translocation in Plants

CHAPTER: 12(A)

Translocation in Plants
(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.

1. Downward translocation: leaves to stems, roots
2. Upward translocation:            From seed bulb to buds, leaves flowers and fruits.
3. Radial translocation:   From the cell of pith to cortex

Path of Translocation:

1. 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:

1. 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.

2. 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.

3. Effect of Blocking the phloem:

•      The blocking of sieve tubes by the deposition of callose (Complex Polysaccharides) results in the stoppage of translocation process.

4. 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.

5. 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.

2. 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:

1. In plant A: ringing was done as described in ringing experiment.
2. In plant B: Xylem was injured in a ring but phloem was left intact.
3. 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:

1. Sieve Tube      2. Companion Cells     3. Phloem Parenchyma            4. Phloem Fibres

1. 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.

2. 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

3. Phloem Parenchyma:

•      Living and often cylindrical in shape.

•      Mostly absent in monocots.

•      These mainly store food materials in form of starch and fats.

4. 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:

1. Phloem Loading:

•      Phloem loading can be performed by the following four steps:

1. Diffusion of triose phosphates from the matrix of the chloroplast into the cytosol where it is finally converted into sucrose.
2. Sucrose travels a few cells from the mesophyll to near the sieve elements (short-distance transport).
3. Sugars enter the sieve elements-Companion cell complex
4. 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.

2. 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:

1. Transport of sugar out of the sieve elements.
2. The sugars diffuse into the storage tissues (Sink).
3. The sugars are usually hydrolyzed and converted into starch for the purpose of storage. 

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