Phloem Translocation


 Phloem translocation of photosynthesis products (photosynthates)


Description of the Phloem system,

Within Vascular bundles, surrounded by vascular sheath

·        Sieve tube

·        Companion cells

·        Phloem parenchyma

·        Phloem fibers

·        Sclerids

·        Laticifers


Phloem Functions;

a-     Translocation of materials from origin

b-    Redistribution of previously transported material


Note; 'Sieve tube' or the 'Sieve Element' only, involved in translocation and 'Bundle Sheath' for water conduction.

Evidences to transport inside the phloem;

1-     Girdling of stem

2-     Autoradiography with atomic isotopes, 14C, 11C, 14CO2 and 32


The Sieve Element;

Characterized by;

1-     Loss of its nucleus, tonoplast as it develops and mature

2-     No mitochondria, microtubules, Golgy apparatus, ribosomes

3-     Retains plasmamembrane, microtubules, plastids, smooth endoplasmic reticulum (SER)

4-     Modifies cell wall


Note; Sieve tube is always under HIGH TURGOR pressure

Sieve Area; 0r Sieve Plate: Pore size ranges from 1um to 15 um

Phloem Protein, or P-Protein;

a-     it is found in all dicotyledonous plants, Rare in monocotyledonous plants, But never found in Gymnosperms

b-    In dicotyledonous it is more significant in Immature sieve tube

c-     P-protein functions in; sealing off damaged tube

d-    Most evident in mature sieve tubes

e-     Formed of discrete bodies in the cytosole, of spherical, spindle shape or twisted and coiled particles

f-      P-protein gets disperse into tubules or fibrillation form during cell maturation


The Callose; chemically it is; β - (1-3) glucan. This compound gives positive (+tive) reaction with Analine blue staining.

1-     it is deposited outside of the cell between plasmamembrane and cell wall

2-     Synthesized inside functional sieve tubes only under the effect of a STRESS FACTOR such as high temperature, or mechanical damage.


Types of Callose;

·        Wound Callose; deposited and may be removed of disappear (dissolved)

·        Definitive Callose; associated with sieve tube obliteration and the formation of secondary cell wall. It stays and do not get dissolved.

·        Dormancy Callose; in over wintering plants, perennial, plants dormant in winter. It is dissolved in spring time.


The Companion Cell;

·        Sieve tubes are associated with 1--> more companion cells

·        Plasmadesmata tubules connect between companion cell and the adjacent sieve tube

·         In the Angiosperms the Sieve tube and the companion cell come from a common one mother cell

·        In gymnosperms; companion cell called “Abuminous cell” and arises from separate mother cell other than the mother cell for the sieve tube. i. e. they are not from the same mother cell.

·        It function in Protein synthesis, ATP-generation, as a bridge for photosynthetic products


Transfer cells;

·        present in some herbaceous dicots

·        with cell walls facing inside with ingrowths, to increase the surface area of plasmamembrane.


Patterns of Translocation in Phloem

It follows the rule of source s -->  sinks


a-     mature leaves

b-    storage organs


Sinks; areas of metabolism


Generalizations about the source to sink relations;

1-     The proximity of source to the sink; such as

·        upper mature leaves --> younger growing shoot tip and younger immature leaves.

·        Lower leaves --> root system

·        Intermediate leaves --> to both directions


2-     The importance of various sinks; may shift such as;

·        root and shoot tips are sinks during growth

·        fruit become dominant sinks


3-     Source leaves supply sinks with which they have direct vascular connection.. or Orthostichy

4-     Interfering with the translocation pathway by;

·        wounding or

·        pruning


Note; removal of the lower leaves can urge the upper ones to translocate to the lower region of the plant and vice versa


Material Translocated in the Phloem

 Collecting Phloem sap for analysis;

a-     cutting the phloem tissue and collect the sap

b-    Honey due of Aphids



1-     water, most abundant substance in the phloem sap

2-     Carbohydrates (CH2O) Mainly sucrose in most plants,

Note; Reducing sugars; Glucose, Fructose and Mannose generally do NOT found in phloem sap.

3-     Sucrose is the most commonly transported CH2O in phloem, Sieve element contains about 0.3-0.9 M sucrose solution.

4-     Other organic solutes such as;

* Nitrogenous compounds as;

  a- Amino acids, especially Glutamate, and Glutamines, and Aspartate and Asparagine

  b- Amides; Ureids in nitrogen fixing plants

  c- Auxin, Gibberellins, Cytokines, and Abscisic Acid

  d- Nucleotides phosphates

  e- Enzymes

5- Inorganic compounds such as ions; K+, Mg+2, PO4-2, Cl-

   (Note; NO3, and Ca+2 do not exist in phloem)

Note: Sucrose (as a unit of Glucose-Fructose molecule) has to be  the first part of a chain of saccharide of non-reducing sugar polymer to be translocated inside phloem.


This table show the Swanson & Shishing data about sugar translocation in phloem. Experiment made on Grapes

(Vitis labrucana )

Average transport in Grape stem of labeled 14C sucrose, glucose and fructose

Distribution of translocation in mm

Counts/ min/ mg Dry wt. Of Bark



















































Conclusion; Translocation happens in form of sucrose


Sucrose is; glucose-fructose

Rafinose is; sucrose- galactose

Stachyose is; sucrose- galactose-galactose

Verbacose is; sucrose-galactose-galactose-galactose


High Positive Pressure in Phloem

 This positive pressure as a driving force in phloem transport

* Sap in the phloem exudes in a phenomenon of “Phloem Exudation” when phloem sieve element is severed or cut.

* Honey Dew associated with Aphid feeding on some plants. The Aphid pierces its stylet into single sieve element. The sap flow under pressure through the gut of the insect and appears at the rear end of its elementary tract ad drops of “Honey Dew”.

* Some plant species do not show exudation because P-protein plugging of the cut sieve element 

* Rate of flow is increased by the application of EDTA

   EDTA is a chelating agent for the ions of Ca, and then prevents Callose formation.


Collecting Phloem sap

·        Making advantage of the positive pressure in the phloem


1-     Direct cut the tissue and collect the sap, But problem of P-protein plugging and contamination of the sap with other cytoplasm material

2-     Direct cut the tissue and add EDTA to cut point to keep the flow of sap, But problem of contamination of the sap with other cytoplasm material

3-     Using Aphids in process of collecting Honey dew.

Make the Aphid pierces its stylet into single sieve element,

Apply anesthetic material to paralyze the Aphid, such as CO2

Cut the stylet of the insect by removing the insect leaving its stylet in position.

Use that stylet to collect the flowing sap.



Phloem Loading and Unloading.


* Loading at the source point and Unloading at the Sink point

* Loading and Unloading in an Active Process

* Source loading is derived by Proton Gradient at the expense of ATP

* Process of Loading involves;

   1- Sucrose hydrolyzed in the appoplast to glucose and fructose and go into the Companion cell

   2- Sucrose go in as it is without hydrolysis as a non-charged molecule, but may get hydrolyzed inside the cell

   3- Sucrose goes in as it is via Plasmadesmata

Therefore the loading process involves both Passive and Active action in transport.



Pressure Flow Hypothesis

The theory put out for the explanation of the movement of the sap inside the phloem

'Translocation in Phloem is derived by the positive pressure gradient in the phloem from the source to the sink'


Predictions for this theory;

·        Sieve plate, with open pores not normally blocked by the p-protein or resistance

·        No bidirectional flow within any one single sieve tube

·        Passive no great Expenditure of energy. Energy at loading and Unloading sites only.

·        Positive pressure gradient between loading point and the unloading point

·        Passive flow in the pathway in between those points


Consider the Model to show the principles of actual flow in plant tissue.