Transport in the Phloem
I. Transport of Organic Solutes
A. Girdling Experiments: showed that the phloem is important for movement of photosynthate and
that xylem was important for movement of mineral nutrients and water.
1. Transpiration stream occurs in the wood (xylem)
a. If a tree is girdled there will be no immediate effect on growth of the shoot or transpiration from
b. Xylem sap contains dissolved minerals from soil plus small amounts of organic compounds,
sugars, and amino acids
c. Water with its dissolve minerals moves upward in the plant through xylem tissues.
2. Nutrients and photosynthate which are essential to the life of the plant move in the bark (phloem)
B. Assimilates: products of assimilation and metabolism
1. Inorganic ions (phosphate, sulfate, potassium) remain chemically unchanged during transport.
Sometimes phosphate and sulfate are incorporated into nucleotides and amino acids.
2. CO2 is first incorporated into charbohydrates and is then metabolized into other organic
compounds in the plant
a. Photosynthates: usually glucose. Transported as sucrose
b. Assimilates, including photosynthates, move long distances through sieve tubes in the phloem.
This is phloem transport.
c. Leaves constitute the source. In the spring of the year storage organs such as stems, roots,
tubers, cotyledons, and seed endosperm are sources. Any growing, storing, or metabolizing
tissue like fruits, stems, roots, corms, tubers, flowers, or young leaves, can be sinks.
Assimilates move from source to sink.
A lab model showing two osmometers that
illustrates the pressure-flow
theory of solute translocation in phloem
II. Pressure-Flow Mechanism
A. Two osmometers connected to each other. Osmometers immersed in solution with the same
water potential. First osmometers contains a solution that is more concentrated than its
surrounding solution. Second osmometer contains a solution less concentrated than that in the
first osmometer. Water moves osmotically into the first osmometer, resulting in a pressure build
up. Pressure is transferred. Increasing pressure in the second osmometer causes a more
positive water potential. Water diffuses out through the membrane. Releases the pressure in the
system. Results in bulk flow of solution (water with its solutes)
B. Bulk Flow (mass flow). The sieve elements near source cells (photosynthesizing leaf mesophyll
cells) have a high concentration of assimilates. Assimilate concentration of the phloem system at
the sink end is kept low because assimilates are transferred to the storage cells. Assimilates are
metabolized, incorporated into protoplasm or stored, usually as starch. The channel that connects
the source to the sink is the phloem system with sieve tubes (symplast) and the apoplast (cell
walls and in the xylem).
C. Flow through sieve tubes is passive. The flow occurs in response to a pressure gradient caused
by the osmotic diffusion of water. Water moves into the sieve tubes at the source end of the
system and out of the sieve tubes at the sink end of the system.
D. Cytoplasmic streaming does not occur in mature sieve elements and does not play a role in
III. Testing the Hypothesis
A. Phloem Anatomy
1. Phloem Tissue
Sieve elements or sieve tube members (no nuclei)
Angiosperms: sieve plates between cells. Forms sieve tubes.
Gymnosperms: sieve plates are not as clearly displayed., sieve cells
Companion cells (Angiosperms) -- Albuminous cells, (Gymnosperms) (nuclei, with copious
plasmodesmata, exact function not known)
Phloem parenchyma: storage and lateral transport
Phloem fibers: support
Minor and Major leaf veins. No mesophyll cell is the leaf is separated from a minor vein by
more than two or three other mesophyll cells.
Transfer Cells: Companion cells that have wall ingrowthes that increase the membrane
surface area. Found throughout the plant (xylem and phloem, parenchyma of leaf nodes,
reproductive structures, interface between the gametophyte and the sporophyte of both
lower and higher plants) as well as in phloem. Correlated with active transport over short
distances. Involved in movement of unloaded sugars into the endosperm of developing
seeds. Has not yet be demonstrated that they play a role in phloem loading.
2. Phloem Development
Slime Bodies: phloem protein (P-protein)
3. Phloem Ultrastructure
a. Companion cells have dense cytoplasm with small vacuoles. Mitochondria, dictyosomes,
and endoplasmic reticulumare abundant. Nucleus is well defined.
b. Sieve tubes have smooth endoplasmic reticulum in a continuous network along the inner
surface of the plasmalemma. Contain mitochondria . In addition to slime bodies they have
amorphous material in the lumen made of proteinaceous fibrillar material. This stuff is called
P-protein or slime bodies (P-protein bodies). Probably important in sealing off damaged
cells. Sieve-tube pores are open in normal growing plants.
B. Rates of Phloem Transport:
1. Mass Transfer Rate: quantity of material passing through a given cross section of sieve tubes
per unit of time.
2. Velocity: measure of the linear distance traversed by an assimilate molecule per unit of time.
3. Osmotic potentials of -2 to -3 MPa are common in intact sieve tubes. Equivalent to 20 - 30 %
C. Transported Solutes:
1. Bleeding is rapidly stopped as P-protein and other particulate matter clog the sieve pores.
2. 90% or more of the material transloacted in phloem consists of carbohydrates. The sugars
transported in the phloem are nonreducing sugars, sucrose being the most abundant.
3. Major nonreducing carbohydrates transported in higher plants belong either to: (1) raffinose
series of sugars (sucrose, raffinose, stachyose, verbascose), or to the sugar alcohols (mannitol,
sorbitol, galacitol, and myo-inositol).
4. Nonreducing sugars are less reactive and less labile to enzymatic destruction in sieve elements.
5. Inorganic nitrogen is usually transported in the xylem as nitrate (NO3-), which is never present in
phloem. Nitrogen can be transported in the xylem as ureides, amides, or other nitrogen-rich
6. In some species nitrogen can also occur in xylem as alkaloids or non-protein amino acids.
7. Relative nutritional completeness of sieve-tube sap: plant parts with no or minimal transpiration
are dependent on the phloem for organic and inorganic nutrients during part or all of their growth.
D. Phloem Loading:
Mesophyll cells of trees have an osmotic potential of -1.3 to -1.8 MPa. Sieve elements in leaves
have an osmotic potential of about -2.0 to -30. MPa. The osmotic potential is caused by the
presence of sugars. Sugar concentration is approximately 1.5 to 3 times higher in sieve
elements than in surrounding mesophyll cells. Phloem Loading - process by which sugars
concentrations are raised to high levels in phloem cells close to a source.
1. Pathway of transport:
a. Sugar is actively secreted from mesophyll cells into the apoplast of the minor veins. From the
apoplast, sugar is adsorbed actively into companion cells of the minor veins. Sucrose then
passes symplastically into the sieve elements.
b. Phloem loading can sometimes occur via the symplast.
c. Some species may use element of both symplast and apoplast paths
d. Active loading of sucrose into the companion cells produces a negative osmotic potential in the
cells. This leads to an up take of water by osmosis. The water plus photosynthate pass by
bulk flow across the plasmodesmatal connections between the companion cells and sieve
element. High pressures and mass flow.
2. Selective loading of sugars
a. Regardless of the pathway, only sugars transported in the phloem are accumulated in minor
veins. Selectivity of phloem loading is based on mutual recognition by the sugars and protein
carriers in the plasmalemma that transports the sugars into the cytoplasm.
b. Preference occurs for compounds that are readily transported in phloem.
(1) Preferential loading also occurs with amino acids.
(2) Minerals: those are readily transported in phloem (phosphorus, potassium) are loaded,
those that are usually not transported (calcium, boron, iron) are not loaded.
c. Many substances (growth regulators) can enter the phloem by passively diffusing along their
own concentration gradients.
3. Sucrose/Proton cotransport mechanism:
a. Transport of organic molecules (sugars and amino acids) is linked with transport of hydrogen
b. Sucrose loading into the phloem occurs by a cotransport system.
c. Protons are pumped out through the plasmalemma using the energy from ATP and an ATPase
carrier enzyme. The pH outside the cell in the apoplast becomes much lower (more acidic) than
inside the cell. As the protons diffuse back into the cell across the membrane they are coupled
to a carrier protein that transports sucrose or other sugars into the cell along with the hydrogen
4. Role of metabolism in transport:
Maintenance of the phloem transport system for bulk flow of sap requires a minimum of
metabolic energy. Metabolic energy is required mainly for phloem loading.
5. Development of Loading Capacity:
a. Change over from an import to an export mode of phloem transport. The development of phloem
loading capacity in the minor veins accounts for the switch from import to export. Once
sucrose begins to be actively loaded into the sieve elements, water enters by osmosis, and
flow begins out of the minor veins. The leaf becomes a source instead of a sink.
E. Phloem Unloading:
1. Solute Unloading: maintains low phloem turgor pressures at the sink.
2. Solute unloaded at the sink can be absorbed into developing fruit or other cells. Solute
concentrations can reach values as high or higher than in the sieve tubes at the source.
F. Pressure in the Phloem:
1. A pressure gradient occurs in the phloem sufficient for the flow from source to sink.
2. Gradients in osmotic potential in the sieve tubes from source to sink have been measured.
The most negative values occur at the source.
G. Two Problems with Pressure Flow:
3. Substances should move in the phloem not only in the same direction but at the same velocity.
Studies contradict this showing that water moves slower than solutes. The following two
explanations support why water moves slower than solutes.
a. Water exchange occurs rapidly along the pathway.
b. Sieve elements are alive and contain cytoplasm and are not inert tubes. This allows for
solutes to be metabolized or otherwise interact along the pathway.
H. Pressure Flow: Summary
1. Munch Osmometer Experiment:
a. An osmotic gradient between the two osmometers
b. Membranes that allow the establishment of a pressure gradient in response to the established
c. A low resistance pathway (tube) between the two osmometers that allows flow
d. The osmometer with the most negative osmotic potential is immersed in a solution with a
higher water potential than that in the other osmometer.
2. The osmotic system (symplast) with surrounding membranes exists in the plant. Pressures
are observed in the transport system. The medium with high water potential around the source
phloem tissues is the hydrated apoplast.
IV. Partitioning and Control Mechanisms:
A. Photosynthesis and Sink Demand:
1. Photosynthesis of leaves is influenced by sink demands.
2. When sink demand is low, sucrose levels increase in the leaves, causing a product inhibition
of photosynthetic reactions.
B. Metabolically Driven Gradients:
1. Sucrose gradient that drives phloem transport is produced by metabolism of sucrose in the
2. In storage tissues sucrose may be metabolized into starch or some other insoluble product
that has a lower effect on osmosis than sucrose.
C. Phloem Unloading:
1. Removal of sucrose or other solutes from sieve cells makes the osmotic potential in the sieve
cells less negative. Pressure transmitted from the source areas also raises the water potential.
Water diffuses out into the apoplast.
2. Decreased pressure at the sink end of the sieve tubes increases the pressure gradient between
source and sink and leads to further flow toward the sink region.
3. Sucrose unloading occurs via the apoplast and plasmodesmata (symplast) to sink cells.
In growing and respiring sinks (meristems, roots, and young leaves) unloading is typically
symplastic since the sucrose is metabolized.
4. Since there is no symplastic connection between the phloem of the mother plant and a
developing seed embryo, phloem unloading is into the apoplast.
5. Evidence indicates that assimilate unloading is under metabolic control and may involve carriers
located in membranes.
D. Growth Regulator Directed Transport
1. Growth regulators help direct translocation.
2. New sinks depend on hormones but also on increased concentrations of sucrose.
3. Growth regulators induce formation of new growth regions (sinks). Growth regulators that are
released from the new sinks and act as strong mobilizing agents.
E. Turgor Sensing in Sugar Transport:
1. Strong sinks have high concentrations of solutes (usually sucrose) in their apoplasts.
2. High negative solute potentials result in diffusion of water out of the phloem cells and a
subsequent reduction in phloem turgor. This reduction in phlem turgor makes the pressure
gradient from source to sink steeper, which increases flow. Reduced turgor in phloem cells at
the source promotes more rapid phloem loading, which also increases that rate of transport.
F. Control of Fruit and Vegetable Composition:
1. Developing seeds, fruits, or other food storage organs transpire at a low rate. These organs
subsist on phloem sap.
2. Phloem sap changes significantly in composition during transport from source to sink.