Transport in the Phloem

Transport of Organic Solutes

  1. Girdling Experiments:
  2. Assimilates:  products of assimilation & metabolism
The Pressure-Flow Mechanism
  1. Study how the laboratory system with the two osmometers is set up and how it works.  This system shows how bulk flow occurs in the phloem of plants.
  2. Bulk flow (mass flow).
  3. Flow through the sieve tubes is passive, occurring in response to the pressure gradient caused by osmotic diffusion of water into sieve tubes at the source end of the system & out of the sieve tubes at the sink end of the system.
  4. Cytoplasmic streaming does not occur in mature sieve elements & thus cannot play a role in phloem transport.
Two osmometers illustrating the pressure-flow theory of solute translocation
A lab model showing two osmometers that illustrates the pressure-flow theory of solute translocation in phloem

Testing the Hypothesis

  1. Phloem anatomy
  2. Rates of Phloem Transport
  3. Transported Solutes
  4. Phloem Loading:  Mesophyll cells have osmotic potentials of -1.3 to -1.8 MPa.  Sieve elements in leaves have osmotic potentials of -2.0 to -30. MPa.  Osmotic potential is caused by presence of sugars.  Sugar concentration is approximately 1.5 - 3 times higher in sieve elements than surrounding mesophyll cells.
  5.  Phloem Unloading:  maintains low phloem turgor pressures at the sink.  Solute unloaded at sink can then be absorbed into developing fruit or other cells.  Concentrations can reach values higher than in sieve tubes at source.
  6. Pressure in the Phloem:  a pressure gradient occurs in phloem sufficient to account for flow from source to sink.  Gradients in osmotic potential in sieve tubes from source to sink have often been measured, with most negative values at source.
  7. Two Problems with Pressure Flow:  Substances should move in the phloem not only in the same direction but at the same velocity.  Studies seem to contradict this showing that water moved slower than solutes.  What accounts for this problem?  (see the 2 explanations)
  8. Pressure Flow:  Summary:  Munch osmometer experiment reviewed.  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.
Partitioning & Control Mechanisms
  1. Photosynthesis & Sink Demand
  2. Metabolically Driven Gradients
  3. Phloem Unloading
  4. Growth Regulator Directed Transport
  5. Turgor Sensing in Sugar Transport
  6. Control of Fruit & Vegetable Composition