1. Phloem Translocation and Assimilate Partitioning
HORT 301 – Plant Physiology
November 2, 2009
Taiz and Zeiger, Chapter 10
Assimilate translocation in the phloem from source-to-sink is bidirectional

2. Phloem sap composition

3. Phloem tissue is adjacent to the xylem
Stem (perennial)
Leaf

4. Sieve elements and companion cell

5. Companion cell types
Ordinary companion cell
Transfer cell
Intermediary cell

6. Source to sink translocation
Proximity
Vascular interconnection
Sink strength

7. Pressure flow model for phloem translocation from source to sink
Source – phloem loading increases ψp
Sink – phloem unloading decreases ψp
Ψp gradient between source and sink drives solution movement

8. Phloem loading from source cells and unloading into sink cells
Photo-assimilate movement
Symplast (active) and apoplastic (active) phloem loading

9. Apoplastic (active) phloem loading
Summary of apoplastic and symplastic phloem loading

10. Phloem unloading and storage in sinks

11. Photosynthetic product allocation and partitioning
Allocation – regulation of carbon assimilate distribution into various metabolic pathways or sink cells
Partitioning – distribution of carbon assimilates within plants

12. Translocation from source to sink