1. Translocation in the phloem - Ch. 10.
Moving around all that photosynthate (and other stuff)
Translocation in the phloem - Ch. 10.
What does it do?
II. Pathways and conduits for translocation
sieve elements
sieve tube elements (angiosperms)
sieve cells (gymnosperms)
III. Source-sink relationships
IV. The mechanism and water relations of phloem
translocation.

2. The phloem is the vascular system for moving (translocating)
sugars produced in photosynthesis (photosynthate) and other
substances throughout the plant.
0.5M
PP10T020.jpg

3. Fig. 10.2 Phloem Bark Secondary phloem Vascular cambium
Growth ring 3 (current year)
Xylem
Ring 2 (one year older)
Ring 3 (two years older)
PP1002.jpg

4. Vascular bundle of clover
Fig. 10.1
Phloem
xylem
PP1001.jpg

5. “Girdling” a woody plant causes swelling of stem
above the point of damage, indicating a blockage
of phloem transport.
A classic experiment - girdling

6. More experimental evidence that phloem is the transport
tissue for carbohydrates.
Radioactive
labeling with
14CO2 can trace
movement of
sugars in the
phloem, and from
source leaves
to sinks throughout
the plant.

7. Sampling the
phloem for
chemical analysis
Aphids insert a
feeding stylet into
phloem and this can
be used to collect
phloem exudate for
chemical analysis.

8. Figure 10.3 Sieve tube elements Tubular cells with end wall pores
and lateral sieve areas
Membrane bound
Have some organelles
Have adjacent companion cells
PP10030.jpg
Figure 10.3

9. Sieve element features
living, membrane-bound cells (compare to tracheary
elements of xylem)
lack some structures and organelles in most living
cells - no nuclei, vacuole, Golgi, ribosomes,
microtubules, microfilaments
associated with companion cells that have full
set of structures and organelles
have sieve areas or pores that interconnect
adjacent sieve elements
angiosperm s.e. are called sieve tube elements, while
gymnosperms’ are called sieve cells (see Table 10.1 for differences).

10. Fig. 10.5 Cell wall between sieve elements Sieve plate pore
PP1005.jpg
Sieve plate pore
Companion cell

11. In what direction does phloem transport
substances throughout the plant?
From an area of carbohydrate supply to an
area of carbohydrate demand.
Source ----> Sink

12. Source-sink relationships can explain the direction of
phloem translocation within the plant.
Source - produces more carbohydrates than required for its own needs
Sink - produces less carbohydrates than it requires
PP10080.jpg

13. Anatomical and developmental determinants of the direction of source-sink translocation.
Proximity - sinks tend to be supplied by closer sources
Vascular connections may cause distinct source-sink
patterns that counter proximity
3. Source-sink relationships may shift during development

14. Young leaf is completely dependent on carbohydrates from
other sources. It is
a strong sink.
PP1019a.jpg

15. increasingly provides for its own carbohydrate needs.
As the leaf grows it
increasingly provides
for its own
carbohydrate needs.
PP1019b.jpg

16. PP1019c.jpg

17. a carbohydrate exporter (source)
Mature leaf is largely
a carbohydrate exporter (source)
PP1019d.jpg

18. Phloem transport
Velocities ≈ 1 m hour-1 , much faster than diffusion
What is the mechanism of phloem transport?
What causes flow?, What’s the source of energy?

19. The pressure-flow model (Münch, 1930s)
Phloem solution moves along a gradient of pressure generated by a solute concentration difference between source and sink ends of the pathway
PP10100.jpg
Fig

20. Sugars are moved from photosynthetic cells and
actively (energy) loaded into companion cells.
Fig
PP10141.jpg

21. The concentrating of sugars in sieve cells drives the
Sugars are moved from photosynthetic cells and actively (energy) loaded into companion & sieve cells.
The concentrating of sugars in sieve cells drives the
osmotic uptake of water.
Fig
PP10142.jpg

22. proton/sucrose symport. Fig. 10.16
Phloem loading uses a
proton/sucrose symport.
Fig
PP10160.jpg

23. The pressure-flow model (Münch, 1930s)
PP10100.jpg
Fig

24. The pressure-flow model of phloem translocation
At source end of pathway
Active transport of sugars into sieve cells
Ys and Yw decrease
Water flows into sieve cells and turgor increases
At sink end of pathway
Unloading (active transport again) of sugars
Ys and Yw increase
Water flows out of sieve cells and turgor decreases

25. Some key elements of phloem transport
Flow is driven by a gradient of pressure, YP.
Energy is required to establish the pressure gradient,
but energy is not required by cells of the pathway itself.
Flow is in direction of higher total water potential,
(counter to direction water tends to flow passively).

26. Yw
-1.1MPa
-0.4MPa