Moving around all that photosynthate and other stuff

1

• 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
3

Fig. 10.2
Bark
Phloem Secondary phloem Vascular
cambium Growth ring 3 (current year)
Xylem Ring 2 (one year older) Ring 3 (two
years older)
4

Vascular bundle of clover
Fig. 10.1
Phloem xylem
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

• Sieve tube elements
• Tubular cells with end wall pores
• and lateral sieve areas
• Membrane bound
• Have some organelles
• Have adjacent companion cells

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
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
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.
15

As the leaf grows it increasingly provides for
its own carbohydrate needs.
16

17

Mature leaf is largely a carbohydrate exporter
(source)
18
Phloem transport Velocities 1 m hour-1 , much
faster than diffusion What is the mechanism of
phloem transport? What causes flow?, Whats 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

Fig. 10.10
20

Sugars are moved from photosynthetic cells and
actively (energy) loaded into companion cells.
Fig. 10.14
21

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. 10.14
22

Phloem loading uses a proton/sucrose symport.
Fig. 10.16
23

The pressure-flow model (Münch, 1930s)

Fig. 10.10
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

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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).
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Yw
-1.1MPa
-0.4MPa