PHLOEM TRANSLOCATION

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PHLOEM TRANSLOCATION

• THE EVOLUTION OF AERIAL SHOOTS AND SUBTERRANEAN
  ROOTS NECESSITATED A MECHANISM FOR LONG-DISTANCE
  TRANSPORT OF SUGARS.

2. THE PRIMARY FUNCTION OF THE PHLOEM IS TO
CARRY OUT THE LONG DISTANCE TRANSLOCATION OF
SUGARS AND OTHER PHOTOSYNTHETIC PRODUCTS.
3. PHLOEM TRANSLOCATION OCCURS IN EITHER SIEVE
TUBE ELEMENTS STACKED INTO SIEVE TUBES
(ANGIOSPERMS) OR IN SIEVE CELLS (GYMNOSPERMS).
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1o PHLOEM
1o XYLEM
VASCULAR BUNDLE
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Cork
2o PHLOEM
VASCULAR CAMBIUM
2o XYLEM
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P-PROTEIN (aka Slime)

• P-protein found in all dicots many monocots. It
  is absent in gymnosperms.

2. Occurs in different forms tubular
fibrillar granular and crystalline.
3. Begin as discrete spherical bodies (P-protein
bodies) which gradually disperse during
maturation of sieve tube member.
4. Function in sealing off damaged sieve tube
elements by plugging up the sieve plate pores.
5. In Cucurbita it consists to two major
proteins PP1 (the filament protein) and PP2 (a
lectin or sugar-binding protein).
6. Callose (-13-glucan) used for long term
plugging of damaged or senescing sieve tube
elements.
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Companion cell
Sieve tube elements
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Parenchyma cell Unobstructed sieve plate
pores Sieve element
Parenchyma cell
Sieve plate
Companion cell
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Sieve cell
P
Sieve cell
SER
Sieve area
P
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THREE TYPES OF COMPANION CELLS IN THE MINOR VEINS
OF MATURE EXPORTING LEAVES

• ORDINARY COMPANION CELLS - Have chloroplasts and
  a smooth cell wall relatively few plasmodesmata
  connect ordinary companion cell to cells other
  than adjacent sieve element symplastically
  isolated.

2. TRANSFER CELLS - Like ordinary companion
cells except have finger-like wall ingrowths
which increase surface area. Both ordinary
companion cells and transfer cells are
symplastically isolated and are therefore
specialized for taking up sugars from the
apoplast.
3. INTERMEDIARY CELLS - Connected to surrounding
cells via numerous plasmodesmata and are thus
suited for taking up solutes via the symplast
lack well-developed chloroplasts.
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Sieve Elements
Intermediary Cell
Ordinary Companion Cell
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Sieve element
Wall ingrowths
Transfer cell
Plasmodesmata
Parenchyma cell
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Vascular parenchyma cell
Intermediary cell
plasmodesmata
Sieve elements
Bundle sheath cells
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PATTERNS OF TRANSLOCATION SOURCE TO SINK
1. Phloem sap is not translocated exclusively in
either an upward or downward direction and is not
influenced by gravity phloem translocation
occurs from source to sink.
2. Sources include any exporting organ typically
mature leaves exporting photosynthate.
3. Storage organs (roots tubers seeds etc.)
can also serve as sources.
4. Sinks include any nonphotosynthetic organ or
tissue that does not produce sufficient
photosynthate to support its own metabolic needs
roots underground stems buds immature leaves
flowers fruits etc.
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SOURCE TO SINK PATHWAYS FOLLOW ANATOMICAL AND
DEVELOPMENTAL PATTERNS
1. Proximity is important upper mature leaves
supply photosynthate to growing shoot tip lower
leaves supply the root middle leaves supply both.
2. Development influences transport
a. Young leaves begin as sinks gradually become
sources.
b. Reproductive structures become dominant sinks
during flowering.
3. Vascular connections important source leaves
preferentially supply sinks to which they have
vascular connections typically sources leaves
supply sinks along the same vertical row or
orthostichy.
4. Phloem interconnections (anastomoses) can
provide alternative pathways in the event of
wounding or pruning.
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CHANGES IN SOURCE-SINK RELATIONS DURING LEAF
DEVELOPMENT
Young leaf
Mature Leaf



SINK
SOURCE
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TRANSITION FROM SINK TO SOURCE LEAF
AUTORADIOGRAPHIC EVIDENCE USING SUMMER SQUASH
(Cucurbita pepo)
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APHIDS
Honey dew
stylet
Phloem Sieve Tubes
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RATES OF PHLOEM TRANSPORT
1. The rate of phloem transport can be expressed
as the linear velocity (m/hr) or as the mass
transfer rate (g/hr/cm2).
2. A typical velocity of transport is 0.3 - 1.5
m/hr much faster than the rate of diffusion.
3. A typical mass transfer rate is 1-15
g/hr/cm2. (See Web Topic 10.4 for methods for
measuring the mass transfer rates.)
4. Aphids can be used to study transport rates as
well as the composition of phloem sap.
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THE PRESSURE-FLOW MODEL OF PHLOEM TRANSLOCATION
1. ACCORDING TO THE MÜNCH PRESSURE FLOW MODEL
SUGARS MOVE BY BULK FLOW IN SIEVE TUBES IN
RESPONSE TO AN OSMOTICALLY GENERATED PRESSURE
GRADIENT (p) BETWEEN THE SOURCE AND THE SINK.
TRANSLOCATION IS THUS PASSIVE.
2. ATP-DEPENDENT PHLOEM LOADING OF SUGARS OCCURS
AT THE SOURCE ATP-DEPENDENT PHLOEM UNLOADING
OCCURS AT THE SINK. LOADING AND UNLOADING ARE
THUS ACTIVE.
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PREDICTIONS OF THE PRESSURE-FLOW MODEL HAVE BEEN
CONFIRMED
1. Sieve plate pores must be unobstructed for
pressure-flow to occur between sieve tube members.
CONFIRMED BY ELECTRON MICROSCOPY
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PREDICTIONS OF THE PRESSURE-FLOW MODEL HAVE BEEN
CONFIRMED
2. True bidirectional transport in a single sieve
tube cannot take place.
CONFIRMED BYAPHID STUDIES USING RADIOACTIVE
TRACERS AND DYES.
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PREDICTIONS OF THE PRESSURE-FLOW MODEL HAVE BEEN
CONFIRMED
3. PHLOEM TRANSLOCATION SHOULD BE A PASSIVE
PROCESS NOT DIRECTLY DEPENDENT ON ATP.
CONFIRMED BY PHYSIOLOGICAL STUDIES USING COLD
TREATMENT ANOXIA AND INHIBITORS.
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PREDICTIONS OF THE PRESSURE-FLOW MODEL HAVE BEEN
CONFIRMED
4. THERE MUST BE PRESSURE GRADIENTS BETWEEN
SOURCE AND SINK SUFFICIENT TO DRIVE BULK FLOW.
CONFIRMED BY STUDIES USING APHID STYLETS AS
MICROMANOMETERS.
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Vascular parenchyma cells
Companion cells
Sieve elements
Bundle Sheath Cell
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TWO PATHWAYS OF PHLOEM LOADING
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POLYMER-TRAPPING MODEL OF PHLOEM LOADING
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TYPES OF PHLOEM UNLOADING PATHWAYS
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