PHLOEM TRANSLOCATION
1. 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).
1o
PHLOEM
1o
XYLEM
VASCULAR
BUNDLE
Cork
2o
PHLOEM
VASCULAR
CAMBIUM
2o XYLEM
P-PROTEIN (aka: Slime)
1. 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 (-1,3-glucan) used for long term plugging of damaged
or senescing sieve tube elements.
Companion
cell
Sieve tube
elements
Parenchyma cell
Unobstructed sieve
plate pores
Sieve
element
Parenchyma cell
Sieve
plate
Companion
cell
Sieve cell
P
Sieve cell
SER
Sieve area
P
THREE TYPES OF COMPANION CELLS IN THE MINOR
VEINS OF MATURE EXPORTING LEAVES
1. 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.
Sieve
Elements
Intermediary
Cell
Ordinary
Companion
Cell
Sieve element
Wall ingrowths
Transfer cell
Plasmodesmata
Parenchyma cell
Vascular
parenchyma cell
Intermediary cell
plasmodesmata
Sieve
elements
Bundle sheath cells
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.
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.
CHANGES IN SOURCE-SINK RELATIONS DURING LEAF
DEVELOPMENT
Mature Leaf
Young leaf




SINK
SOURCE
TRANSITION FROM SINK TO SOURCE LEAF: AUTORADIOGRAPHIC
EVIDENCE USING SUMMER SQUASH (Cucurbita pepo)
APHIDS
“Honey dew”
“stylet”
Phloem Sieve Tubes
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.
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.
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
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.
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.
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.
Vascular parenchyma cells
Sieve elements
Companion cells
Bundle Sheath
Cell
TWO PATHWAYS OF PHLOEM LOADING
POLYMER-TRAPPING MODEL OF PHLOEM LOADING
TYPES OF PHLOEM UNLOADING PATHWAYS