Translocation

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Translocation
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How the growing parts of the plant are provided
with sugar to synthesize new cells
A system of vascular tissue runs through all
higher plants. It evolved as a response to the
increase in the size of plants, which caused an
progressing separation of roots and leaves in
space. The phloem is the tissue that
translocates assimilates from mature leaves to
growing or storage organs and roots.
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Sources and sinks

• Direction of transport through phloem is
  determined by relative locations of areas of
  supply, sources and areas where utilization of
  photosynthate takes place, sinks.
• Source any transporting organ capable of
  mobilizing organic compounds or producing
  photosynthate in excess of its own needs, e.g.,
  mature leaf, storage organ during exporting phase
  of development.
• Sink non photosynthetic organs and organs
  that do not produce enough photoassimilate to
  meet their own requiements, e.g., roots, tubers,
  develpoping fruits, immature leaves.

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Multiple sources and sinks
The flow of water in plants is almost always from
roots to leaves. Translocation of sucrose can be
in any direction depending on source and sink
location and strength.
Source
Developing apex
Sink
Source
Translocation
Examples Beta maritima (wild beet) root is a
sink during the first growing season. In the
second season the root becomes a source, sugars
are mobilized and used to produce a new shoot.
In contrast, in cultivated sugar beets roots
are sinks during all phases of development.
Source
Sink
Sink
Sink
Sink
Sink
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Girdling experiments
Girdling a tree, i.e., removing a complete ring
of bark and cambium around a tree, has no
immediate effect on water transport, but sugar
accumulates above the girdle and tissue swells
while tissue below the girdle dies.
Girdling is sometimes used to enhance fruit
production.
Radio active tracer experiments
Application of 14CO2 to a photosynthesizing
leaf, or application of 14C-sucrose, then
visualization of the path of the radioactive
tracer through photographing cross sections of
the plats stem indicates that photosynthate
moves through phloem sieve elements.
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Aphids
A technique for analyzing phloem sap chemistry
is the use of aphid stylets. A feeding aphid is
anesthetized and its stylet severed The phloem
sap is under positive pressure and is collected.
http//members.ozemail.com.au/lblanco/Ap1.htm
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Aphid stylet procedure
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Collecting phloem exudate
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Typical Phloem Sap Chemistry
Xylem and Phloem Sap Compositions from White
Lupine (Lupinus albus)
Xylem Sap (mg/l)
Phloem Sap (mg/l) Sucrose

154,000 Amino acids 700

13,000 Potassium 90
1,540
Sodium 60

120 Magnesium 27
85
Calcium 17

21 Iron 1.8

9.8 Manganese 0.6

1.4 Zinc 0.4

5.8 Copper Trace

0.4 Nitrate 10

pH
6.3
7.9
http//forest.wisc.edu/forestry415/INDEXFRAMES.HTM
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Nasty things animals do to plants!
Aphids transmit plant viruses. In Circulative
transmisson the virus circulates in the body of
the insect. In Persistent transmission the aphid
retains the virus in its body for days or weeks
spreading it to many plants as it moves and
feeds. Winged aphids often develop as host
plants begin to deteriorate or when the aphid
population is overcrowded.
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Sucrose
The sugar that is most important in translocation
is sucrose
Sucrose is a disaccharide, i.e., made up of two
sugar molecules an additional synthesis
reaction is required after photosynthesis
Sucrose - is not a rigid structure, but mobile in
itself.
http//www.biologie.uni-hamburg.de/b-online/e16/16
h.htmsucr
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There are two parts to translocation
The physiological processes of loading sucrose
into the phloem at the source and unloading it at
the sink.
Control of pressure flow of the sap in the phloem
driven by osmosis.
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General diagram of translocation
Physiological process of loading sucrose into the
phloem
Pressure-flow Phloem and xylem are coupled in an
osmotic system that transports sucrose and
circulates water.
Physiological process of unloading sucrose from
the phloem into the sink
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Diagram of loading
Sugar produced at a source must be loaded into
sieve-tube members. Sucrose follows a
combination of two routes symplastic, though
the cells, and apoplastic , in solution
outside cells. Some plants have transfer
cells, modified companion cells with numerous
ingrowths of their walls that increase the cells'
surface area and enhance solute transfer between
apoplast and symplast. Physiological transport
accumulates sucrose in sieve-tube members to two
to three time the concentration in mesophyll
cells. Proton pumps power this transport by
using ATP to create a H gradient.
Transfer cell
The same type of proton pump you saw in the
chloroplast. membrane
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Pressure flow schematic
The pressure-flow process Build-up of pressure
at thesource and release of pressure at the sink
causes source-to-sink flow. At the source
phloem loading causes high solute concentrations.
y decreases, so water flows into the cells
increasing hydrostatic pressure. At the sink y
is lower outside the cell due to unloading of
sucrose. Osmotic loss of water releases
hydrostatic pressure.Xylem vessels recycle water
from the sink to the source.
Fig. 32.5B
Velocity up to 100 cm/hour.
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Film clip
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Phloem structure
Top
Translocation is through sieve tubes, comprised
of sieve-tube elements SE in the diagram, (sieve
cells in gymnosperms). The perforated end walls
of each member are called sieve plates, SP, that
are open when translocation occurs, see .
Each sieve-tube member has a companion cell,
CC, (albuminous cell in gymnosperms). While
both sieve tube elements and companion cells are
alive at maturity, only the companion cell has a
nucleus, and seems to control the metabolism and
functioning of the sieve-tube member.
At a phloem transport velocity of 90 cm/hour a
0.5 cm long sieve element reloads every two
seconds.
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Branched plasmodesmata
Companion cell
Cell wall
Sieve element
Longitudinal section between cells in the phloem
including a branched plasmodesma. (Echium
rosulatum petiole)  
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The evil sweetner!
Corn syrup
Sugar beet
Sugar cane
The U.S. is the worlds largest consumer of
natural sweeteners. We consume about 9.3 million
tons of refined sugar each year from sugar beet
and sugar cane, and about 12 million tons of corn
sweeteners. 100 lbs per person per year.
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An Exception in Sucrose Transport
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Storage ray
Storage ray
Sliced vertically but off-center, i.e., in
tangential section, the rays, which run from the
phloem through the xylem towards the center of
the tree, are seen in cross (transverse) section
in wood of sugar maple (Acer saccharum).
Photomicrograph T. A. Dickinson
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Tapping the spring flow of sugar maple
Many large-scale producers have thousands of
taps, some up to 20,000
Spiles are inserted into the tree gently by hand
and then "seated" with a mallet or hammer.
Tubing networks should be laid out so that sap
flows directly to the sugar house or a storage
tank.
http//www.dnr.cornell.edu/ext/maple/Maple20Tour/
maple_tour.htm
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Sections you need to have read
32.5
Courses that deal with this topic
Botany 371/372 Plant physiology laboratory