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Translocation
How the growing parts of the plant are
provided with sugar to synthesize new cells
Photosynthesis
Translocation
New growth
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.
Sources and sinks
Photosynthesis provides a
sugar source
Translocation
New growth is a
sugar sink
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.
Source
Multiple sources and sinks
Developing
apex
Sink
Source
Translocation
Source
Sink
Sink
Sink
Sink
Sink
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.
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.
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 plat’s stem indicates that photosynthate moves through
phloem sieve elements.
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.
Aphids
http://members.ozemail.com.au/~lblanco/Ap1.htm
Aphid stylet procedure
Collecting phloem exudate
Typical Phloem Sap Chemistry
Xylem and Phloem Sap Compositions from White Lupine (Lupinus albus)
Xylem Sap (mg/l)
Sucrose
Amino acids
Potassium
Sodium
Magnesium
Calcium
Iron
Manganese
Zinc
Copper
Nitrate
pH
*
700
90
60
27
17
1.8
0.6
0.4
Trace
10
6.3
http://forest.wisc.edu/forestry415/INDEXFRAMES.HTM
Phloem Sap (mg/l)
154,000
13,000
1,540
120
85
21
9.8
1.4
5.8
0.4
*
7.9
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.
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/e16/16h.htm#sucr
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.
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
Transfer
cell
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.
The same type of proton pump you
saw in the chloroplast. membrane
The pressure-flow process
Build-up of pressure at the
source and release of pressure at
the sink causes source-to-sink
flow.
Pressure flow schematic
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.
Film clip
Top
Translocation is through sieve tubes,
comprised of sieve-tube elements SE in
the diagram, (sieve cells in gymnosperms).
Phloem structure
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).
At a phloem transport
velocity of 90 cm/hour
a 0.5 cm long sieve
element reloads every
two seconds.
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.
Companion cell
Cell wall
Branched plasmodesmata
Sieve element
Longitudinal section between cells in the phloem including a
branched plasmodesma. (Echium rosulatum petiole)
Corn syrup
The evil sweetner!
Sugar beet
Sugar cane
The U.S. is the world’s 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.
An Exception in Sucrose Transport
Most people associate plant sugar with phloem and assume that
sugar maple sap comes from the phloem. Not so! Sugar here
comes from the wood!! In late summer and before it loses its
colorful leaves in the fall, the tree stores large quantities of starch
in
wood parenchyma in the rays.
When temperatures rise in late winter, the starch is broken down
and converted into sucrose, which is released into the wood
vessels. The high concentration of sugar in the vessels causes soil
water to be brought into the roots, building up pressure and
forcing the sugary sap upwards toward the unopened, dormant
buds.
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
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.
Sections you need to have read
32.5
Courses that deal with this topic
Botany 371/372 Plant physiology laboratory