Transcript Sieve element Companion cell

Translocation
How the growing parts of the plant are
provided with sugar to synthesize new cells
Photosynthesis
Translocation
New growth
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
Direction of transport is
determined by relative locations
of areas of supply, and sources
and areas where utilization of
photosynthate takes place, sinks.
Source:
Translocation
New growth is a
sugar sink
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
Water flow in plants is almost
always from roots to leaves.
BUT translocation of sucrose
can be in any direction –
depending on source and sink
location and strength.
Examples:
In Beta maritima (wild beet) the
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
Sucrose
Sucrose is the most important sugar in translocation
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.
The pressure-flow process
SOURCE
(e.g., mature
leaf cells)
Pressure flow schematic
WATER
Build-up of pressure at the
source 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.
Bulk
Flow
SINK
(e.g., developing
root cells)
Fig. 30.18
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.
Velocity up to 100 cm/hour.
Loading at source
Translocation
sieve
tube
Upper epidermis
Photosynthetic cell
Sieve tube
Companion cell
Section from a stem
Lower epidermis
Sieve tube
Section from a leaf
Section from a root
Unloading at sink
Fig. 30.18a, p. 527
Transfer
cell
Diagram of loading
Sugar produced at a source must be
loaded into sieve-tube members.
Sucrose follows a combination of two
routes:, apoplastic in solution outside the
cells, symplastic through the 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
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.
Plasmodesmata
Plasmodesmata seen in Transverse Section:
They are not simple openings as they have a
complex internal structure.
Plasmodesmata
Plasmodesmata
Companion cell
Cell wall
Branched plasmodesmata
Sieve element
Longitudinal section between cells in the phloem including a
branched plasmodesma. (Echium rosulatum petiole)
The Evil Sweetener!
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.