Translocation

How the growing parts of the plant are provided with sugar to synthesize new cells

Photosynthesis

New growth

Translocation

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 sinksDirection 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.

Photosynthesis provides a

sugar source

New growth is a

sugar sink

Translocation

Multiple sources and sinks

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.

Translocation

Source

Source

Source

Sink

Sink

Sink

SinkSink

Sink

Developing apex

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.

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.

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.

Radio active tracer experiments

Girdling is sometimes used to enhance fruit production.

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

Aphid stylet procedure

Collecting phloem exudate

Xylem and Phloem Sap Compositions from White Lupine (Lupinus albus) Xylem Sap (mg/l) Phloem Sap (mg/l) Sucrose * 154,000Amino acids 700 13,000 Potassium 90 1,540 Sodium 60 120Magnesium 27 85 Calcium 17 21Iron 1.8 9.8Manganese 0.6 1.4 Zinc 0.4 5.8Copper Trace 0.4Nitrate 10 *pH 6.3 7.9http://forest.wisc.edu/forestry415/INDEXFRAMES.HTM

Typical Phloem Sap Chemistry

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.

Nasty things animals do to

plants!

SucroseThe 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

Control of pressure flow of the sap in the phloem driven by osmosis.

The physiological processes of loading sucrose into the phloem at the source and unloading it at the sink.

There are two parts to translocation:

General diagram of translocationPhysiological process of loading sucrose into the phloem

Physiological process of unloading sucrose from the phloem into the sink

Pressure-flowPhloem and xylem are coupled in an osmotic system that transports sucrose and circulates water.

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

Pressure flow schematic

Fig. 32.5B Velocity up to 100 cm/hour.

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.

decreases, so water flows into the cells increasing hydrostatic pressure.

At the sink 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.

Film clip

Phloem structure

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.

Top

At a phloem transport velocity of 90 cm/hour a 0.5 cm long sieve element reloads every two seconds.

Branched plasmodesmata

Cell wall

Longitudinal section between cells in the phloem including a branched plasmodesma. (Echium rosulatum petiole)  

Sieve element

Companion cell

The evil sweetner!

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.

Sugar cane

Corn syrup

Sugar beet

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.

An Exception in Sucrose Transport

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

Storage ray

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.

Tapping the spring flow of sugar maple

http://www.dnr.cornell.edu/ext/maple/Maple%20Tour/maple_tour.htm

Sections you need to have read

32.5

Courses that deal with this topic

Botany 371/372 Plant physiology laboratory