PowerPoint Presentation

Translocation

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

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Photosynthesis

New growth

Translocation

A system of vascular tissueruns through all higherplants. It evolved as aresponse to the increase inthe size of plants, whichcaused an progressingseparation of roots andleaves in space. The phloemis the tissue thattranslocates assimilatesfrom mature leaves togrowing or storage organsand roots.

Sources and sinks

Direction of transport throughphloem is determined by relativelocations of areas of supply, sourcesand areas where utilization ofphotosynthate takes place, sinks.

Source: any transporting organcapable of mobilizing organiccompounds or producingphotosynthate in excess of its ownneeds, e.g., mature leaf, storageorgan during exporting phase ofdevelopment.

Sink: non photosynthetic organsand organs that do not produceenough photoassimilate to meet theirown requiements, e.g., roots, tubers,develpoping fruits, immature leaves.

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Photosynthesis provides a

sugar source

New growth is a

sugar sink

Translocation

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Multiple sources and sinks

Examples:

Beta maritima (wild beet) root isa sink during the first growingseason. In the second season theroot becomes a source, sugarsare mobilized and used toproduce a new shoot.

In contrast, in cultivated sugarbeets roots are sinks during allphases of development.

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Translocation

Source

Sink

Developingapex

The flow of water in plants isalmost always from roots toleaves.

Translocation of sucrose canbe in any direction –depending on source and sinklocation and strength.

Girdling experiments

Girdling a tree, i.e., removing a complete ring of bark andcambium around a tree, has no immediate effect on watertransport, but sugar accumulates above the girdle and tissueswells while tissue below the girdle dies.

Application of 14CO2 to a photosynthesizing leaf, orapplication of 14C-sucrose, then visualization of the path ofthe radioactive tracer through photographing cross sections ofthe plat’s stem indicates that photosynthate moves throughphloem 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 aphidstylets. A feeding aphid is anesthetized and its stylet severed Thephloem sap is under positive pressure and is collected.

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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 * 154,000

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

Typical Phloem Sap Chemistry

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Aphids transmit plant viruses. In Circulative transmisson thevirus circulates in the body of the insect. In Persistenttransmission the aphid retains the virus in its body for days orweeks spreading it to many plants as it moves and feeds.

Winged aphids often develop as host plants begin to deteriorateor when the aphid population is overcrowded.

Nasty thingsanimals do toplants!

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

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Sucrose - is not a rigidstructure, but mobilein itself.

http://www.biologie.uni-hamburg.de/b-online/e16/16h.htm#sucr

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

The physiological processes of loadingsucrose into the phloem at the sourceand unloading it at the sink.

There are two parts to translocation:

General diagram of translocation

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Physiological process ofloading sucrose into thephloem

Physiological processof unloading sucrosefrom the phloem intothe sink

Pressure-flow

Phloem and xylem arecoupled in an osmotic systemthat transports sucrose andcirculates water.

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Diagram of loading

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Sugar produced at a source must be loadedinto sieve-tube members.

Sucrose follows a combination of tworoutes: symplastic, though the cells, andapoplastic , in solution outsidecells. Some plants have transfer cells, modifiedcompanion cells with numerous ingrowthsof their walls that increase the cells' surfacearea and enhance solute transfer betweenapoplast and symplast. Physiological transport accumulatessucrose in sieve-tube members to two tothree time the concentration in mesophyllcells. Proton pumps power this transportby using ATP to create a H+ gradient.

Transfercell

The same type of proton pump yousaw in the chloroplast. membrane

Pressure flow schematic

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Fig. 32.5B

Velocity up to 100 cm/hour.

The pressure-flow process

Build-up of pressure at thesource and release of pressure atthe sink causes source-to-sinkflow.

At the source phloem loadingcauses high solute concentrations. decreases, so water flows intothe cells increasing hydrostaticpressure.

At the sink  is lower outside thecell due to unloading of sucrose. Osmotic loss of waterreleases hydrostatic pressure.Xylem vessels recycle water fromthe sink to the source.

Film clip

Phloem structure

Translocation is through sieve tubes,comprised of sieve-tube elements SE inthe diagram, (sieve cells in gymnosperms).

The perforated end walls of each memberare called sieve plates, SP, that are openwhen translocation occurs, see .

Each sieve-tube member has acompanion cell, CC, (albuminous cell ingymnosperms).

While both sieve tube elements andcompanion cells are alive at maturity,only the companion cell has a nucleus,and seems to control the metabolism andfunctioning of the sieve-tube member.

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Top

At a phloem transportvelocity of 90 cm/houra 0.5 cm long sieveelement reloads everytwo seconds.

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Branched plasmodesmata

Cell wall

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

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Sieve element

Companion cell

The evil sweetner!

The U.S. is the world’s largest consumer of natural sweeteners. We consume about9.3 million tons of refined sugar each year from sugar beet and sugar cane, andabout 12 million tons of corn sweeteners. ~100 lbs per person per year.

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Sugar cane

Corn syrup

Sugar beet

Most people associate plant sugar with phloem and assume thatsugar maple sap comes from the phloem. Not so! Sugar herecomes from the wood!! In late summer and before it loses itscolorful leaves in the fall, the tree stores large quantities of starchin wood parenchyma in the rays.

When temperatures rise in late winter, the starch is broken downand converted into sucrose, which is released into the woodvessels. The high concentration of sugar in the vessels causes soilwater to be brought into the roots, building up pressure andforcing the sugary sap upwards toward the unopened, dormantbuds.

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 xylemtowards the center of the tree, are seen in cross(transverse) section in wood of sugar maple (Acersaccharum).

Photomicrograph: T. A. Dickinson

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