1. Translocation

2. 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.

3. Photosynthesis provides a
sugar source
New growth is a
sugar sink
Translocation
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.

4. 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.
Multiple sources and sinks
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

5. Girdling experiments Radio active tracer 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.

6. 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

7. Aphid stylet procedure

8. Collecting phloem exudate

9. Typical Phloem Sap Chemistry
Xylem and Phloem Sap Compositions from White Lupine (Lupinus albus)
Xylem Sap (mg/l) Phloem Sap (mg/l)
Sucrose * ,000
Amino acids ,000
Potassium ,540
Sodium
Magnesium
Calcium
Iron
Manganese
Zinc
Copper Trace
Nitrate * pH

10. 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.

11. 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.

12. 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.

13. 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

14. The same type of proton pump you saw in the chloroplast. membrane
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
Diagram of loading
The same type of proton pump you saw in the chloroplast. membrane

15. Pressure flow schematic
The pressure-flow process
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.
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.
Pressure flow schematic
Fig. 32.5B
Velocity up to 100 cm/hour.

16. Film clip

17. 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.
Phloem structure
At a phloem transport velocity of 90 cm/hour a 0.5 cm long sieve element reloads every two seconds.

18. Branched plasmodesmata
Companion cell
Cell wall
Branched plasmodesmata
Sieve element
Longitudinal section between cells in the phloem including a branched plasmodesma. (Echium rosulatum petiole)  

19. Corn syrup Sugar beet Sugar cane
The evil sweetner!
Corn syrup
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.

20. 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.

21. 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

22. 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.

23. Sections you need to have read
32.5
Courses that deal with this topic
Botany 371/372 Plant physiology laboratory