Translocation

Translocation is the movement of solutes from source to sink. Photosynthesis: 6CO2 + 6H2O  C6H12O6 + 6O2 Glucose is a photosynthetic product (photosynthate)of the process which is converted to sucrose. The phloem is the tissue that translocates assimilates (sucrose) from mature leaves to growing or storage organs and roots. Translocation is the movement of solutes from source to sink. Photosynthesis New growth Translocation

Photosynthesis provides a sugar source New growth is a sugar sink Translocation Sources and sinks Direction of transport through phloem is determined by areas of supply i.e. sources and areas where utilization of photosynthate takes place i.e sinks. Source: any transporting organ capable of mobilizing organic compounds or producing photosynthate in excess of its own needs, e.g., mature leaf. Sink: non photosynthetic organs and organs that do not produce enough photo-assimilates to meet their own requirements, e.g., roots, tubers, developing fruits, immature leaves.

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

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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 w 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.

Mass flow Hypothesis Is the bulk of substances (sugars and amino acids) due to differences in pressure. It explains their movement from a high pressure region to a lower one through phloem sieve tubes.

Comparison of bulk Flow in Vascular Tissue

figure 35-15b.jpg Figure 35.15 – Part 2 Active Process

Sucrose Entering the Phloem Active process (requires energy) Companion cells use ATP to transport hydrogen ions out of their cytoplasm As hydrogen ions are now at a high concentration outside the companion cells, they are brought back in by diffusion through special co-transporter proteins, which also bring the sucrose in at the same time As the concentration of sucrose builds up inside the companion cells, they diffuse into the sieve tubes through the plasmodesmata (gaps between sieve tubes and companion cell walls)

Sucrose movement through phloem Sucrose entering sieve tube lowers the water potential (more negative) so water moves in by osmosis, increasing the hydrostatic pressure (fluid pushing against the walls) at the source Sucrose used by cells surrounding phloem and are moved by active transport or diffusion from the sieve tube to the cells. This increases water potential in the sieve tube (makes it less negative) so water moves out by osmosis which lowers the hydrostatic pressure at the sink

Movement along the phloem Water entering the phloem at the source, moving down the hydrostatic pressure gradient and leaving at the sink produces a flow of water along the phloem that carries sucrose and other assimilates. This is called mass flow. It can occur either up or down the plant at the same time in different phloem tubes

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

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.

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.

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

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.

Evidence for translocation summary Radioactively labelled carbon from carbon dioxide can appear in the phloem Ringing a tree (removing a ring of bark) results in sugars collecting above the ring An aphid feeding on the plant stem contains many sugars when dissected Companion cells have many mitochondria Translocation is stopped when a metabolic poison is added that inhibits ATP pH of companion cells is higher than that of surrounding cells Concentration of sucrose is higher at the source than the sink

Evidence against translocation Not all solutes move at the same rate Sucrose is moved to parts of the plant at the same rate, rather than going more quickly to places with low concentrations The role of sieve plates is unclear

The evil sweetner! ELABORATE 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.

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

Overall review and Quiz https://www.youtube.com/watch?v=MxwI63rQubU http://bcs.whfreeman.com/thelifewire/content/chp36/36020.html (Experiment and quiz)