2. Translocation

3. How the growing parts of the plant are provided with sugar to synthesize new cells Photosynthesis New growth Translocation The phloem is the tissue that translocates assimilates from mature leaves to growing or storage organs and roots. 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.

4. Sources and sinks 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 Direction of transport is determined by relative locations of areas of supply, and 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.

5. Multiple sources and sinks 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. Translocation Source Sink Developing apex BUT translocation of sucrose can be in any direction – depending on source and sink location and strength. Water flow in plants is almost always from roots to leaves. In contrast, in cultivated sugar beets roots are sinks during all phases of development. Sink

6. 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 14 CO 2 to a photosynthesizing leaf, or application of 14 C-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.

7. 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

8. Aphid stylet procedure

9. Collecting phloem exudate

10. Xylem and Phloem Sap Compositions from White Lupine (Lupinus albus) Xylem Sap (mg/l) Phloem Sap (mg/l) Sucrose * Amino acids 700 Potassium 90 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 154,000 13,000 1,540

11. 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

12. 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:

13. Pressure flow schematic Fig. 30.18 Velocity up to 100 cm/hour. The pressure-flow process SOURCE (e.g., mature leaf cells) WATER SINK (e.g., developing root cells) Bulk Flow Build-up of pressure at the source and release of pressure at the sink causes source-to-sink flow.  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. At the source phloem loading causes high solute concentrations.

14. Fig. 30.18a, p. 527 Section from a leaf Section from a stem Section from a root Upper epidermis Photosynthetic cell Sieve tube Companion cell Lower epidermis sieve tube Sieve tube Loading at source Unloading at sink Translocation

15. Diagram of loading Sucrose follows a combination of two routes:, apoplastic in solution outside the cells, symplastic through the cells. Transfer cell The same type of proton pump you saw in the chloroplast. membrane Sugar produced at a source must be loaded into sieve-tube members. 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. 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.

16. Film clip

17. Phloem structure Each sieve-tube member has a companion cell, CC, (albuminous cell in gymnosperms). Top At a phloem transport velocity of 90 cm/hour a 0.5 cm long sieve element reloads every two seconds. 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. 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.

18. Plasmodesmata seen in Transverse Section: They are not simple openings as they have a complex internal structure. Plasmodesmata

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

21. 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 The Evil Sweetener!