Translocation in the phloem - Ch. 10. Moving around all that photosynthate (and other stuff) Translocation in the phloem - Ch. 10. What does it do? II. Pathways and conduits for translocation sieve elements sieve tube elements (angiosperms) sieve cells (gymnosperms) III. Source-sink relationships IV. The mechanism and water relations of phloem translocation.

The phloem is the vascular system for moving (translocating) sugars produced in photosynthesis (photosynthate) and other substances throughout the plant. 0.5M PP10T020.jpg

Fig. 10.2 Phloem Bark Secondary phloem Vascular cambium Growth ring 3 (current year) Xylem Ring 2 (one year older) Ring 3 (two years older) PP1002.jpg

Vascular bundle of clover Fig. 10.1 Phloem xylem PP1001.jpg

“Girdling” a woody plant causes swelling of stem above the point of damage, indicating a blockage of phloem transport. A classic experiment - girdling

More experimental evidence that phloem is the transport tissue for carbohydrates. Radioactive labeling with 14CO2 can trace movement of sugars in the phloem, and from source leaves to sinks throughout the plant.

Sampling the phloem for chemical analysis Aphids insert a feeding stylet into phloem and this can be used to collect phloem exudate for chemical analysis.

Figure 10.3 Sieve tube elements Tubular cells with end wall pores and lateral sieve areas Membrane bound Have some organelles Have adjacent companion cells PP10030.jpg Figure 10.3

Sieve element features living, membrane-bound cells (compare to tracheary elements of xylem) lack some structures and organelles in most living cells - no nuclei, vacuole, Golgi, ribosomes, microtubules, microfilaments associated with companion cells that have full set of structures and organelles have sieve areas or pores that interconnect adjacent sieve elements angiosperm s.e. are called sieve tube elements, while gymnosperms’ are called sieve cells (see Table 10.1 for differences).

Fig. 10.5 Cell wall between sieve elements Sieve plate pore PP1005.jpg Sieve plate pore Companion cell

In what direction does phloem transport substances throughout the plant? From an area of carbohydrate supply to an area of carbohydrate demand. Source ----> Sink

Source-sink relationships can explain the direction of phloem translocation within the plant. Source - produces more carbohydrates than required for its own needs Sink - produces less carbohydrates than it requires PP10080.jpg

Anatomical and developmental determinants of the direction of source-sink translocation. Proximity - sinks tend to be supplied by closer sources Vascular connections may cause distinct source-sink patterns that counter proximity 3. Source-sink relationships may shift during development

Young leaf is completely dependent on carbohydrates from other sources. It is a strong sink. PP1019a.jpg

increasingly provides for its own carbohydrate needs. As the leaf grows it increasingly provides for its own carbohydrate needs. PP1019b.jpg

PP1019c.jpg

a carbohydrate exporter (source) Mature leaf is largely a carbohydrate exporter (source) PP1019d.jpg

Phloem transport Velocities ≈ 1 m hour-1 , much faster than diffusion What is the mechanism of phloem transport? What causes flow?, What’s the source of energy?

The pressure-flow model (Münch, 1930s) Phloem solution moves along a gradient of pressure generated by a solute concentration difference between source and sink ends of the pathway PP10100.jpg Fig. 10.10

Sugars are moved from photosynthetic cells and actively (energy) loaded into companion cells. Fig. 10.14 PP10141.jpg

The concentrating of sugars in sieve cells drives the Sugars are moved from photosynthetic cells and actively (energy) loaded into companion & sieve cells. The concentrating of sugars in sieve cells drives the osmotic uptake of water. Fig. 10.14 PP10142.jpg

proton/sucrose symport. Fig. 10.16 Phloem loading uses a proton/sucrose symport. Fig. 10.16 PP10160.jpg

The pressure-flow model (Münch, 1930s) PP10100.jpg Fig. 10.10

The pressure-flow model of phloem translocation At source end of pathway Active transport of sugars into sieve cells Ys and Yw decrease Water flows into sieve cells and turgor increases At sink end of pathway Unloading (active transport again) of sugars Ys and Yw increase Water flows out of sieve cells and turgor decreases

Some key elements of phloem transport Flow is driven by a gradient of pressure, YP. Energy is required to establish the pressure gradient, but energy is not required by cells of the pathway itself. Flow is in direction of higher total water potential, (counter to direction water tends to flow passively).

Yw -1.1MPa -0.4MPa