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this file about cell, tissues, and organ in plant in anatomical aspect
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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
Fig. 10.2 Bark Phloem Secondary phloem Vascular cambium Growth ring 3 (current year) Xylem Ring 2 (one year older) Ring 3 (two years older)
Vascular bundle of clover Fig. 10.1 Phloem xylem
“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.
Sieve tube elements • Tubular cells with end wall pores • and lateral sieve areas • Membrane bound • Have some organelles • Have adjacent companion cells 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 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 itrequires
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.
As the leaf grows it increasingly provides for its own carbohydrate needs.
Mature leaf is largely a carbohydrate exporter (source)
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 Fig. 10.10
Sugars are moved from photosynthetic cells and actively (energy) loaded into companion cells. Fig. 10.14
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
Phloem loading uses a proton/sucrose symport. Fig. 10.16
The pressure-flow model (Münch, 1930s) 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
