Moving around all that photosynthate (and other stuff)

Translocation in the phloem - Ch. 10.

I. What does it do?

II. Pathways and conduits for translocationsieve 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

PhloemSecondary phloem

Vascular cambium

Growth ring 3 (current year) Xylem

Ring 2 (one year older)

Ring 3 (two years older)

Fig. 10.2

Bark

Vascular bundle of clover

Phloem

xylem

Fig. 10.1

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

Radioactivelabeling with14CO2 can tracemovement of sugars in the phloem, and fromsource leaves to sinks throughoutthe plant.

More experimental evidence that phloem is the transporttissue for carbohydrates.

Aphids insert a feeding stylet intophloem and this canbe used to collectphloem exudate for chemical analysis.

Sampling the phloem for chemical analysis

Figure 10.3

Sieve tube elements• Tubular cells with end wall poresand lateral sieve areas• Membrane bound• Have some organelles• Have adjacent companion cells

Sieve element features•living, membrane-bound cells (compare to tracheary

elements of xylem)

•lack some structures and organelles in most livingcells - no nuclei, vacuole, Golgi, ribosomes, microtubules, microfilaments

•associated with companion cells that have fullset 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).

Sieve plate pore

Cell wall betweensieve elements

Companion cell

Fig. 10.5

In what direction does phloem transportsubstances throughout the plant?

From an area of carbohydrate supply to anarea of carbohydrate demand.

Source ----> Sink

Source-sink relationships can explain the direction ofphloem translocation within the plant.Source - produces more carbohydrates than required for its own needsSink - produces less carbohydrates than it requires

Anatomical and developmental determinants of the direction of source-sink translocation.

1. Proximity - sinks tend to be supplied by closer sources

2. Vascular connections may cause distinct source-sinkpatterns that counter proximity

3. Source-sink relationships may shift during development

Young leaf is completely dependenton carbohydrates fromother sources. It isa strong sink.

As the leaf grows itincreasingly providesfor its owncarbohydrate needs.

Mature leaf is largely a carbohydrate exporter (source)

Phloem transportVelocities ≈ 1 m hour-1 , much faster than diffusion

What is the mechanism of phloem transport?What causes flow?, What’s the source of energy?

Fig. 10.10

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

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

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

Fig. 10.10

The pressure-flow model (Münch, 1930s)

The pressure-flow model of phloem translocation

At source end of pathway• Active transport of sugars into sieve cells• s and w decrease• Water flows into sieve cells and turgor increases

At sink end of pathway• Unloading (active transport again) of sugars• s and w increase• Water flows out of sieve cells and turgor decreases

Flow is driven by a gradient of pressure, P.

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).

Some key elements of phloem transport

-1.1MPa

-0.4MPa

w