How do substances move in the phloem?

Translocation is the movement of organic solutes through plants.

These solutes are generally produced at photosynthetic sites ( source) and are moved to either growing or storage sites depending on the time of the year ( sink).

As growth or storage sites may exist anywhere in the plant simultaneously, flow must be able to be bi-directional within a phloem element.

The first evidence that phloem carried these solutes occurred back in the 1600's through the studies of Marcello Malphigi. When a ring of bark of striped off of a tree the tissue above became swollen, he correctly interpreted this to mean that the solute solution coming from the leaves above was trapped by the break in the bark. This phenomenon of stripping is called girdling and is used to kill trees. Unintentionally it also occurs when animals such as deer desperate for food in the winter eat bark and then kill off the trees.

In 1800s Theodore Hartig, a German botanist studied the same phenomenon. Through an anatomical analysis he studied the cells in the inner bark which appeared to carry the sap and called them sieve tube members.

Physiologists in this century used aphids to gain more insights on phloem flow. Aphids insert their proboscis into phloem when feeding. Turgor pressure forces sap through their digestive tract and out their rear ends, exuding as droplets of "honey dew". A number of ants utilize these 'pipe line' effect feeding off of the exudate of the ants.

Physiologists learned how to anestatize and chop off the insect using a mini guillotine leaving the head/proboscis to tap the flow for hours. Fro this they learned:

10-25% of the exudate were solutes - 90% which was sucrose and the remainder sugar alcohols ( sorbitol, mannitol) as well as hormones, viruses, alkaloids etc.-- less than 1% amino acids and N-compounds.

Flow rate could go as high as 2 m./hr. ( equivalent to 20 liters of sap /day; or 1 gm of sugar/cm2/20 seconds) but generally more likely a rate of 100 cm/ hr. Why is this rate important... this is a relatively rapid rate!

More recent studies included the use of radioisotope tracers. Labeled C¹⁴O₂ gas is given to the plants. Once incorporated into sugars, the flow of these same sugars can be followed through the plant from mesophyll to sink sites, and through the sieve tubes themselves.

Many different models have been proposed for the movement of sugars through the plant

1. Diffusion, although energetically inexpensive, is too slow

2. Cytoplasmic streaming or cyclosis in which the protoplasm moves through the cell would also transfer sugars, but again this transport would be to slow according to the rates indicated by the aphid analysis

3. Munich's "Pressure Flow" model (1927).

Munich's model best describes the movement in the phloem. His model suggests that there is a turgor-pressure gradient that drives the directional mass flow of the solutes and water through sieve tubes of the phloem.

Munich's model is testable: - This can be demonstrated with an osmometer permeable only to water filled with a high concentration of solutes in one arm as shown below. When the osmometer is put in distilled water, the water potential is less than that of surrounding water in the one arm, and water will enter by osmosis which the generates a turgor pressure, The solutes are carried by bulk flow when the water moves in. Eventually this process will stop when the pressure throughout equalizes.

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Using this analogy, phloem transport can be explained as follows:

1. Sucrose is loaded into sieve tubes by active transport. 2. Water potential decreases and water enters sieve tubes by osmosis, turgor pressure. 3. Turgor pressure pushes solutes to sink, were it is needed while water moving in and out of the sieve tubes. 4. Removal of sucrose at sink increases water potential causing water to move out of the sieve tube at the sink. 5. Solutes move to sink cells and water goes back to xylem.

In this model, there is both passive and active transport. Passive: Long distance. Osmosis, water movement into and pushing solutes down the tube. Active: Short distance movement, loading and unloading the sugar.

How does the Phloem load and unload?

Loading: Loading of sieve tubes from the cell walls requires energy which is derived indirectly by the proton gradient.

1. ATP and H+-carrier in the cell membrane are used to pump protons out of the sieve tube.

2. A proton gradient forms across the membrane with high H+ concentration on the outside of the membrane, K+ enters to keep charge balance.

3. Diffusion of proton back into the sieve tube, through ATP-ase, is coupled to a carrier and powers the transport of sugar into the sieve tube.

ATP is important in this process because: - It increases Phloem loading; - Sugars are pumped into sieve tube against a concentration gradient: the concentration of sucrose in the chlorenchyma cell is typically 10-50 mM, while that in a sieve tube of a minor vein of a leaf may be as high as 1M.

This process of phloem loading increases pH in the sieve tubes to 8 and decreases pH out to 5.5, that is why K+ is used to maintain a balance.

Unloading: Unloading may occur symplastically or apoplastically. It may vary in different species.