Translocation of sucrose by mass flow under pressure
Diagrammatic representation of mass flow in the phloem
Mass flow is from source to sink, but sink could be above source
Movement in the phloem is mass flow of water and dissolved solutes generated by (positive) hydrostatic pressure
, and there is a pressure gradient from source to sinks. There are effectively three regions, the loading zone, the (fairly long) transport zone and the unloading zone at the other end.
The source is usually the leaves
which are photosynthesising, but it may be an organ of perennation, such as a tuber, mobilising stored reserves at the beginning of the growing season. The sinks are any areas of growing tissue
where cells are dividing, so they could be the tips of roots or shoots, flowers or fruits, which may be above or below the leaves. So transport in phloem is (potentially) bidirectional
The product of photosynthesis is glucose, but it is converted to sucrose
. In some plants this is converted into other oligosaccharides such as raffinose, stachyose, and verbascose
- all of which consist of sucrose with extra galactose. Sucrose is actively transported
, in conjunction with hydrogen ions (H+
), into the phloem
by a co-transporter system operating in the companion cells
. A separate ATP-powered H+
pump establishes a concentration gradient to power this loading process.
The presence of the sucrose lowers the water potential
in the sieve tube element, and water flows in by osmosis from surrounding tissue and indirectly from the nearby xylem.
This extra volume produces higher pressure
in the liquid, causing it to flow towards the sink.
It is probably worth mentioning that water is resistant to compression forces, so it transmits this force without losing volume. The molecules are already close together, unlike gases.
Of course there is usually more than one sink, and so the liquid can flow both upwards and downwards from the source.
It is said that during transport some organic substances are lost from the phloem to surrounding cells which use it for their own purposes, e.g. respiration and secondary growth. Conceivably companion cells along the length of the transport section work on retrieving sucrose that has 'leaked out' and replacing it in the phloem.
At its destination, cells remove the sucrose from the phloem and absorb it, for respiration or storage. The sucrose is said to be unloaded
In storage organs it is usually converted into other, less soluble carbohydrates, reducing osmotic effects and maintaining a concentration gradient which assists in unloading sucrose from the phloem.
Heat and chemical treatments
Movement of fluid in the phloem may be slowed down by cooling or stopped by extreme (higher) heat treatment. This presumably affects active transport in the companion cells or the cytoplasm in the sieve tube elements.
Active transport is affected by respiratory inhibitors. Sodium azide and cyanide cause great reduction in the tranlocation of sucrose.
The compound PCMBS (p-chloromercuriphenylsulphonic acid) acts as an inhibitor to sucrose transporters so it practically prevents the loading of sucrose into the phloem. This movement may take place via the companion and other cells (symplastic pathway) or through the interstitial spaces/cell walls (apoplastic pathway).
Typical contents of phloem
/ mg cm-3
Non-reducing sugars / ∽ 80-120
(There is very little reducing sugar - glucose etc - transported)
Amino acids / ∽ 5
Proteins / ∽ 2
Organic acids / ∽ 3
Inorganic ions / 5
This shows that phloem also transports other organic compounds than sucrose, as well as inorganic ions (recycled from the leaves, after removal of nitrates etc for production of proteins etc).
A classic ringing experiment
Removing a ring of bark from a tree takes away the phloem, which forms a ring on the outside of the trunk.
Some time later, swelling can be seen just above the cut because of the accumulation of organic solutes that came from higher parts of the tree. These cause increased growth of cells just above the removed section. This sort of swelling may also be seen if a loop of wire is left around a tree and its growth causes the wire to cut into the bark.
Beneath this, supply of sugars are stopped due to the disruption of the phloem. Roots die from lack of 'food' from above, and the whole plant also dies..
Single leaf labelling with CO2 radioisotope to trace translocation of photosynthate
Radiocarbon labelling experiment
From Koning, Ross E. 1994. Translocation. Plant Physiology Information Website. http://plantphys.info/plant_physiology/translocation.shtml. (6-13-2020).
This diagram shows how the products of photosynthesis are translocated in the plant.
A mature leaf from the plant (numbered 14 on the diagram, seen from above) was supplied with carbon dioxide containing radioactive carbon (14
The red shading indicates how much radioactivity is found in the other leaves of the plant, a week later.
What can you conclude from this observation?
CO2 is converted ('fixed') into organic compounds (sugars)
that are moved up the plant
to younger leaves that are immediately above
but not on the other side
Suggest what you would see if you measured the radioactivity in the roots of the plant by spreading them out over photographic film for some time (then developing it).
This is called an autoradiogram
Film would be exposed where radioactive isotope moves to
Roots would show radioactivity (on the same side as the leaves)
but not on the other side