THE MECHANISM OF PHLOEM TRANSLOCATION:

Pressure-Flow Theory:

The theory called, Pressure-Flow Theory, is the most acceptable theory for the transport in the phloem of angiosperms.

Evidence to Support this Theory:

We have considerable evidence to support this theory.

Two Categories:

There were two main categories of theories to account for movement of sap in phloem.

(i)   Active Theories:

The Active theories involving the use of energy for the movement of materials in phloem.

(ii)   Passive Theories:

The passive theories in which no use of energy was involved. The active theories have all been abandoned (alone, with out support) as there is not much evidence to support these theories.

Now we are left with Passive theories of transport/translation.

These Include:

(i) Diffusion

(ii) Pressure flow theory

(i)  Diffusion:

Diffusion is far too slow, to account for the velocities of sugar movement in phloem, which on the average is 1 meter per hour, while the rate of diffusion is 1 meter per eight years.

So we are left with pressure flow theory.

Pressure Flow Theory:

A hypothesis was first proposed by Ernst Munch in 1930.

It states that the flow of solution in the sieve elements is driven by an osmotically generated pressure gradient between source and sink.

Now this hypothesis has been given status of a theory.

See figure, the following steps, explain pressure flow theory.

(i) The glucose formed in the photosynthesizing cells, is used within the cell (for respiration etc) and the rest is converted in to non-reducing sugar i.e., sucrose.

(ii) This source is actively transported through the bundle sheath cells to the companion cell of the smallest vein in leaf a short distance transport (involving 2-3 cells).

Thus sucrose diffuses through plasmodesmata to sieve tube cell or sieve element, raising the concentration of sucrose in it.

The pathway taken by sucrose is symplast in most cases; but in some, apoplastic movement does take place.

The sucrose is actively transported to the sieve elements.

(iii) The water moves by osmosis from the nearby xylem in the leaf vein.

This increase the hydrostatic pressure of the sieve tube element.

(iv) Hydrostatic pressure moves the sucrose and other substances in the sieve tube cells, and moves to sinks e.g. fruits.

In the storage sinks, such as sugar beet root and sugarcane stem, sucrose is removed into apoplast prior to entering symplast of the sink.

(v) Water moves out of the sieve tube cell by osmosis, lowering the hydrostatic pressure.

In symplastic, pathway sucrose (or sugars) moves through plasmodesmata to the receiver cell.

Thus according to pressure flow theory, the pressure gradient is established as a consequence of entry of sugars in the sieve elements at the source; and removal of sugars (sucrose) at the sink

The energy driven energy of sugars in sieve tube elements, generate high osmotic pressure in the sieve tube elements of the source causing a steep drop in the water potential.

(vi) The pressure of sieve plants greatly increase the resistance along the pathway and results in the generation and maintenance of a substantial (considerable) pressure gradient in the sieve elements between source and sink.

The sieve element's contents are physically pushed along the transportation pathway by bulk flow, much like water flowing through a garden hose.

The pressure flow theory accounts for the mass flow of molecules within phloem.

It may be noted that the photosynthate or carbohydrates from the mesophyll cells to phloem tissue involves diffusion and active transport (carrier mediated transport).

Then in phloem tissue (sieve tubes) the movement of materials is according to pressure flow theory.

Again in the sink cells when the sugar and the carbohydrates are passed from the phloem tissue, diffusion and carrier mediated transport, either passive or active, takes place.