Phloem Transportation in Plants (Translocation)

Phloem transportation is a crucial process in plants that correlated to the movement of sugars (primarily sucrose) produced by photosynthesis, from source to sink, this vital process is known as " Translocation ".

Before entering the process of translocation, we need to know some basic structure of Phloem tissues.

The phloem tissues are comprised by four different components, these are -

Sieve Elements : These are the staple conducting cells, which have some net like structure, called sieve plates that allows the flow of nutrients and water (Sap).
Companion Cells : These cells aids sieve elements by giving metabolic functions and facilitating the transportation of sap.
Phloem Parenchyma : These kind of cells are involved with storing and transporting of nutrients.
Phloem Fibers : Give structural support to the phloem tissue.

Source and Sink

A source is any structure in a plant that can be produced or released sugars for the growing areas (it can be developing leaves, blooming, fruits or seeds) or storing areas ( like storing in roots, tubers and bulbs).

A sink is any structure in plant that can use or store food for growing, developing, differentiating etc.

Notice that a storage area can be either a sink or a source, it relies on the seasons or the plant’s stage of development.

Mechanism of Translocation

There are several theories for explaining the mechanism of phloem transportation in plants, but the Pressure Flow Hypothesis is widely accepted theory which interprets how sucrose moves from source (high concentration areas of solute) to sink (low concentration areas of solute) through pressure gradients and osmosis.

Pressure Flow Hypothesis : Sugars which are produced during photosynthesis or stored in any parts of the plants are actively transported (by using ATP) into sieve elements . When the sugars enters to the sieve elements, it's creates a high concentration of sugars. This high concentration of sugar at the source creates a low solute potential (Ψs), which allows water to enter into the phloem from the nearby xylem through osmosis. Entering of water into the phloem creates a high pressure potential (Ψp), which can lead to a high turgor pressure in the phloem tissue.

The high turgor pressure in phloem forces movement of phloem sap (water + nutrients) from source to sink by a process, called Bulk Flow. The sucrose (or sugars) moved through bulk flow are then briskly removed from the phloem at the sink.

Removal of the sugars at the sink then increases the solute potential (Ψs), which makes water to leave the phloem and return to the xylem ( which then declines pressure potential (Ψp) at the sink.

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