What Is Translocation in Plants?

Translocation is the movement of organic substances, mainly sucrose, through a plant’s phloem from a source to a sink. A source supplies sucrose, such as a photosynthesising leaf. A sink uses or stores it, such as a growing root or a developing fruit.

How does it work?

The mass flow hypothesis explains how sucrose moves through the phloem:

1. At a source, sucrose is loaded into phloem sieve tubes. Companion cells help with this process, which uses energy from ATP.
2. The sucrose lowers the water potential inside the sieve tube, so water enters from the nearby xylem by osmosis. This raises the pressure in the phloem.
3. At a sink, sucrose is removed for use or storage. Water potential rises, and water can move back towards the xylem. Pressure is therefore lower at the sink.
4. The pressure difference drives the phloem contents from source to sink as a mass flow.

A sieve tube is a chain of living cells that forms a transport pathway. Water potential describes the tendency of water to move: water moves by osmosis towards a lower water potential.

For example, sucrose made in a leaf can be transported to a growing root, where it is used in respiration or stored. A storage organ can later become a source when its reserves are mobilised. Translocation can therefore occur in different directions within a plant, depending on where the sources and sinks are.

Evidence for translocation comes from tracing labelled carbon in sucrose: it appears in phloem and later at sinks. In a ringing experiment, removing a ring of bark containing phloem interrupts transport through that point, while xylem remains intact.

Exam mistake to avoid: Do not confuse translocation with transpiration. Translocation carries organic substances through phloem; transpiration is the loss of water vapour from leaves, which helps draw water up the xylem.