Translocation in phloem

Plants move sucrose and amino acids from parts that release them to parts that need or store them. This movement through the phloem is called translocation.

A source is a part of the plant that releases sucrose or amino acids into the phloem. A sink is a part that receives these substances from the phloem for use or storage. For example, a leaf can be a source of sucrose, while a growing shoot or a storage root can be a sink.

Sources and sinks can change

Source and sink describe what a plant part is doing at a particular time, not what it is permanently. A storage root acts as a sink while it receives and stores substances. Later, it can become a source when it releases stored substances into the phloem for transport to other parts.

As the plant’s growth and storage needs change, the parts acting as sources and sinks can change too.