Translocation is the movement of sugar (such as sucrose), produced during photosynthesis, to other areas of the plant. These sugars are used immediately for respiration or for storage.

  • Translocation occurs in the phloem, one of the two main types of transport tissue in the vascular bundle.

Transporting substances from sources to sinks

The ‘source‘ in a plant is the part where substances, such as sugars, are produced or enter. Whereas the ‘sink’ is the area of growth or storage, so it is the destination of translocation.

  • Companion cells provide energy for sieve cells

During translocation, substances typically move from the source to the sink. However, the organs that are considered the source change throughout the year and throughout the plant’s life.

Seasonal changes

During the summer:

  • The source is the leaves
  • The sink is the roots

In the summer, the source of sucrose is the mature leaves, which is where photosynthesis mostly occurs. During summer, there are longer daylight hours, so the leaves produce excess sugars. The excess sugars, in the form of sucrose, are transported to the roots, which act as the sink.

Sucrose is a disaccharide made of glucose and fructose, and plants move sugars in the form of sucrose and store them in the form of starch.

During the winter and spring:

  • The source is the roots
  • The sink is the leaves

With less sunlight available for photosynthesis in winter and spring, the roots, which stored sugars during the summer, now act as the source for translocation, transporting sucrose to the leaves, which serve as the sinks.

Growth periods

During a growth period, the storage organs (e.g. the roots) act as the source, while the areas where the plant is growing serve as the sinks.

Once the plant has grown, the photosynthesising leaves produce large amounts of sugars. This allows them to become the source. Therefore, the roots become the sinks and store sucrose as starch until it is needed.

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