Translocation

Source and sink

A source is any part of the plant that produces or releases food faster than it uses it, most commonly a mature, photosynthesising leaf, or a storage organ releasing its reserves. A sink is any part that consumes or stores food, such as growing roots, buds, flowers, fruits or developing seeds.

Which organ acts as a source or sink can change with the plant's stage of growth; for example, a storage root can be a sink while it fills up and later act as a source when its reserves are used.

How translocation happens

Sugar made in the source is actively loaded into the sieve tubes of the phloem, which lowers the water potential inside the sieve tube. Water then moves in from the nearby xylem by osmosis, raising the pressure at the source end.

At the sink end, sugar is removed and used, raising the water potential there, so water follows it out. This pressure difference between the source and sink ends pushes the sugar solution through the phloem, a mechanism known as mass flow.

Translocation compared with transport in xylem

How it is examined

You may be asked to identify which parts of a plant are acting as source and sink in a given scenario, to explain the mass flow mechanism, or to interpret experimental evidence, such as ringing (removing a strip of bark and phloem) causing sugar to accumulate above the ring.

Worked exam-style question

Question. A ringing experiment is carried out on a tree: a complete ring of bark and phloem is removed from around the trunk, leaving the xylem intact, and the tree is left standing in the field. After several weeks, the bark above the ring is observed to swell.

(a) Explain why removing the ring of phloem does not immediately kill the tree. (b) Explain why the bark above the ring swells.

(c) Predict what would eventually happen to the roots if the ring is not repaired.

Model answer. (a) The xylem, which transports water and mineral salts, is left intact, so the tree can continue to photosynthesise and its leaves do not immediately wilt. (b) Sugar produced by photosynthesis above the ring can no longer move down through the phloem (which has been removed), so it accumulates above the ring, causing the tissue there to swell.

(c) The roots, which act as a sink and rely on the phloem for a sugar supply, would eventually starve of food, weaken, and could die, since they cannot photosynthesise for themselves and no longer receive sugar from the shoot.

Practice question

Try this. During fruit ripening, a developing fruit imports large amounts of sugar from the leaves. (a) State whether the developing fruit is acting as a source or a sink at this stage.

(b) Explain, using the mass flow hypothesis, how sugar moves from a mature leaf to the fruit. (c) State what would happen to the water potential inside the sieve tubes near the fruit as sugar is unloaded there.

Exam tip

Key terms

• Translocation, the transport of food substances, mainly sugars, through the phloem to all parts of the plant.
• Phloem, the living plant tissue, made of sieve tubes and companion cells, through which translocation occurs.
• Transpiration stream, the continuous movement of water from the roots through the xylem to the leaves, distinct from the movement of sugar in translocation.
• Transpiration, the loss of water vapour from a plant, mainly through the stomata, which drives the transpiration stream but not translocation.

Source:SRC-DSKP-EN

Frequently asked questions

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