Sugar Transport in Plants: Phloem, Source–Sink Relationships, and Storage

Xylem vs. Phloem: A Quick Comparison

Plants use two long-distance transport systems. You have already learned how water moves through xylem; here we focus on how sugars and other organic molecules move through phloem.

How to read “source → sink”

Unlike xylem, phloem does not have a single permanent direction. A leaf can export sugars to roots in one season, and later export to developing fruits or storage organs. The direction depends on where sugars are being produced versus where they are being used or stored.

Sources and Sinks: Where Sugars Come From and Where They Go

Sources (net exporters)

A source is a tissue that produces more sugar than it needs and therefore exports it. The most common sources are mature, fully photosynthesizing leaves. Other tissues can become sources at certain times, such as storage organs when they are being mobilized (e.g., a potato tuber feeding new shoots).

• Mature leaves: export sucrose made from photosynthesis.
• Mobilized storage tissues: export sugars when reserves are being broken down (early spring growth is a classic case).

Sinks (net importers)

A sink is a tissue that imports sugars because it is growing, storing, or consuming them faster than it can produce them. Common sinks include:

• Roots: growth, maintenance, and storage (especially in root crops).
• Fruits (e.g., apples): sugar import supports growth and later sweetness.
• Seeds: filling with starch, oils, and proteins during development.
• Growing shoots and young leaves: building new tissues before they can photosynthesize strongly.
• Bulbs (e.g., onion, tulip): storage leaves that import sugars to build reserves.
• Tubers (e.g., potato): underground stems that store carbohydrates.

Concrete examples of shifting source–sink roles

• Potato plant: During tuber formation, mature leaves are sources and tubers are strong sinks (starch storage). At sprouting time, the tuber can become a source, exporting sugars to the growing shoot.
• Apple tree: Leaves are sources; developing apples are sinks. As fruit ripens, it often becomes an even stronger sink for sugars and water, depending on variety and conditions.
• Bulb plants: After flowering, leaves often act as sources that refill the bulb (bulb is a sink). Before leaves expand in spring, the bulb can act as a source feeding the shoot.

Phloem Structure: Why Sieve Tubes Need Companion Cells

Phloem transport happens through sieve tube elements, which align end-to-end to form sieve tubes. Their end walls have sieve plates with pores that allow sap to flow. Sieve tube elements are living cells but have reduced internal structures (for example, they lack a nucleus at maturity), which makes them efficient conduits.

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Companion cells sit next to sieve tube elements and handle much of the metabolic work, including many steps of sugar loading and maintaining the sieve tube’s function. Think of the sieve tube as the pipeline and the companion cell as the control-and-power unit.

What Exactly Is Moving in Phloem Sap?

The main transported sugar is usually sucrose because it is soluble and relatively stable for transport. Depending on the species and situation, phloem sap can also carry:

• Amino acids (nitrogen transport in organic form)
• Hormones (growth and developmental signals)
• Small RNAs and proteins (signaling and regulation)
• Mineral nutrients in smaller amounts

Phloem sap is often described as “sugar-rich,” but it is better to think of it as a nutrient-and-information stream connecting sources and sinks.

Pressure-Flow (Mass Flow) Concept: A Qualitative Model

The most widely used explanation for long-distance sugar transport is the pressure-flow concept. You do not need equations to use it effectively; focus on the cause-and-effect chain.

Core idea

Loading sugars at a source increases solute concentration in phloem, which draws in water, raising pressure. Unloading sugars at a sink lowers solute concentration, water leaves, lowering pressure. Sap flows from high pressure to low pressure.

Step-by-step: from a mature leaf (source) to a fruit (sink)

1. Sucrose is produced in the leaf and moved toward the phloem in leaf veins.
2. Phloem loading: sucrose is transported into sieve tube elements (often via companion cells). This increases solute concentration in the phloem near the source.
3. Water enters the phloem by osmosis from nearby xylem/vascular tissues, because the phloem now has lower water potential.
4. Turgor pressure rises in the source region of the phloem (imagine inflating a water-filled hose).
5. Bulk flow occurs: phloem sap moves through sieve tubes toward regions with lower pressure.
6. Phloem unloading at the sink: sucrose leaves the phloem into sink cells (for growth, respiration, or storage as starch).
7. Water exits the phloem (often back toward xylem), decreasing pressure at the sink end.
8. Pressure difference is maintained as long as sources keep loading and sinks keep unloading/using sugars.

Two important clarifications

• Bulk flow is passive once the pressure gradient exists. The plant typically spends energy on loading/unloading (and on maintaining companion cell function), not on “pumping” sap along the tube.
• Different sieve tubes can carry different directions at the same time. In one stem, some phloem pathways may be exporting to roots while others export to fruits, depending on vascular connections and local source–sink demands.

Storage: Turning Transported Sugars into Reserves

Many sinks do not just burn sugars immediately; they convert them into storage forms that are less osmotically active and more compact.

• Potato tubers: import sucrose and convert much of it into starch stored in amyloplasts.
• Bulbs: store carbohydrates in fleshy leaf bases; forms vary by species (often fructans or starch).
• Seeds: convert imported sugars into starch, oils, and storage proteins.
• Fruits (e.g., apples): store sugars in vacuoles; sweetness depends on import rate, metabolism, and dilution by water.

Storage strength matters for transport: a sink that rapidly converts sucrose into starch (or uses it for growth) keeps sucrose concentration low in sink cells, which helps maintain unloading and continued flow.

Scenario-Based Activity: Predicting Changes in Sugar Transport

Use the pressure-flow model plus source–sink thinking. For each scenario, follow this practical prediction routine:

1) Identify current sources (net exporters). 2) Identify current sinks (net importers). 3) Decide which sinks are strongest (fast growth, active storage, ripening fruit). 4) Predict loading/unloading changes. 5) Predict direction and relative amount of phloem flow. 

Scenario A: Fruit ripening on an apple tree

Situation: Apples are enlarging and then ripening while leaves are still photosynthesizing.

• Step 1–2 (sources/sinks): Mature leaves are sources; fruits are major sinks; roots and developing buds may also be sinks.
• Step 3 (sink strength): As fruits ripen, they often increase sugar accumulation (variety-dependent). If the fruit is actively converting and storing sugars, it remains a strong sink.
• Step 4 (loading/unloading): Continued loading in leaves maintains high pressure near leaves; active unloading into fruit maintains low pressure near fruit.
• Step 5 (flow prediction): Expect strong phloem flow toward fruits. If fruit demand rises (more unloading/storage), flow to fruits tends to increase relative to other sinks.

Check-your-thinking prompt: If a fruit is shaded and photosynthesizes less, does it become more dependent on imported sugars? How would that affect its sink strength compared with a sun-exposed fruit?

Scenario B: Pruning (removing a portion of shoots/leaves)

Situation: A gardener prunes a plant, removing some leaves and/or young shoots.

• Case 1: Removing mature leaves (sources)
  • Sources decrease → less sugar available for loading.
  • Pressure at source regions drops because less sucrose is loaded.
  • Prediction: Reduced phloem flow overall; remaining sinks (roots, fruits) may receive less sugar, potentially slowing growth or storage.
• Case 2: Removing young shoots (sinks)
  • Sink demand decreases because a strong growing sink is removed.
  • Prediction: More sugar may be redirected to other sinks (e.g., fruits, roots, storage organs). Remaining sinks may enlarge faster or store more, depending on the plant’s developmental stage.

Step-by-step mini-task: Pick one plant (tomato, apple, potato). Write down (1) what was removed (source or sink), (2) which organs remain as strongest sinks, and (3) where you predict sugars will be redirected over the next week.

Scenario C: Early spring growth before leaves fully expand

Situation: Buds break dormancy; shoots and flowers start growing, but leaves are not yet fully photosynthetic.

• Step 1–2 (sources/sinks): New shoots, flowers, and developing leaves are strong sinks. The main sources are often stored reserves (e.g., roots, stems, bulbs, tubers) that are being mobilized.
• Step 3 (sink strength): Rapidly growing tissues have high demand; they are dominant sinks.
• Step 4 (loading/unloading): Storage tissues load sugars into phloem (acting as sources). Growing buds unload sugars quickly (acting as sinks).
• Step 5 (flow prediction): Phloem flow is directed from storage organs toward developing shoots/buds. As leaves mature, the main source gradually shifts from storage tissues to the new mature leaves.

Apply it to bulbs: In a tulip, the bulb often starts as a source feeding the shoot. After flowering, leaves become sources that refill the bulb (bulb becomes a sink again).