Phloem Translocation and Source-Sink Dynamics

Translocation is the transport of organic compounds, mainly sucrose, through the phloem from sources (where sugars are made or released, such as mature leaves) to sinks (where they are used or stored, such as roots, fruits and growing shoots), and it is explained by the pressure-flow hypothesis. In brief: sucrose is actively loaded into the sieve tubes at the source, which lowers the water potential, water enters by osmosis and raises the pressure, and the sap flows toward the sink, where the sucrose is unloaded and water leaves. Minerals absorbed by roots are carried in the xylem, but some mineral ions are redistributed in the phloem.

Translocation distributes the products of photosynthesis to all parts of the plant, and is the reason that the sink strength of fruits and grains affects crop yield. This guide defines sources and sinks, explains the pressure-flow mechanism and the evidence for it, describes phloem loading and unloading, covers the mineral nutrition of plants, works through a pressure and concentration example and corrects common mistakes.

What are sources and sinks?

A source is any organ that produces or releases more sugar than it uses: mature leaves, which photosynthesize, and storage organs that are releasing their reserves, such as a germinating seed, a tuber in spring or a tap root. A sink is any organ that uses or stores more sugar than it makes: growing roots, young leaves, flowers, developing fruits and seeds, and storage organs that are filling.

The roles can change during the life of the plant. A potato tuber is a sink in summer, when it fills with starch, and a source in spring, when it supports the growth of new shoots. Phloem flow therefore goes in both directions in the plant, but in any one sieve tube the sap moves in one direction. The “sink strength” of an organ, which depends on its size and activity, affects where most sugar goes. In crops, the sink strength of the grain is one of the factors that determines yield.

How are sugars loaded into the phloem?

At the source, sucrose made in mesophyll cells must move into the sieve tubes. It travels from cell to cell through plasmodesmata or through the cell walls (the apoplast), and then into the companion cells and sieve tube elements. In many plants, this step is active loading:

1. Proton pumps in the companion cell membrane use ATP to pump H⁺ out of the cell into the cell wall space.
2. The H⁺ ions flow back into the cell through a cotransporter protein, bringing sucrose with them against its concentration gradient.
3. The sucrose moves into the sieve tube element through plasmodesmata.

This explains why phloem transport depends on the metabolism of the plant: metabolic poisons that stop ATP production, or cooling, stop loading. It also explains why companion cells have many mitochondria. In some plants (especially trees), sugar moves through plasmodesmata by diffusion down a concentration gradient (passive symplastic loading).

How does the pressure-flow hypothesis work?

1. Loading at the source: sucrose concentration in the sieve tubes becomes very high, so their water potential falls.
2. Water enters: water moves from the neighboring xylem into the sieve tubes by osmosis. This raises the hydrostatic pressure (turgor) in the sieve tubes at the source end.
3. Mass flow: the pressure at the source is higher than the pressure at the sink, so the sap flows from high to low pressure along the sieve tubes, through the sieve plate pores, carrying the sugar. This is bulk flow, not diffusion, and it is much faster than diffusion.
4. Unloading at the sink: sucrose is removed from the sieve tubes into the sink cells, where it is converted into starch (which does not affect the osmotic balance) or used in respiration. The sieve tube water potential rises, water leaves by osmosis (some into the xylem), and the pressure falls.
5. Recycling: the water returns to the xylem and recirculates.

What is the evidence for translocation by pressure flow?

• Aphid stylets: an aphid inserts its mouthpart into a sieve tube. If the aphid is cut away, sap continues to flow out of the stylet, which shows that the sap is under pressure. The sap contains a high concentration of sucrose, and the rate of flow is highest near the source.
• Ringing experiments: removing a ring of bark (phloem) from a stem causes sugar to accumulate above the ring, which swells, and the roots below starve.
• Radioactive tracers: when a leaf is supplied with ¹⁴CO₂, radioactive sucrose can be traced in the phloem and moves to sinks at speeds of 30 to 100 cm per hour.
• Metabolic inhibitors: they stop loading and flow, showing that active processes are involved.

There are some problems with the hypothesis. It is not obvious how the sieve plates, which could create great resistance, allow flow, and the observed direction of flow within a single sieve tube is always one-way, as predicted. Most physiologists consider pressure flow the best explanation for the evidence.

Xylem transport and phloem transport compared

What is the role of mineral nutrition?

Plants need macronutrients in larger amounts (nitrogen, phosphorus, potassium, calcium, magnesium and sulfur) and micronutrients in trace amounts (iron, manganese, zinc, copper, boron, molybdenum and chlorine). They are absorbed from the soil by root hairs, by active transport and diffusion, and carried upward in the xylem. Some are important for specific roles.

• Nitrogen: for amino acids, proteins, nucleotides and chlorophyll. Deficiency causes yellow older leaves and stunted growth.
• Phosphorus: for ATP, DNA and phospholipids. Deficiency gives purplish leaves and poor root growth.
• Potassium: for enzyme activation, guard cell function and osmotic balance. Deficiency causes yellow leaf margins.
• Magnesium: a component of chlorophyll. Deficiency causes yellowing between the veins (chlorosis).

Mobile elements such as nitrogen, phosphorus, potassium and magnesium can be moved in the phloem from old to young leaves, so deficiency shows first in the older leaves. Immobile elements such as calcium and iron cannot be moved easily, so deficiency shows first in young leaves.

Worked example: pressure difference and speed

In a sieve tube, the sap at the source has a sucrose concentration of 0.8 mol/L, and at the sink, 0.3 mol/L. Estimate the osmotic pressure difference and the sap flow speed.

1. Osmotic pressure: use π = cRT, where c is the concentration in mol/m³, R = 8.31 J/(mol K) and T = 293 K (20 °C). For sucrose (does not dissociate): at the source, c = 800 mol/m³, π = 800 × 8.31 × 293 = 1.95 × 10⁶ Pa, or 1.95 MPa. At the sink, c = 300 mol/m³, π = 0.73 MPa.
2. Difference: 1.95 − 0.73 = 1.22 MPa. This is the approximate pressure difference available to drive the flow, if the xylem water potential is the same at both ends.
3. Speed: suppose the sap moves 20 m along the stem in 30 hours. The speed is 20 ÷ 30 = 0.67 m/h, within the observed range.
4. Mass transport: if the sieve tubes have a total cross-section of 5 mm² and the sap has a sucrose concentration of 0.6 mol/L (205 g/L), then at 0.67 m/h the flow is 5 mm² × 0.67 m/h = 5 × 10⁻⁶ m² × 0.67 m/h = 3.35 × 10⁻⁶ m³/h = 3.35 mL/h, carrying 3.35 × 0.205 ≈ 0.69 g of sucrose per hour.

The calculation shows that bulk flow can carry large amounts of sugar even in narrow tubes. Diffusion alone over 20 m would take years, because the time for diffusion grows with the square of the distance. For sucrose, with a diffusion coefficient of about 5 × 10⁻¹⁰ m²/s, the time is roughly L² ÷ D = 400 ÷ (5 × 10⁻¹⁰) = 8 × 10¹¹ s, or about 25 000 years.

How was translocation studied?

Marcello Malpighi did ringing experiments in the 1670s. Ernst Münch proposed the pressure-flow hypothesis in 1930. In the 1950s, Martin Zimmermann and others used aphid stylets to sample the contents of sieve tubes, which provided direct evidence. In the 1950s and 1960s, radioactive carbon allowed researchers to follow the movement of sugars. Modern studies use fluorescent dyes and genetic markers to follow transporters, such as the SWEET and SUT proteins for sucrose.

Which mistakes are common?

Mistake 1: Saying that translocation is by diffusion

It is a mass flow driven by a pressure difference. Diffusion would be far too slow.

Mistake 2: Thinking that sources are only leaves and sinks only roots

Sources and sinks are defined by whether the organ makes or uses sugar. They can change with the season.

Mistake 3: Describing phloem loading as passive in all cases

In many plants, it is an active process that needs ATP.

Mistake 4: Mixing up xylem and phloem transport

Xylem is pulled by transpiration in dead tubes. Phloem is pushed by a pressure difference in living tubes.