Phloem Dynamics: The Osmotic Pressure Flow Model

The most widely accepted hypothesis explaining phloem transport is the Pressure Flow Mechanism, first proposed by Münch in 1927. This model describes how nutrients, primarily sucrose, are transported from the source (where they are produced) to the sink (where they are stored or used) within plants.

Source and Sink in Phloem Transport

Plants have two distinct regions involved in nutrient transport:

• Source: The area where sugar is produced, such as green leaves and stems.
• Sink: The area where sugar is stored or utilized, such as young leaves, fruits, seeds, and roots.

Mechanism of Phloem Transport

According to the Pressure Flow Mechanism, sugar in solution moves under pressure through the phloem. This process involves several key steps:

1. Sugar Production and Conversion

• Glucose production: Photosynthesis in mesophyll cells of green leaves produces glucose.
• Utilization and conversion: Some glucose is used for respiration, while the rest is converted into sucrose (a non-reducing sugar).

2. Active Transport of Sucrose

• Sucrose concentration in sieve tubes of leaves ranges between 10-30%, whereas in photosynthetic cells, it is only 0.5%.
• Sucrose is actively transported into the companion cells of the smallest veins in the leaf.
• It then diffuses through plasmodesmata into sieve tube elements, increasing sucrose concentration.

3. Water Uptake and Pressure Generation

• Water moves osmoscially from the nearby xylem into the sieve tubes, increasing hydrostatic pressure.
• The increased pressure propels sucrose and other substances through sieve tubes toward the sink.

4. Sugar Unloading at the Sink

• In storage sinks like sugar beet roots and sugarcane stems, sucrose is removed into the apoplast before entering the symplast of the sink.
• Water exits sieve tube cells by osmosis, lowering hydrostatic pressure.
• A pressure gradient forms due to sugar entry at the source and removal at the sink.

5. Role of Sieve Plates

• Sieve plates significantly increase resistance along the transport pathway.
• This resistance helps maintain a substantial pressure gradient between the source and the sink.

Bulk Flow and Sugar Utilization

• The contents of sieve elements move by bulk flow along the transport pathway.
• As sap moves down the phloem, sugar is removed by the cortex of both the stem and root.
• The removed sugar is either consumed or converted into starch, which is insoluble and does not create osmotic pressure.
• Consequently, the osmotic pressure in phloem decreases, leaving relatively pure water.
• This water is thought to either exit by osmosis or be reabsorbed into xylem due to the transpiration pull.

Key Factors in the Pressure Flow Mechanism

The process depends on:

1. Turgor Pressure
2. Osmotic Pressure Gradient between the source and the sink

Limitations of the Pressure Flow Mechanism

A major limitation of this model is its failure to explain bidirectional movement, where different substances move in opposite directions simultaneously.

Experiments on Bidirectional Movement

• Scientists tested bidirectional movement by applying two different substances at separate points in the phloem and tracking their movement.
• If the Pressure Flow Mechanism were the only process, bidirectional movement in a single sieve tube should not be possible.
• However, experimental results are mixed:
• Some studies suggest bidirectional movement does occur in a single sieve tube.
• Others indicate that movement happens in separate sieve tubes running parallel.

Final Thoughts

The Pressure Flow Mechanism remains the most accepted explanation for phloem transport. However, its inability to fully explain bidirectional movement highlights the need for further research. While the bulk flow hypothesis effectively describes mass flow in phloem, additional mechanisms may also be involved in plant nutrient transport.