Phloem transports sap primarily through a mechanism called pressure flow, which involves the bulk flow of sap driven by positive pressure within specialized angiosperm sieve tubes.
The Mechanism: Bulk Flow Driven by Positive Pressure
Researchers have concluded that phloem sap travels through angiosperm sieve tubes via bulk flow driven by positive pressure, also known as pressure flow. This process is highly efficient for moving sugars and other solutes over long distances within the plant.
Here’s a breakdown of the key elements involved in this transport:
- Bulk Flow: This refers to the mass movement of a fluid (phloem sap) in response to a pressure gradient. Unlike the slower process of diffusion, which relies on random molecular motion, bulk flow ensures a rapid, directed movement of sap.
- Positive Pressure: This internal pressure, also known as turgor pressure, builds up within the sieve tubes. It acts as the driving force, pushing the sap from regions of high pressure to regions of lower pressure.
- Pressure Flow Hypothesis: This widely accepted model explains how plants create and maintain the necessary pressure gradient to facilitate the continuous flow of sap.
The Source-to-Sink Dynamic
The continuous movement of phloem sap is powered by the difference in pressure between "source" and "sink" regions within the plant. As highlighted by research, "Sap flows from source to sink due to pressure buildup and pressure reduction at the sink."
Let's understand these critical components:
- Source: A plant part where sugars (primarily sucrose) are produced in excess (e.g., through photosynthesis in mature leaves) or where stored sugars are mobilized (e.g., from storage roots or tubers).
- At the source, sugars are actively loaded into the sieve tubes. This process lowers the water potential inside the sieve tubes, causing water to move in from adjacent xylem vessels by osmosis. The influx of water increases the volume and, consequently, the hydrostatic (positive) pressure within the sieve tubes.
- Sink: A plant part where sugars are consumed for energy, growth, or storage. Examples include actively growing regions like young leaves, developing fruits, root tips, or storage organs accumulating carbohydrates.
- At the sink, sugars are actively unloaded from the sieve tubes into the surrounding cells. This increases the water potential inside the sieve tubes, causing water to move out by osmosis, typically back into the xylem. The outflow of water reduces the hydrostatic pressure within the sieve tubes.
| Feature | Source | Sink |
|---|---|---|
| Role | Sugar production or release | Sugar consumption, growth, or storage |
| Pressure | High positive pressure (due to water influx) | Lower positive pressure (due to water efflux) |
| Water Movement | Water enters sieve tubes via osmosis | Water exits sieve tubes via osmosis |
| Examples | Mature leaves, starch-releasing storage organs | Growing shoots/roots, fruits, developing seeds |
This dynamic interplay of pressure buildup at the source and pressure reduction at the sink creates a continuous pressure gradient that drives the bulk flow of phloem sap, ensuring that vital sugars and other organic compounds are efficiently distributed throughout the plant wherever they are needed for metabolic activities and development.
