Mass Flow in Phloem Sieve Tubes

Mass flow is the primary mechanism by which sugars (mainly sucrose) are transported in the phloem of plants. This process relies on a hydrostatic pressure gradient established between a source (where sugars are produced) and a sink (where sugars are utilized or stored).

Hydrostatic Pressure Gradient

The movement of phloem sap is driven by differences in hydrostatic pressure. Hydrostatic pressure is the pressure exerted by a fluid at a given depth. In the phloem, the pressure is higher at the source and lower at the sink. This pressure difference creates the driving force for mass flow.

Process of Mass Flow

1. Loading at the Source: Sugars (primarily sucrose) are actively loaded into the sieve tubes at the source. This process requires energy and is often mediated by companion cells. The loading of sugars increases the solute concentration within the sieve tubes.
2. Water Uptake: The increased solute concentration lowers the water potential within the sieve tubes. This causes water to move into the sieve tubes by osmosis from the surrounding xylem.
3. Pressure Build-up at the Source: The influx of water increases the turgor pressure within the sieve tubes at the source, leading to a higher hydrostatic pressure.
4. Flow Through the Phloem: The high pressure at the source drives the phloem sap through the sieve tubes towards the sink. The sap moves due to the pressure gradient.
5. Unloading at the Sink: At the sink, sugars are actively unloaded from the sieve tubes. This process also requires energy and is often mediated by companion cells. The unloading of sugars increases the water potential within the sieve tubes.
6. Water Removal: Water moves out of the sieve tubes by osmosis into the surrounding sink tissues. This reduces the turgor pressure within the sieve tubes at the sink, leading to a lower hydrostatic pressure.

Mathematical Representation

The hydrostatic pressure ($P$) in a fluid is given by the equation:

$$P = \rho g h$$

where:

• $\rho$ is the density of the fluid (phloem sap).
• $g$ is the acceleration due to gravity.
• $h$ is the height of the fluid column above a reference point.

In the context of mass flow, the pressure difference ($ \Delta P $) between the source and the sink is crucial for driving the movement of phloem sap.

Table Summarizing Mass Flow

Stage	Location	Process	Effect on Water Potential	Effect on Hydrostatic Pressure
Loading	Source	Active transport of sugars into sieve tubes	Decreases	Increases
Water Uptake	Source	Osmosis of water into sieve tubes	Increases	Increases
Flow	Phloem	Movement of phloem sap due to pressure gradient	N/A	N/A
Unloading	Sink	Active transport of sugars out of sieve tubes	Increases	Decreases
Water Removal	Sink	Osmosis of water out of sieve tubes	Decreases	Decreases