What is the principle of mass flow or pressure flow system of food translocation in plants?

Pressure Flow Hypothesis: Principle

The mass flow (or pressure flow) hypothesis, proposed by Ernst Münch (1930), explains how organic solutes (primarily sucrose) are translocated from source to sink through phloem tissue.

Core Principle:

Organic solutes move en masse (in bulk) through phloem sieve tubes, driven by an osmotically generated pressure gradient between source (high turgor) and sink (low turgor) regions.

Fundamental Physics:

• Movement: From region of high osmotic pressure → low osmotic pressure
• Driving Force:Turgor pressure gradient (ΔΨ_p)
• Flow Type:Bulk flow (not diffusion), carrying water and dissolved solutes together

Mechanism—Step by Step:

SOURCE (Photosynthetic Leaf)

↓

[Sugar Loading]

↓

High Turgor Pressure

↓

[Phloem Sieve Tube]

↓

Pressure-Driven Flow

↓

Low Turgor Pressure

↓

[Sugar Unloading]

↓

SINK (Root, Fruit, Seed)

Detailed Process:

At the SOURCE (e.g., mature leaf):

1. Sugar Production:
   • Photosynthesis produces glucose
   • Glucose → Sucrose (transport sugar)
2. Phloem Loading (Active Transport):
   • Sucrose actively pumped into companion cells (requires ATP)
   • Moves through plasmodesmata into sieve tube elements
   • Mechanism: Proton-sucrose cotransporter (symport)
3. Osmotic Water Entry:
   • High sucrose concentration → low water potential (Ψ_w decreases)
   • Water enters from adjacent xylem by osmosis
   • Turgor pressure increases (Ψ_p ↑)

In the PHLOEM:

1. Pressure-Driven Flow:
   • High Ψ_p at source pushes sap toward lower Ψ_p at sink
   • Bulk flow through sieve tube pores
   • Flow rate: 1 m/hour (much faster than diffusion)

At the SINK (e.g., growing root, fruit):

1. Phloem Unloading (Active Transport):
   • Sucrose actively transported out of sieve tubes into sink cells
   • Used for respiration or converted to starch (storage)
2. Osmotic Water Exit:
   • Sucrose removal → osmotic pressure decreases
   • Water moves out into xylem
   • Turgor pressure decreases (Ψ_p ↓)

Continuous Cycle:

• Water returns to xylem, recirculates to leaves
• Maintains pressure gradient as long as source produces and sink consumes sugars

Evidence Supporting the Hypothesis:

1. Girdling experiments: Removing phloem stops translocation; xylem removal doesn't
2. Radioactive tracer studies: ¹⁴C-labeled sucrose moves from source to sink
3. Pressure measurements: Phloem sap is under positive pressure (exudes when cut)
4. Bidirectional flow: Different sieve tubes can transport in opposite directions simultaneously

Critical Requirements:

• Living sieve tubes: Metabolic energy needed for loading/unloading
• Source-sink gradient: Active source and sink necessary
• Functional sieve pores: Blocked pores stop translocation

Biological Significance: This mechanism allows plants to efficiently distribute photosynthetic products to non-photosynthetic tissues, supporting growth, storage, and reproduction.