Phloem transport — What is the widely accepted mechanism explaining long-distance movement of sugars in the phloem of vascular plants?

Introduction / Context:
Products of photosynthesis (primarily sucrose) must be distributed from sources (mature leaves) to sinks (developing tissues, roots, storage organs). The pressure flow hypothesis, proposed by Münch, explains how this bulk movement occurs efficiently through the phloem sieve-tube system.

Given Data / Assumptions:

• Sucrose is loaded into sieve-tube elements at sources, often via active processes.
• Loading reduces water potential in phloem, causing water to enter osmotically from xylem.
• Sinks unload sucrose, raising water potential and allowing water exit.

Concept / Approach:
Osmotically generated pressure differences between source and sink regions drive a mass flow of phloem sap. This does not rely on transpiration pull (a xylem phenomenon), nor on cytoplasmic streaming over long distances. It is a bulk flow under pressure gradients along sieve tubes with companion cells maintaining loading/unloading.

Step-by-Step Solution:

Verification / Alternative check:
Experiments show turgor pressure differences along phloem and rapid movement rates consistent with bulk flow; girdling and aphid stylet studies support the hypothesis.

Why Other Options Are Wrong:

• Root pressure: insufficient and episodic; mainly affects xylem.
• Transpiration: drives xylem ascent, not phloem flow.
• Cytoplasmic streaming: local phenomenon, too slow for long-distance transport.
• Electro-osmosis: not the dominant mechanism in intact plants.

Common Pitfalls:
Confusing xylem and phloem drivers; transpiration pull is not responsible for sugar transport in phloem.

Final Answer:
Pressure flow (mass flow) hypothesis