Phloem Transport: Pressure Flow Hypothesis Explained

Phloem transport is the movement of sugars and other organic nutrients from leaves, where they are made, to growing or storage parts like roots, fruits, and seeds. The pressure flow hypothesis, proposed by Ernst Münch, explains this movement as bulk flow driven by a pressure gradient between source and sink.

Understanding phloem transport is important because it decides how food reaches the parts of the plant that cannot photosynthesize. Crop yield, fruit sweetness, and tuber size all depend on efficient transport from source leaves to sink organs.

Key Concepts

A source is any organ that produces or releases sugars, typically a mature photosynthesizing leaf or a storage organ during mobilization. A sink is any organ that consumes or stores sugars, such as roots, developing fruits, seeds, and young leaves. Sieve tube elements are living phloem cells joined end to end with sieve plates that allow bulk flow. Companion cells load and unload sugars using ATP. Turgor pressure is high at the source end where sugars are loaded, and low at the sink where sugars are removed, creating the gradient for flow.

How It Works

1. Companion cells actively load sucrose into sieve tubes at the source, lowering water potential there.
2. Water enters sieve tubes by osmosis from nearby xylem, raising turgor pressure at the source end.
3. At the sink, sucrose is unloaded actively or passively and used or stored, raising water potential.
4. Water leaves the phloem at the sink, lowering turgor pressure there.
5. The pressure difference pushes the sugar solution in bulk from source to sink, like water through a pipe, and xylem recycles the water.

Worked Example

Imagine phloem at a leaf source has solute potential Psi_s = -1.5 MPa because of high sucrose, and after water entry, pressure potential Psi_p = +1.2 MPa, so Psi_w = -0.3 MPa. At a root sink, sugars are removed, so Psi_s = -0.6 MPa and Psi_p = +0.3 MPa, giving Psi_w = -0.3 MPa as well at equilibrium for water, but the key difference is pressure: 1.2 MPa at source versus 0.3 MPa at sink. The gradient of 0.9 MPa drives bulk flow. Sap velocities measured in plants are about 0.5 to 1.5 metres per hour, much faster than diffusion could achieve over metres.

Common Misconceptions

• Students think phloem transport is always downward. It is bidirectional in the plant as a whole; each sieve tube flows from its own source to its sink, which can be upward to fruits or downward to roots.
• Many believe sugars move by diffusion. Diffusion is far too slow over long distances; pressure-driven bulk flow carries all solutes together at the same speed.
• Some think phloem sap is just sugar water. It also carries amino acids, hormones, RNAs, and signalling molecules essential for coordination.

Real-World Applications

Girdling experiments, where a ring of bark including phloem is removed, show swelling above the ring because sugars cannot move down, proving phloem's role. Farmers manage source-sink balance by pruning leaves or thinning fruits to increase fruit size. Virus and phytoplasma diseases spread through phloem and affect whole plants. Aphids feed on phloem sap, and studying their stylet exudate revealed phloem composition. Improving phloem loading is a target for increasing crop yields.

The pressure flow hypothesis therefore explains long-distance food transport as an osmotically generated pressure push from where sugar is loaded to where it is needed most.