Cohesion-tension theory

Water moves through the stem to the leaves in continuous columns inside xylem vessels. These vessels are hollow and have lignified walls. The cohesion-tension theory explains how water loss from leaves creates a pull that draws these columns upwards.

How transpiration creates tension

Water evaporates from the surfaces of mesophyll cells inside a leaf, and water vapour leaves through the stomata. This water loss is part of transpiration. It lowers the water potential in the leaf, so water is drawn from the xylem to replace the water lost.

As water leaves the xylem in the leaf, it creates tension, or negative pressure, in the water column. This tension pulls on the water below it.

Cohesion and adhesion in the water column

Water molecules form hydrogen bonds with one another. This attraction, called cohesion, helps the molecules remain together, so the tension pulls the continuous column upwards rather than simply separating the molecules.

Water also attracts the xylem vessel walls. This attraction, called adhesion, helps maintain the water column within the vessels.

Because the water column is continuous, the pull is transmitted down the plant and draws water into the xylem from the roots. Water movement is therefore driven by transpiration at the leaves, not by the xylem vessels pumping water upwards.

Mass flow hypothesis

Phloem transports organic substances, especially sucrose, from places where they are supplied to places where they are used or stored. This transport is called translocation. A supplying region is a source; a region that uses or stores the substances is a sink.

The mass flow hypothesis explains translocation through phloem sieve tubes as the bulk flow of sap from higher to lower hydrostatic pressure. Hydrostatic pressure is the pressure exerted by the fluid within the sieve tubes.

How the pressure gradient develops

At a source, sucrose is loaded into the sieve tubes. This lowers the water potential of the sap, so water enters the phloem from the xylem by osmosis. The added water raises the hydrostatic pressure in the sieve tubes.

At a sink, sucrose is unloaded from the sieve tubes for use or storage. This raises the water potential of the sap. Water can then leave the phloem, lowering the hydrostatic pressure at the sink.

The resulting pressure difference pushes phloem sap through the sieve tubes from source to sink. The sucrose and other substances dissolved in the sap are carried with it as a bulk flow. Loading and unloading can require metabolic energy, but the bulk-flow step itself is driven by the pressure gradient.

Evidence for and against mass flow

Higher pressure near sources than near sinks is consistent with the pressure gradient predicted by the hypothesis. Labelled organic substances have also been observed moving through phloem, showing that these substances are transported along that pathway. These observations support mass flow, but they do not by themselves establish every detail of its mechanism.

Some observations require careful interpretation. Substances may appear to move at different rates, whereas substances carried together in the same bulk flow would be expected to move with the sap. Also, metabolic inhibition can affect transport. This does not necessarily mean that bulk flow requires metabolic energy: inhibition may instead disrupt the loading or unloading that helps maintain the pressure gradient.

Tracer experiments in plant transport

A tracer is a detectable label incorporated into a substance so that the substance’s movement can be followed. In a plant transport experiment, a radioactive isotope can be used to label an organic substance. Researchers then examine plant tissues to find where the label appears.

One detection method is autoradiography. Radiation from the labelled material produces an image showing where the radioactive label is present in a tissue. By examining material at different times, researchers can build up a picture of where the label moves and which tissues are associated with its movement.

Tracers and translocation

Translocation is the transport of organic substances through the phloem. A source is a region that supplies these substances, while a sink is a region that uses or stores them. If a label introduced at a source is later detected in the phloem and then towards a sink, its distribution is evidence consistent with translocation through the phloem.

The timing matters: detecting the label at a source and later in tissues towards a sink supports an interpretation of movement between those locations. Detecting it only in a sink would not, by itself, establish the route it took.

Interpreting tracer evidence

A tracer shows where the label is detected. It does not automatically establish that the originally labelled substance remains unchanged throughout the experiment: the label may become part of another substance. Interpretation therefore depends on what was labelled, where the label was detected and when each observation was made.

A pattern associated with the phloem is a correlation between the label and that tissue. Its strength as evidence for phloem transport depends on whether the observations distinguish that route from other possible explanations. The pattern alone also does not prove what physically drives the movement.

Tracer results can be assessed alongside other evidence when evaluating the mass flow hypothesis, which proposes that a pressure difference drives the bulk flow of phloem contents from source to sink. A source-to-sink pattern is consistent with that hypothesis, but it does not by itself demonstrate the proposed driving mechanism.

Ringing experiments in plant transport

A ringing experiment tests what happens when a transport pathway in a plant stem is interrupted. A ring of bark containing phloem is removed from around the stem, while the xylem is left largely intact. Phloem carries organic substances, such as sugars, through the plant.

Organic substances moving down the phloem cannot pass normally across the removed ring. They can accumulate just above it, often causing the stem to swell there. Tissues below the ring may receive less of these substances.

The interruption of the phloem and the accumulation above it support the conclusion that phloem transports organic substances. Leaving the xylem largely intact helps distinguish the interrupted phloem pathway from the xylem pathway.

Interpreting ringing experiments

Ringing is an intervention: the experimenter changes the stem rather than merely observing an association. The result nevertheless needs careful interpretation. Removing the ring damages tissues and can have effects beyond interrupting phloem, so an observed change cannot automatically be attributed to a single cause.

Ringing provides evidence about the pathway taken by organic substances. It does not, by itself, prove that pressure-driven mass flow is the mechanism that moves them.