Trees Look Serene, But Operate Under Very High Pressure

Xylem, phloem, turgor ... what the heck is going on in tall plants so they can drink from dizzying heights?

Atmospheric pressure is 76 cm of mercury, or 10.3 meters of water, or 0.1 MPa. That means that even a vacuum can't pull water up higher than 10 meters from the earth's surface. Yet Redwoods grow to 115 meters. How does that happen? There are two problems- where does the pulling force come from, and even with the requisite force, how can anything conduct water to far greater heights than a vacuum on bulk water would allow?

The answer to the second issue is capillary action. Plants have a circulatory system somewhat analogous to ours, with two parts going in opposite directions connected by tiny selective membranes. But there is no heart- no pump. Fluid is moved around by high pressure differentials, driven by osmotic differentials, strong cellular architecture, and active molecular transport at the sub-microscopic level.

One part (phloem, maybe analogous to arteries) is for the sugars produced by the leaves, called sap. The second is xylem, (maybe analogous to veins), conducting water and minerals up from the roots. Xylem is a dead, porous tissue inside the cambium. Using tiny, hydrophilic vessels allows water tension to "hold it all together", while the negative pressure at the top of a plant pulls continuously upwards. How much pressure is needed at the top of a plant? Well, about 19 atmospheres of negative pressure for a hundred-meter tall tree, or ~2MPa. (Compared to this, our blood pressure is negligible, at 0.015 MPa.) The xylem is carefully constructed to prevent bubbles from forming, and from spreading if they do form, helping the natural cohesion of water (which causes the phenomena of capillary action and surface tension) to keep the water column intact under such intense pressures.

Xylem, full of small conducting pores, and carefully isolated within the plant to sustain high negative pressures. The pores  also stand ready to break the spread of air, if any is introduced.

Just as interesting, however, is the phloem system, which generates high positive pressures and is integral to the whole circulatory regime. Phloem is living tissue- just outside the cambium (in trees) and specialized to conduct sugar fluid to sites that need it, like fruits and roots, all over the plant. The xylem and phloem systems are selectively interconnected. The key "capillary" point in the leaves where xylem fluid is drawn towards the phloem fluid, due to the osmotic pressure from a low-solute (xylem) to a high solute (sugary phloem) fluid, forms the pressure differential that makes the whole system work, by what is called the Münch hypothesis.

It has taken decades to test this hypothesis, since it was proposed almost a century ago, for technical reasons- the second you cut into a plant to look at its phloem, the pressure drops in that region. A recent paper (review) used fluorescence tracers and microneedle probes as new methods to observe pressures and flow rates in living, whole plants- the morning glory vine.

Phloem, while largely empty space, has a complex internal surface, including walls (and companion cells) which contain the selective ionic barriers that keep sugars in while letting water and minerals from the xylem fluid enter, at least in locations like the leaves where that is useful. They also have open pores (sieve plate pores) between successive cells (called sieve tubes) that allow flow, but can quickly restrict it in case of injury. Sieve tubes even have their own type of plastid, whose function is entirely unknown.

Phloem sieve tube plates, aligned with a plant and locations from which they were sampled. The lower plates are more tilted to allow more flow. Their pores are also larger, though that is not so apparent here.

The morning glory vine was used for obvious reasons. It is highly accessible, easy to grow, grows to great heights, is easily manipulated, and has a thin stem that is easily dissected. In its stem, it has phloem both inside and outside of a xylem zone, which reinforces the idea that the xylem needs to be carefully protected from the atmosphere.

A stem section taken at four meters height. Phloem- dashed arrows, xylem- yellow and green arrows.

The researchers calculated the sap viscosity, its flow rate, and the phloem volume and cell structural characteristics, to come up with the pressures and other parameters required to achieve it. They also used micro-pressure guage needles and fluorescent dyes in the sap to attempt direct measurements on the phloem channels. Results where that the sap had a sugar concentration of 18%, and about 0.2MPa are needed per meter to drive sap flow. Over a seven meter plant, which was what they were studying at first, this amounts to about +1.4 MPa overall at the leaves, to drive fluid movement to the roots, which was indeed observed overall.

Next, they grew the plants to 17 meters high, removed the lower leaves to simplify the analysis, and measured again. The phloem pressure at the top was +2.2 MPa, which is high, but not enough by their prior analysis to get the fluid all the way to the roots. So they took a second look at the anatomy, and found that the plant had instituted structural changes to ease sap flow. The lower you are on the plant, the larger the pores between successive phloem cells, the larger and more tilted the pore plates themselves, and the higher the sap conductivity, up to six-fold. They conclude that the phloem system is pressure-driven, with the pressure carefully raised, along with other physiological parameters, to adapt to longer lengths of transport.

This leads to a unified picture of plant fluid transport, where very high pressures, both positive and negative, and clever anatomy, allow the transport of both key fluids- the water from the roots, and the precious photosythesized food from the leaves. Osmotic pressure is key at both ends, since not only does the high sugar content of the phloem sap drive root water towards it and supply its pressurization, but in the roots, a modest level of sugar and/or other concentrated ions drives water from the surrounding soils into the root, the pressure of which varies in different plants.

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