You have probably never tried drinking a glass of water from ground level while standing on the roof of your house with an extremely long straw. But if by any chance you have, you’ll have learned one thing: it doesn’t work. “After ten meters at the very most, the suction power won’t be enough to maintain the hydrostatic head – the column of water – and it will collapse,” explains Roland Netz, a professor of biophysics at Freie Universität Berlin. So how do trees manage to transport the water they need from the ground right up to their highest branches without it getting lost on the way? North American redwoods, for example, can grow up to 110 meters tall.

Trees do this by using what physicists call “negative pressure.” The unit used to measure this type of pressure is the bar. Normal pressure equals 1 bar, and pressure falls by one bar for every ten meters of height. This means that a 110 meter tree would have to use a pressure of minus 10 bars to pump water up to its crown. But research has shown that a tree of this height can achieve suction power of up to minus 20 bars. Meanwhile, trees in very dry regions have to employ much greater negative pressure. Junipers use minus 60 bars of pressure, while the creosote bush, a desert shrub, achieves an astonishing minus 80 bars.

The Straw Effect

But minus 80 is probably the maximum level of negative pressure that a plant can bring to bear. At first glance, this seems strange. You would think that minus 100, or even minus 150, would be ideal for plants that grow in the desert – or indeed for any plant in the age of climate change. But this question is related to a problem that has puzzled scientists for a long time. Vascular plants – plants which are capable of conducting water via a special kind of tissue known as xylem – are seemingly exempt from the “straw effect,” where an air bubble inevitably forms after the liquid has reached a certain height, forcing it to shoot back down again. Why is this?

Two years ago, precisely this problem was preoccupying biologists Jochen Schenk (California State University) and Steven Jansen (Ulm University). To find an answer, they approached Roland Netz and his colleagues Matej Kanduc and Emanuel Schneck in the Freie Universität physics department. The physicists were able to model the behavior of the different components of water using high-performance computer clusters. This way, the team as a whole could figure out what happens when you place 100,000 water molecules in a cuboid vacuum chamber and subject them to rising negative pressure of up to minus 100 bars.

Based on the hydrogen bonds between the water molecules, the computer calculated how the molecules move when subject to increasing negative pressure, creating a timeline of these movements within a given period. “The result is a bit like a movie in miniature,” says Roland Netz. “It shows us nanosecond changes in the molecular structures in the vacuum chamber.” Philip Loche, a doctoral student supervised by Netz who co-authored the publication, explains further: “The water molecules move at random depending on temperature, and this regularly creates tiny voids in the liquid. But because water exerts a strong cohesive force, these voids close up again almost immediately.”

This explains why water columns are able to resist relatively high gravitational forces. The tiny voids – technically known as cavitations – described by the researchers would need an unimaginably long time to expand to the point where they would be capable of forcing pure or salt-containing water molecules apart from each other. Speaking mathematically, the number of seconds it would take is 10 to the power of 2,000, a period of time longer than the universe has existed. The likelihood that this will happen is therefore pretty small. It doesn’t become more likely even when negative pressure reaches minus 100 bar, as the computer simulation shows.

Nonetheless, plants do occasionally suffer from embolisms, when air bubbles in the water column block their systems. In these cases, cavitations interrupt the flow of the water as it is being conducted through the plant, similarly to how a blood clot can block a vein in humans. This can lead to the water column collapsing. Probably this is caused by lipids in the plant sap. When such lipids are dissolved in water, they spontaneously build lipid bilayers, similarly to how they behave in cell membranes. The long “fatty acid tails,” which are not soluble in water, push inward while the water-soluble polar groups protrude outward.

Further simulations by the team showed that when negative pressure is applied to water containing lipids, cavitations form much faster. In this scenario it takes only days, or even hours, for the cavitations to form, while the lipid bilayers separate at more or less exactly minus 80 bars. “A simple way to describe it is that it is much easier to separate two lipid layers than to separate a group of water molecules,” says Emanuel Schneck. It is therefore the lipids in their sap that prevent plants from exerting stronger negative pressure.

The research team believes that the interior “walls” of xylem fibers are lined with lipids that make them incredibly smooth, a theory which they now want to put to the test. “We think that this lipid layer helps to prevent roughening of the xylem walls and thus prevents fissures, where nanometer-sized cavitations could form and potentially get bigger,” says Roland Netz. This explains why straws and pipes are not capable of transporting liquids for long distances via suction. Their interior surfaces are not perfectly smooth, and this allows tiny air bubbles to form, which can quickly become large bubbles under negative pressure.

The immense negative pressure found in xylem actually comes about because of an invisible process that takes place in the green leaves or needles of the plant. The leaves and needles have tiny pores, stomata, which open to let in carbon dioxide from the air – a necessary part of photosynthesis. Each time the pores open, a small amount of water evaporates from the plant. This has to be replaced by drawing up water from the roots via xylem. In the end, what happens at leaf level affects the entire tree.

 “You can hear the water column in the tree collapsing”

Plants are most likely to suffer from embolisms in very hot summers, when air humidity is at its lowest. Despite the lack of water, they still have to open their stomata, otherwise photosynthesis would stop. “If you hold a microphone to a tree over a long period of time, you can actually hear the water column inside the tree collapsing,” says Roland Netz. The moment of collapse creates vibrations as the wooden walls of the xylem snap together, causing a “click” that can be heard via ultrasound. One-off embolisms generally do no damage to fully-grown trees, but if a drought continues for a long time and more water evaporates from the leaves than can be replaced from the roots, branches or even entire trees can die.

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This text originally appeared in German on June 21, 2020, in the Tagesspiegel newspaper supplement published by Freie Universität.