What limits the suction power of plants?

Plants suck water from the soil by means of negative pressure. So far, it has been an unsolved mystery why the pressure value does not fall below about -100 bar. An interdisciplinary and international research group has now shown, using atomistic simulations and model calculations, that this is apparently due to the so-called lipid aggregates that are contained in the plant sap. The modeling shows that their presence leads to the formation of growing cavities in the water column if the negative pressure is too strong. The limited suction power affects how plants can get water from drying soil.

April 27, 2020
Cross-section of an expanding cavity in the centre of a lipid bilayer in a plant supply channel. The formation of such cavities prevents the maintenance of negative pressures in the plant sap that are stronger than about -100 bar.

Hydraulic systems use pressure differences for transmission of signals, force, and energy. Many machines work based on this principle - but there is also a prominent example of this in nature: the water absorption of plants. The suction power of the roots is based on the negative pressure in the plant supply channels, which is created by the evaporation of water from the cell walls of the leaves.

Experts call a plant’s hydraulic system the "xylem" - a cellular tissue traversed by tiny conduits in which water and minerals flow through the plant. The negative pressure in this network is typically somewhere between minus 5 and minus 50 bar. Desert plants reach the strongest negative pressures of around minus 80 bar. But there has been no conclusive explanation so far for why plants cannot go below the limit of around minus 100 bar. A higher suction power obviously would be an advantage for a plant, since it could pull water from dry soils more effectively and transport the liquid higher and thus become larger. Furthermore, basic physical reasons did not seem to argue against stronger negative pressures in plants.

An interdisciplinary research team of plant biologists and physicists from the Jožef Stefan Institute in Ljubljana, Slovenia, the Max Planck Institute of Colloids and Interfaces in Potsdam, the Free University of Berlin, the University of Ulm, the Technical University of Darmstadt (all in Germany), and the California State University, Fullerton, USA, provided an explanation: Using computer simulations, the scientists were able to show that apparently water-insoluble natural substances, so-called lipids, are responsible for the phenomenon in the plant sap. In the event of negative pressure, the lipids promote the formation of rapidly expanding cavities - experts call this cavitation. If cavities become too large, the water column tears off. This dramatically reduces the strength of the maximum tolerable negative pressure, from below minus 1000 bar in pure water to less than minus 100 bar in the plant sap. The researchers conclude that the value predicted by the models is in excellent agreement with the strongest negative pressure measured in plants.

In living organisms, lipids mainly serve as structural components in cell membranes, as energy stores, or as signaling molecules. It is known from recent biochemical studies that such lipids also occur in the plant vascular system - especially those lipids that form double layers in an aqueous solution. From a chemical point of view, this behavior is based on the two completely different ends of the lipids: one repels water molecules, the other attracts them. Experts speak of hydrophilic (water-attracting) and hydrophobic (water-repellent). The hydrophilic head group thus turns outwards towards the water, while the hydrophobic tail assembles with a similar end of another lipid. Such lipid pairs then sort of arrange themselves to form a so-called lipid bilayer in the manner of the cell membranes.

In their work, the researchers combined extensive atomistic computer simulations of molecular dynamics with model calculations on the formation rate of cavities. This allowed them to come to conclusions based on microscopic processes about behavior on biologically relevant length and time scales. "Due to the temperature-induced random movements of water molecules, tiny voids regularly form in the liquid," explains Philip Loche, a doctoral student at the Department of Physics at the Free University of Berlin. However, the cohesive forces of the water usually ensure that they close quickly. “In a way, the molecules in liquids stick together, unlike in a gas,” Loche says. For this reason, water columns withstand comparatively high pulling forces without separating. However, the lipid bilayers mean that cavities can be created much more easily, which can grow quickly instead of dissolving again. "Put simply, it is much easier to tear apart two lipid layers than a group of water molecules," explains Emanuel Schneck, professor at the Technical University of Darmstadt and until recently a researcher at the Max Planck Institute of Colloids and Interfaces in Potsdam.

The simulations revealed that due to the lipid aggregates, cavities form more frequently at negative pressures below minus 100 bar. With the typically prevailing negative pressure in plants of minus 5 to minus 50 bar, however, this hardly ever happens. The researchers also found that small, water-soluble components of the plant sap hardly favor the formation of cavities. Apparently, the pressure limit observed in the plant world is based on the accumulated lipids in the hydraulic system. "For the first time, our results provide a plausible explanation of why plants cannot maintain negative pressures of below minus 100 bar for long," says Schneck. Since plants cannot suck water from the soil unlimitedly, this limits the ability of plants to take up water from drying soil and ultimately where plants can survive and grow.

Matej Kanduč, physicist at the Jožef Stefan Institute in Ljubljana, Slovenia, notes that the results are also of interest in connection with climate change. "The greatest negative pressures in plants are found in areas where water is scarce," he reports. And due to climate change, soils are drying out in more and more regions of the world. "Water has to be drawn there from the soil against the greatest resistance," says Kanduč.

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