Lipids limit the suction power of plants

Models confirm observations

2020/04/27 by

Plants use negative pressure to draw water from the soil. Why the pressure value does not fall below minus 100 bar was previously an unsolved mystery. An interdisciplinary and international research group with the participation of the Technical University of Darmstadt now reports in the journal Proceedings of the National Academy of Sciences (PNAS) that apparently so-called lipid aggregates in the plant saps are responsible for the phenomenon. Simulations and model calculations show how the lipids lead to the formation of expanding cavities that cause the fluid column to break off when the negative pressures become too great.

Professor Emanuel Schneck.

Hydraulic systems use pressure differences to transmit signals, power and energy. Many machines work on the basis of this principle, and 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 results from the evaporation of water on the cell walls of the leaves.

Experts call the plant hydraulic system “xylem” – a cell tissue traversed by tiny conduits in which water and minerals flow through the plant. The negative pressure in this network is typically between minus 5 and minus 50 bar. The strongest negative pressures of about minus 80 bar are found in desert plants. However, there has not yet been any conclusive answer as to why the limit of roughly minus 100 bar is typically not exceeded. After all, there generally seemed to be no physical reasons preventing stronger negative pressures. And greater suction power would be advantageous for the plant, as it could draw water more effectively from dry soils.

With negative pressure the water column breaks off

An interdisciplinary research team of botanists and physicists from the Jožef Stefan Institute in Ljubljana, the Max Planck Institute of Colloids and Interfaces in Potsdam, the Freie Universität Berlin, Ulm University, the Technical University of Darmstadt, and the California State University Fullerton, USA, has now come up with an explanation: Using simulations at the atomic level, the researchers were able to demonstrate that apparently water-insoluble natural substances, so-called lipids, in plant fluids are responsible for the phenomenon. Under negative pressure, they promote the formation of rapidly expanding cavities. If these become too large, the water column breaks off.

Cross-section of a growing 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.

This dramatically reduces the strength of the maximum tolerable negative pressures, from more than minus 1000 bar in pure water to less than minus 100 bar in the plant saps. The value that the models predict corresponds perfectly with the strongest negative pressures measured in botany, the researchers report.

In living organisms, lipids mainly serve as structural components in cell membranes, as energy stores or as signal molecules. Recent biochemical investigations have shown that such lipids also occur as a lipid bilayers in aqueous solutions in the plant vascular system.

When cavities are formed

In their work, the researchers combined extensive atomistic computer simulations of molecular dynamics with model calculations on the formation rate of cavities. This enabled them to draw conclusions about behaviour at biologically relevant length and time scales from microscopic processes.

Temperature-induced movements of water molecules create tiny cavities in the liquid, which normally close again quickly due to the cohesion forces of water. This is why water columns can withstand comparatively high tensile forces without breaking. However, the presence of the lipids now means that it is much easier for cavities to form that grow rapidly instead of disappearing again. “Put simply, it is much easier to break apart two lipid layers than a group of water molecules”, explains Emanuel Schneck, Professor of Physics of Biological Soft Matter at the Department of Physics 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 cavities form very frequently due to the lipid aggregates at negative pressures of more than minus 100 bar. However, with the negative pressures of minus 5 to minus 50 bar that are typically found in plants, this almost never happens. The researchers also found that small, water-soluble components of the plant sap have little effect on the formation of cavities. Apparently, the pressure limit observed in the plant world is indeed based on the aggregated lipids. “For the first time, our results provide a plausible explanation as to why negative pressures of more than minus 100 bar cannot be maintained for long by plants”, says Schneck.

Matej Kanduč, physicist at the Jožef Stefan Institute in Ljubljana, Slovenia and lead author of the study, notes that the results are also of interest in the context of climate change. “The greatest negative pressures in plants are found in areas where water is scarce”, he reports. And as a result of climate change, the soil in more and more regions of the world is drying out. “In these areas, water must be drawn from the ground against the greatest resistance”, says Kanduč.

The publication:

Kanduč, M. ; Schneck, E. ; Loche, P. ; Jansen, S. ; Schenk, H. J. ; Netz, R. R.: Cavitation in lipid bilayers poses strict negative pressure stability limit in biological liquids, Proceedings of the National Academy of Sciences of the United States of America