Plants need water to grow. The supply of liquid is provided by the roots. How much water is absorbed is determined by a hydraulic system that works similar to machines. A negative pressure ensures that plants suck the water out of the soil. The suction power is based on the negative pressure in the plant supply channels, which is created by the evaporation of water on the cell walls of the leaves. But the pressure in this network is usually limited to minus 100 bar for plants. Until now it was unclear why this is so. An international research team of botanists and physicists is now providing an answer.

Lipids ensure the formation of cavities in the plant sap

„Due to the random movements of water molecules caused by temperature, tiny cavities are regularly formed in the liquid“, explains Philip Loche, doctoral student in the Department of Physics at Freie Universität Berlin and co-author of the study. Normally, the cohesion forces of the water ensure that the caves close again. Using atomistic simulations, the scientists were able to show that apparently water-insoluble natural substances, so-called lipids, prevent this.They are responsible for the formation of cavities, also called cavities, in the plant fluids.

Lipid aggregates responsible for pressure limit

As the team reports in the journal Proceedings of the National Academy of Sciences (PNAS), lipid aggregates cause the cavities to expand extremely quickly under negative pressure and the water column to break off. According to the study, this dramatically reduces the strength of the maximum tolerable pressures, from more than minus 1,000 bar in pure water to less than minus 100 bar in plant juices. As a result, the molecules in the plant sap stick together to a certain extent. According to the study, these agglutinated lipids are the cause of the pressure limit prevailing in plants.

Vacuum makes water absorption more difficult on dry soils

The researchers are thus providing the first explanation as to why plants do not suck as much water out of the soil as they like and can therefore only survive on dry soils to a limited extent. „The greatest negative pressures in plants are found in areas where water is scarce“, adds Matej Kanduč, physicist at the Jožef Stefan Institute in Ljubljana and first author of the study. However, as climate change is causing the soil in many regions of the world to become drier and drier, plants would have to struggle against great resistance to draw water from the soil, Kanduč continues. A higher suction power would be an advantage.

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