The more you suck on a drinking straw, the more liquid comes up. Plants draw water from the soil using a similar principle. But so far there has been a riddle: Why don’t plants exploit the apparently existing negative pressure potential in order to extract water from dry soils even more effectively? A study now shows that certain fats in plant juices prevent higher suction power. Above a certain negative pressure, they form cavities that allow the water flow to break off.
At first glance, the vegetable hydraulic system appears to be quite uncomplicated: the water evaporates in the leaves, which creates a suction effect in the plant’s vascular system that extends right down to the roots. There, the vacuum then absorbs water from the ground. The rule applies to the process: the drier the soil, the more vigorously the plants suck. But as studies have shown, they apparently reach a limit surprisingly early: even the desert plants do not reach more than minus 100 bar. So far, it has not been clear why. Because there was no physical reason to speak against stronger negative pressures: Actually, with water columns up to 1000 bar negative pressure are possible without the connection breaking.
What limits suction power?
An international team of researchers has now uncovered what is the limiting factor in the traces of plants. They used simulations at the atomic level to investigate whether there are ingredients in plant sap that could limit the negative pressure potential. Their results show that certain fats (lipids) are apparently responsible. According to this, these natural substances at more than 100 bar negative pressure in the plant sap cause the formation of rapidly expanding cavities – so-called cavities. If they become too large, the water column tears off and the supply to the plant collapses.
The reason why the lipids make it easier to form cavities is because they form double layers in the sap, as the researchers explain. This effect is due to the two different ends of the lipids: one repels water molecules – it is hydrophobic. The other attracts them – it’s hydrophilic. The hydrophilic part therefore faces the water, while the hydrophobic tail connects to a similar end of another lipid. These characteristics then result in tiny structures made of lipid bilayers in the plant juices.
Fats create cavities in the case of negative pressure
As can be seen from the computer simulations, these structures lead to a significantly increased tendency to form voids under negative pressure. Lipid bilayers in a liquid mean that the cavities are particularly easy to form and do not dissolve as effectively as with pure water. “Put simply, it is much easier to tear two layers of lipid apart than a group of water molecules,” explains co-author Emanuel Schneck from the Technical University of Darmstadt.
As he and his colleagues were able to show, the water column in plant juices breaks off at pressures of more than 100 bar. However, with the conditions typically found in plants of only 5 to 50 bar, this hardly ever happens, the researchers say. The results therefore match the negative pressures measured in botany. “For the first time, our results provide a plausible explanation of why plants cannot maintain negative pressures of more than minus 100 bar for long,” says Schneck.
Plants therefore have limited ability to absorb water from the drying soil. Ultimately, this also determines where plants can survive and grow, the scientists explain. According to them, the results are also relevant against the background of climate change: drought is increasing in more and more regions of the world. Insights into the plant’s ability to absorb water can therefore be helpful, according to the scientists.
Source: Max Planck Institute for Colloids and Interfaces, Technical article: PNAS, doi: 10.1073 / pnas.1917195117