Antifreeze with wax and hair

With different types of plant leaves, certain fine structures lead to different freezing processes. © Elena Gorb and Stanislav Gorb

How do plants protect themselves from damage caused by ice formation? In addition to "antifreeze" in cell juices, tiny surface structures on the leaves also serve this purpose in some species, researchers report: small hairs and layers of wax prevent or control icing there, as cryomicroscopic studies make clear. The scientists say that more precise insights into these patents of nature could also have a technical meaning in addition to the biological one.

Pointed and hard, they pierce sensitive tissue structures: the formation of ice crystals poses a great threat to living beings. To protect themselves from freezing to death, many animals generate their own heat, insulate their bodies or crawl into frost-free niches. But plants cannot escape the frost. "Many of our wild plants have therefore developed protective mechanisms against icing over the course of evolution," says Stanislav Gorb from the University of Kiel. So far, research has mainly focused on the chemical “antifreeze” in plant sap. Above all, a high sugar content and other chemical aspects protect the cells from the formation of destructive ice crystals at sub-zero temperatures. On the other hand, together with his colleague and wife Elena Gorb, Stanislav Gorb shows the role of plant surface characteristics in frost protection.

Examination at minus 140 degrees Celsius

Their results are based on investigations using so-called cryo-scanning electron microscopy. In contrast to other methods, it allows analysis of biological samples in the frozen state. Because the sample chambers of these special microscopes can be cooled down to minus 140 degrees Celsius. In this way, the structures of samples are preserved and certain reactions associated with icing can be investigated. In this way, the researchers were able to understand how certain microscopic structures in plants affect the formation of ice crystals on the surface. The focus was on different leaf types of different wild plants.

As the researchers report, there are two protective mechanisms on the leaves that work almost in opposite directions: "Plants can avoid or at least control the freezing of their direct leaf surface through hairs or a waxy layer on the leaves," says Elena Gorb. As can be seen from the investigations, the effect on the hair structures is based on the water-attracting (hydrophilic) characteristics of these so-called trichomes. As a result, the ice crystals form preferentially on these protruding structures and not on the leaf surface. In this way, the sensitive tissues are protected from the destructive effects of ice formation, the researchers explain.

Natural research with technical potential

In the other concept, a water-repellent (hydrophobic) effect comes into play: Specially structured waxy layers on the leaf surfaces ensure that water droplets roll off effectively. Only very few water molecules can get stuck and form ice crystals. They cannot then penetrate the wax layer and thus deeper leaf tissue remains protected. The researchers found a particularly complex form of wax protection against the cold in a plant with an extremely icy home: Deschampsia antarctica is one of the only two flowering plant species in Antarctica. It has a very complex form of wax coating that could be related to its high resistance to frost damage, the scientists say.

As they emphasize, their research could, in addition to providing clues as to how plants survive extreme sub-zero temperatures, also be of importance for technology - especially for aviation: Because in order to protect surfaces from the problematic formation of ice crystals, among other things, work is being done on the development refined coatings worked. “In the course of evolution, plants have developed many ways of protecting themselves from the cold. What happens when water interacts with leaf surfaces could provide exciting insights into the development of technical 'anti-ice' surfaces,” says Stanislav Gorb. That's why further research is now called for: "Because we still know very little about these processes," emphasizes the scientist.

Source: University of Kiel

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