When temperatures drop below freezing, ice crystals form on hard green leaves. However, they survive (How plants protect themselves from frost) the winter unharmed. Using a special cryogenic scanning electron microscope, researchers at the Zoological Institute of the University of Kiel (CAU) were able for the first time to take high-resolution pictures of the freezing process in wild plants and Antarctic plants. They discovered small features on the leaves of plants that protect themselves from low temperatures. A better understanding of these defense mechanisms can also be of interest to plants or aircraft. The results appear in the scientific journal The Science of Nature.
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To protect (How plants protect themselves from frost) against icing, the plane is treated with a special liquid or prepared to be heated. Science and industry around the world are looking for suitable coatings for aircraft. “On the other hand, many of our land plants have developed their own natural protection against icing during evolution,” said Professor Stanislav Gorb, head of the Functional Morphology and Biomechanics department, explains. For more than 20 years, the zoologist and his wife, the botanist Dr. Elena Gorb, has studied horticulture at CAU.
Research on how plants protect themselves from frost has focused on sugar content and other chemical processes in plant cells. Kiel’s research team has now been able to show that the surface structure of the paper also plays an important role in protecting against the cold. To do this, they looked at protecting ice crystals and different types of leaves of forest plants. “With hair or a waxy coating on the leaves, the plant can avoid or at least control the frost of its leaf surface. Because if the ice layer is there, the cells of the plant will freeze soon,” explains Elena Gorb.
Various leaf surfaces were developed
As a result of research in nature and in the laboratory, the two researchers were able to identify different protection mechanisms in the literature:
The leaves of plants with hairs (“trichomes”), such as those of the daisy (Bellis perennis), are usually hydrophilic. This is where ice crystals form first – they melt again very quickly once the outside temperature rises again. The surface of the leaf has an underlying effect that is free of ice and is not damaged.
On the other hand, the waxy coating on the leaves, such as some types of tulips (Tulipa gesteriana), is superhydrophobic: water immediately escapes, which is also called the “lotus effect“. Ice crystals can form here if water droplets “stick” to spots in the wax layer. But even then, they do not damage the wax layer, and the foil underneath remains intact.
Researchers in Kiel have discovered that wax protects against cold not only in plants in the north of Germany, but also in the south polar circle: Deschampsia antarctica, one of the only two flowers of Antarctica, made look twice. It has a prominent epicuticular layer on both sides of the leaf, which has two overlapping layers. In the journal Beilstein of Nanotechnology, researchers suspect that these two components of wax increase resistance to cold, icing, harmful UV rays and dehydration and thus help the plant to adapt in the harsh environmental conditions of Antarctica.
Trees with soft leaves such as cherry laurel (Prunus laurocerasus) are the first to be seen and the most protected: ice crystals can form on the entire surface. Under the influence of the sun, melted water accumulates in the lower part of the sheet. If the temperature drops again, these fluids freeze and can damage plant cells for a long time. “But since these plants survive the winter, we think they have enough chemical protection,” says Stanislav Gorb.
The microscope allows imaging at minus 140 degrees
The team was able to study nanoscale ice crystals using a so-called cryo electron microscope. Unlike other methods, biological samples are not dried here, but frozen quickly. In this way, their structures are well preserved and can be examined in normal conditions. To do this, the group immerses a small leaf in liquid nitrogen at minus 196 degrees. At room temperature, they began to freeze. A high-resolution image of the ice crystal can be created in the microscope, which is cooled to 140 degrees. The process can be adapted to do long-term thawing – for example from the sun during the day – and refreezing.
“During evolution, plants have developed many ways to protect themselves from the cold”, concludes Stanislav Gorb. In particular, what happens when using water to interact with the surface of the leaf promises exciting research, as well as for the development of “anti-ice” technology. “But we still know very little about these processes.”
