Mint’s “secret weapon” as a bio-herbicide?


With its essential oils, mint keeps weeds away. (Image: Jana Müller)

The essential oils of mint not only have a healing effect – they could also become a source of new bioherbicides: Researchers have found that mint oil contains two substances that naturally weed suicides. The plants stop growing and die. Because this effect is specific to certain groups of plants, this could enable gentler, more targeted weed control.

Some plants are very successful in the competition for soil, nutrients and water: They hinder the growth of their competitors through chemical substances that trigger cell death in the neighboring plant. Scientists at the Botanical Institute of the Karlsruhe Institute of Technology (KIT) are investigating how this effect comes about in order to use it for the development of environmentally friendly bioherbicides.

They are not particularly popular in the home garden or in the field: wild herbs are important for insects and other animals in the field, but at the same time they reduce the yields if they grow in the middle of the crop plants and compete with them for nutrients. “If we didn’t fight the weeds, 30 to 50 percent of the agricultural yield would be lost,” explains Peter Nick from the Botanical Institute of the Karlsruhe Institute of Technology (KIT).

Fragrance as a biological defense weapon

The problem, however, is that conventional herbicides have a significant impact on ecosystems and often kill more plants than necessary. In addition, some of these agents, including glyphosate, which is widely used worldwide, are suspected of being harmful to health. That is why researchers have long been looking for biological, environmentally friendly replacements for common weed killers. A promising approach is to learn appropriate strategies from the plants themselves. There are some types of plants, including walnut, wild garlic or mint, that inhibit the growth of other plants in their vicinity by releasing chemical substances. Experts call this chemical warfare allelopathy.

“These are mostly not poisons, but chemical signals that have an effect on the target plant,” explains Nick. In order to find out exactly how this biological defense works and to look for candidates for agricultural use, Nick and his colleague Mohammed Sarheed took a closer look at various species of mint. “We went through nature with an open eye and asked ourselves whether there could be a connection between the strong growth of mint and its distinctive scent, which is different for every type of mint,” says Nick

Two ingredients with a “suicide effect”

For their study, the scientists first extracted the essential oils of various mint and provided molecular markers on individual potentially bioactive components. Then, in germination tests, they planted the seeds of various other plants, including cress, winter wheat, tomatoes, bindweeds and the stump-leaved steamer (Rumex obtusifolius), a common willow weed. It was shown that two components of the essential mint oils are effective against competing plants: beta-pinene and menthone. These biological messengers trigger the cellular suicide program in the plants they contain and thus lead to the growth stopping and the germination of the seedlings.

In additional cell culture experiments, however, the researchers found differences in the mechanism of the two substances: the compound menthone present in the mint oil activates a process in competing plants through which microtubules – finely branched, tubular protein structures in the cells – self-destruct. “It turned out that Menthon is particularly effective against the weed marsh that occurs on mountain pastures,” says Sarheed. The pinene, on the other hand, trigger cellular suicide through its action on actin filaments – the second stabilizing component of the cell skeleton. These mint oil components inhibit the growth of field winds very effectively, but even seem to have a growth-stimulating effect on crops such as apple or wine.

According to the researchers, the essential oils of mint thus offer new starting points for the development of organic, environmentally compatible herbicides. “The results pave the way for the use of novel bioherbicides, the effects of which are specific for a particular species,” explains Sarheed. “For example, weeds would commit suicide if the signals were given, while crops would ignore the signal.”

Source: Karlsruhe Institute of Technology; Technical article: dissertation KITopen, doi: 10.5445 / IR / 1000099195

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