Primeval leaf sleep in the mirror of scars

Symmetrically arranged feeding marks on modern (right) and fossil leaf structures (left) indicate that insects gnawed on them in a folded nocturnal position. Right image: © Stephen McLoughlin, left: © Current Biology/Feng et al.

A fascinating movement behavior of some of today's plants apparently existed in primeval times, report paleobotanists: Already more than 250 million years ago, some plants of the Permian age folded their leaves at night. The researchers were able to demonstrate this using characteristically symmetrical feeding marks on fossil leaves. Insects apparently once gnawed the holes in the leaves when they were in a temporarily closed position.

Anyone who pays patient attention to them will find that plants also show a sometimes complex movement behavior. This already fascinated the time-honoured pioneer of modern biology, Charles Darwin. In his work "The Power of Movement in Plants" from 1880, he reported, among many examples, on a phenomenon known as nyctinasty. Various plant species bring their leaves into certain resting positions at night.

Some leaves "go to sleep"

This is particularly evident in some species: they carefully fold their leaves together in the evening. The main leaf vein acts as a "fold line" so that the two lateral surfaces lie on top of each other. In the morning, certain motor elements then cause the leaf blade to open again. However, there are still some unanswered questions about the functions of this behavior. Various purposes are suspected: Possibly the plants reduce the nocturnal heat emission by folding or prevent problematic water accumulation on the surfaces. However, they may also make their leaves unattractive to predators at night or provide less cover to make the pests easier for nocturnal predators to prey on.

One might think that this complex-looking concept is a relatively modern development in the evolutionary history of plants. However, as an international team of paleobotanists has now shown using fossil leaves, this is apparently not the case. "Our discovery is based on a rather unorthodox approach," says senior author Stephen McLoughlin of the Swedish Museum of Natural History in Stockholm. The study started with the finding that insects can leave characteristic patterns of food marks on plant leaves that close at night: they gnaw away at both surfaces lying on top of each other. When the sheet then opens in the morning, two mirror-inverted hole structures appear to the right and left of the folding axis.

Chewed through in closed position

As the team reports, they found exactly such patterns in leaf fossils from China. They come from plants from the group of the so-called gigantopterids, which were characteristic of the flora of the Permian period about 300 to 250 million years ago. It is known that their leaves have also been gnawed by insects. Through their extensive investigations, the researchers have now been able to identify fossil leaves that clearly show the symmetrical pattern typical of nyctinastia. These were full-grown leaves that had apparently been eaten by insects in a nocturnal folding position.

“Our results demonstrate that these plants have evolved nyctinastic leaf movements at such an early stage in plant evolution. That really surprised me,” says lead author Zhuo Feng from Yunnan University in Kunming. McLoughlin continues: "It now seems clear that sleep behavior evolved independently in different plant groups and at different times throughout Earth's history, suggesting that it must have important ecological benefits for plants," McLoughlin said.

The scientists now want to continue to devote themselves to this interesting research topic: Perhaps the characteristic traces of "sleeping behavior" can also be detected in other fossil groups from the developmental history of plants.

Source: Cell Press, Article: Current Biology, doi: 10.1016/j.cub.2022.12.043

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