The impact of falling drops on Salvinia molesta leaves was observed by a research team at the University of Tübingen using high-speed cameras.
The tropical floating fern Salvinia molesta has developed sophisticated structures to allow water to roll off its leaves quickly even during heavy rainfall. This relieves the pressure on the leaves floating on the water surface, but above all also keeps the stomata free for air exchange. This allows the fern to absorb carbon dioxide unhindered for photosynthesis. This was discovered in an interdisciplinary cooperation project between Professor James Nebelsick and Dr. Dr. Wilfried Konrad from the Department of Geosciences at the University of Tübingen and Dr. Anita Roth-Nebelsick from the Staatliches Museum für Naturkunde Stuttgart during experiments with artificial rain.
The team tracked the path of water droplets slapping onto leaves with high-speed cameras. The leaf structures, which are efficiently adapted to rainfall, could contribute to the strong growth and rapid spread of the floating fern. Far from its native Brazil, it is considered a dangerous invasive plant in many places, displacing other species. The new study was published in the Journal of the Royal Society Interface.
Salvinia molesta, like all floating ferns, floats freely on the water surface. Of each trifoliate whorl, two floating leaves lie on top, and it extends one root-like leaf downward. It quickly forms dense and thick mats on the water surface. It originates from humid tropical regions in Brazil, where it is regularly exposed to heavy rainfall. The upper surface of the floating leaves is covered with highly water-repellent hairs whose structure resembles tiny whisks.
“These trichomes ensure that the submerged leaves are not wetted with water. However, other aquatic plants achieve this with much less elaborate structures. We therefore asked ourselves what benefit the fern gets from the tiny whisks.”
- Wilfried Konrad
Experiments simulate rainfall
The research team obtained specimens of Salvinia molesta from the botanical gardens in Tübingen and the Wilhelma in Stuttgart. In the experiment, it allowed smaller and larger drops of water from different heights to splash onto the leaves of the floating fern, which were fixed in a partial experiment. “Overall, the leaves quickly shook off most of the artificial rain from the surface. The camera images show that the trichomes respond elastically to the kinetic energy of the falling drops,” Konrad says. But even more important in shaking off the water, he says, is the elasticity of the leaves as a whole. That’s what the comparison of rain experiments of free-floating leaves with fixed leaves revealed, he says. “When the leaves were fixed, it took much longer for their surface to become water-free again.”
The researchers observed how water droplets bounced off the leaves, such as bouncing back and taking on a pancake-like shape or breaking into smaller droplets. “Especially in the experiments with thick water droplets falling from a great height onto the fern leaves, we noticed that there was always some residual water left under the small whisks,” says the physicist. “Surprisingly, the trichomes seemed to rather prevent the water from beading off quickly.”
Water-repellent structures
However, detailed observations revealed that they were very efficient at keeping the leaf base free of water. “The energy state of a droplet is more favorable on the trichomes than between them. This is due to the water-repelling effect of the whisks,” Konrad explains. Now, when a drop of water rolls over the trichomes of the leaf, it literally sucks up the residual water trapped between the trichomes due to its strong surface forces. “In this way, the stomata at the base of the floating fern’s leaf, which is also covered with water-repellent nanocrystals made of wax, remain free of water. That’s where the gas exchange that is vital for the plant takes place.” Possibly this is a decisive factor for the high productivity of the fern Salvinia molesta, which can double its leaf mas-se within a few days under favorable conditions.
