In the scorching heat of the arid desert, plants have evolved a variety of ways to extract and store freshwater from their environment. Some, like cacti, hold onto it in thick, waxy stems. Others grow deep roots to tap into the groundwater.
Now, researchers have discovered an unusual new mechanism that allows some desert shrubs to get water: They can soak it up out of the air using salt crystals atop their leaves.
This method could improve human efforts to collect moisture from the air in dry places, the authors write in their paper published last month in the journal Proceedings of the National Academy of Sciences.
Athel tamarisks (Tamarix aphylla) are salt-excreting plants in a group known as halophytes, which have adapted to live in soils with very high salinities. While the shrubs are native to deserts in Africa and the Middle East, they are often planted as an ornamental in the Southwestern U.S.
The plants soak up salty water through their roots, use what they can and then excrete the excess concentrated saltwater from glands on their leaves. But after this step, scientists did not know what happens next. At first, they thought the water may fall down to the ground, where it can be absorbed by the plants’ roots, reports James Dinneen for New Scientist. But time-lapse videos of the shrub showed that wasn’t true.
“The droplets do not actually fall at all,” Panče Naumov, a study co-author and chemist at New York University Abu Dhabi, tells the publication. “They stick to the surface.”
From there, the water dissipates in the desert heat, leaving behind white salt crystals on the plants’ leaves. And during the night, as the video revealed, these salt crystals swell up with water.
The researchers wanted to test exactly how much water the crystals could absorb, so they placed a freshly cut athel tamarisk branch in an environmental chamber back in the lab—at 95 degrees Fahrenheit and 80 percent humidity, the chamber emulated desert conditions. They weighed the branch every 20 minutes and discovered that after two hours, it had collected about 15 milligrams of water.
Next, the team washed the branch to remove the salt crystals and repeated the experiment. This time, the plant only soaked up 1.6 milligrams of water.
“This result was conclusive to us,” co-author Marieh Al-Handawi, a materials scientist at New York University Abu Dhabi, tells Ariana Remmel of Science News. “It proved salts are the main contributor to the water harvesting, and it’s not the surface of the plant.”
The team ran a variety of other tests, too: In one, they created a model of the leaves, coated in wax extracted from the plant. With it, they measured the adhesion force of a single water droplet and determined that athel tamarisks are almost two times more adhesive to water than Teflon is. The scientists added colored water to the salt-encrusted leaves and watched it diffuse into the plant.
They also analyzed the composition of the salt crystals left on the leaves and found that they were made up of at least ten different minerals, including sodium chloride, gypsum and lithium sulfate. This combination of minerals had the ability to attract moisture from the air, even at relatively low humidity levels of about 55 percent.
What’s more, the lithium sulfate stood out as especially adept at collecting water. It could gather water at the lowest humidities, compared to sodium chloride and gypsum (though these other two salts collected a larger volume of water overall).
Maheshi Dassanayake, a biological scientist at Louisiana State University, tells New Scientist that it’s possible these salt crystals provide a way for the tamarisks to take in water, but she’s not convinced the plant actually uses the water the salt crystals absorb.
“I’m missing the mechanistic basis for how the plant uses energy to get the water,” she tells the publication.
But still, Naumov says in a statement that understanding how this mechanism works could inspire new technologies for harvesting water from the air. Using the salts revealed in this study could create more environmentally friendly collection practices or improve on current methods for cloud seeding, a technique that helps draw precipitation from clouds, per the paper.