An evergreen desert shrub common in the Middle East excretes salt crystals onto its leaves that may help the plant absorb moisture from the air.
“Not only does the plant use some water, it can gain some water,” says Panče Naumov at New York University Abu Dhabi.
The Athel tamarisk (Tamarix aphylla) is part of a group of plants that have adapted to live in extremely salty soil known as recretohalophytes. These plants take in salty water through their roots, using it for nourishment and then excreting the concentrated salty water that remains onto their leaves.
Naumov and his colleagues were curious what happens with this water after it is excreted. At first, they thought the Athel tamarisk might use the droplets it deposits on its leaves to water its own roots. But close observation of time-lapse videos showed this is not the case. “The droplets do not actually fall at all. They stick to the surface,” says Naumov.
The researchers then studied the crystals left behind on the leaves when this water evaporates, collecting salts from plants growing on the outskirts of Abu Dhabi at 5 different parts of the year to account for seasonal differences. X-ray analysis showed the samples were mostly sodium chloride, but also contained more than ten different salt compounds.
The team then tested how the salt stays on the surface of the leaves, using a leaf model made from wax extracted from the plant. While some of the larger crystals easily fell off the waxy surface, they found that smaller lithium sulphate crystals stuck on. Salt with lithium sulphate crystals absorbed water across a wider range of humidity than sodium chloride alone. The team used dyed water to track how the salty liquid on the outside of the leaf can diffuse to the inside of the plant.
Naumov says this suggests the plant may have two mechanisms for getting water from salty soils, first taking in water through its roots during the hotter, drier day, then using the excreted salts to absorb water through its leaves during the cooler, more humid nights. “They work in synergy, day and night,” he says.
Maheshi Dassanayake at Louisiana State University says this is plausible, but she is not convinced by the researcher’s evidence that the plant actually uses the water absorbed by the salt on its leaves. “I’m missing the mechanistic basis for how the plant uses energy to get the water,” she says.
Even if the plant doesn’t make use of the water gathered by salt in this way, Naumov says the salt compounds could be useful for systems that harvest water from the air, or even for seeding clouds to make it rain.