The Atacama Desert, the driest nonpolar desert in the world, is home to Nolana mollis, a shrub with white flowers that is able to survive on less than 25 mm of precipitation a year. Located in Northern Chile, this perennial succulent of the nightshade family thrives in the desert’s dry river valleys and has the unusual capacity to produce brine on its leaves and green stems by condensing atmospheric water vapour.
For nearly 45 years, it has been hypothesized that N. mollis somehow uses this brine coating as a form of hydration. It was assumed that N. mollis is able to either pump water from the epidermal brine directly into shoot tissue or that shallow roots take up the water from brine that drips onto the soil. Either situation would require active water uptake against a strong water concentration gradient.
“For a plant to perform this work would require the existence of a mechanism of plant water uptake unknown to science,” write Gersony et al in their recent paper in Annals of Botany. “If such a mechanism that uses metabolic energy to transport water against its gradient in free energy exists and could be incorporated into agricultural plants, it would afford substantial protection from drought stress to crops.”
And so Gersony et al set out to test the hypothesis that N. mollis can absorb water from the brine into its leaves and stems and conclusively showed that the species cannot. Instead, N. mollis requires access to deep soil water for hydration.
“We conducted a field experiment with three treatments to establish the source of water for N. mollis: control, root cutting to block uptake of all soil moisture, and plastic skirting at the soil surface to block leaf drip of atmospheric water,” write Gersony et al.
Root cutting led to wilting despite the presence of copious amounts of brine on the leaves and stems, while skirted and control plants showed no differences in hydration, excluding the drips from playing an important role.
“This leaves deep soil moisture as the only possible source of water for the observed diurnal maintenance of internal water balance in N. mollis,” write Gersony et al.
These results have led Gersony et al to question the veracity of reports that other salt-exudating halophytes or desert species absorb water against their water potential gradients.
“To our knowledge, while plants that exude salts onto their surfaces do condense water from unsaturated (<90 % RH) atmospheres, such water remains far too dry to be accessed by the plant. Nevertheless, this hypothesized phenomenon continues to be misreported as fact,” write Gersony et al. They suggest that previous reports did not correct for variables such as leaf temperature, xylem osmotic potential or experimental artefacts (e.g. induced damage), leading to positive reports of brine water absorption.
However, the brine could still be important to the ecological success of N. mollis in the Atacama Desert, one of the driest places on Earth. Gersony et al explain that during the day, the brine likely suppresses water loss from leaves by humidifying the leaf boundary layer relative to the air, leading to a lower evaporative demand from the leaf to the air. Also, the brine likely allows for latent cooling of the leaf via evaporation.
“Detailed modelling of these processes is required to test whether these expected effects are likely to be physiologically important in the environment of the coastal Atacama, and to show how brine formation provides sufficient benefits to offset the energetic costs of salt transporters,” write Gersony et al.
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Gersony, J., Manandhar, A., Hochberg, U., Abdellaoui, N., Llanos, P., Dumais, J., Holbrook, N.M. and Rockwell, F.E. (2025) “Making dew in the Atacama Desert: a paradigmatic case of plant water uptake from an unsaturated atmosphere fails a test,” Annals of Botany, 135(5), pp. 841–850. Available at: https://doi.org/10.1093/aob/mcae221
Cover: Nolana mollis by felipecancino – https://www.flickr.com/photos/felipecancino/4942898032/, CC BY-SA 2.0, https://commons.wikimedia.org/w/index.php?curid=11958080
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