Making dew in the Atacama Desert: a paradigmatic case of plant water uptake water from an unsaturated atmosphere fails a test.

Abstract

Nolana mollis is a dominant plant species in the hyperarid Atacama Desert. A previous hypothesis states that N. mollis owes its success to the condensation of atmospheric water from undersaturated air onto its leaf surfaces by exuded salts, and absorption of this water by its leaves, or by shallow roots following drip onto the soil surface; living roots of N. mollis were suggested to only exist near the soil surface. We conducted a field experiment with three treatments to establish the source of N. mollis's water: control, root cutting to block uptake of all soil moisture, and plastic skirting at the soil surface to block leaf drip of atmospheric water. Xylem tensions monotonically increased after root cutting until the plants wilted irreversibly, diverging clearly from the skirted and control treatments showing diurnal patterns of increasing tension in the day followed by recovery overnight. Hydration in N. mollis requires access to deep soil water, motivating an alternative hypothesis: imperfect salt exclusion at the root surface and salt exudation by the leaf results in less root fouling and lower xylem tensions, while during the day evaporation of the surface brine, condensed overnight, increases the water use efficiency of carbon gain.