Coopman, Rafael ENguyen, Hoa ThiMencuccini, MaurizioOliveira, RafaelSack, LawrenLovelock, CatherineBall, Marilyn2023-04-100028-646Xhttp://hdl.handle.net/1885/288195The mangrove Avicennia marina adjusts internal salt concentrations by foliar salt secretion. Deliquescence of accumulated salt causes leaf wetting that may provide a water source for salt-secreting plants in arid coastal wetlands where high nocturnal humidity can usually support deliquescence whereas rainfall events are rare. We tested the hypotheses that salt deliquescence on leaf surfaces can drive top-down rehydration, and that such absorption of moisture from unsaturated atmospheres makes a functional contribution to dry season shoot water balances. Sap flow and water relations were monitored to assess the uptake of atmospheric water by branches during shoot wetting events under natural and manipulated microclimatic conditions. Reverse sap flow rates increased with increasing relative humidity from 70% to 89%, consistent with function of salt deliquescence in harvesting moisture from unsaturated atmospheres. Top-down rehydration elevated branch water potentials above those possible from root water uptake, subsidising transpiration rates and reducing branch vulnerability to hydraulic failure in the subsequent photoperiod. Absorption of atmospheric moisture harvested through deliquescence of salt on leaf surfaces enhances water balances of Avicennia marina growing in hypersaline wetlands under arid climatic conditions. Top-down rehydration from these frequent, low intensity wetting events contributes to prevention of carbon starvation and hydraulic failure during drought.This research was supported by Australian Research Council Discovery Project grant DP150104437 to MCB, CEL, MM, RSO and LS. REC was supported by the project FONDECYT N°1171640 from the Fondo Nacional de Desarrollo Cient ıfico y Tecnol ogico (FONDECYT, Chile). HTN was supported by an Australia Awards PhD Scholarship. We thank Gosia Pilat and Jack Egerton for field assistance, Denver and Jenifer Blake for support of research based at Giralia Station, and Professor John Finnigan for micrometeorological advice on humidity-dependent processesapplication/pdfen-AU© 2021 The AuthorsNew Phytologist © 2021 New Phytologist Foundationdeliquescencefoliar wateruptakehydraulic safetylow intensity leafwettingreverse sap flowwater storage capacitancesalt secretion202110.1111/nph.174612022-01-23