Aerosol deposition affects water uptake and water loss of beech leaves

The deposition of aerosols on leaves could significantly influence plant-atmosphere interaction through the formation of very thin aqueous films that allow the transport of liquid water through the stomata. Such films can be formed by deliquescence and dynamic expansion of hygroscopic aerosols ('hydraulic activation of stomata'; HAS). Two processes that may be associated with stomatal liquid water transport are foliar water uptake (FWU) and the contribution of 'leaky stomata' to minimum epidermal conductance (gmin). We investigated whether ambient aerosols affect FWU and gmin of Fagus sylvatica L. seedlings. Plants were grown in ventilated greenhouses with ambient air or filtered, almost aerosol-free air. gmin was determined using leaf drying curves. The FWU was investigated gravimetrically and with deuterium-enriched water, starting from different leaf water potentials, by spraying freshly cut or pre-dried leaves (60 min). The presence of aerosols in the environment increased gmin by about 47%, confirming previous measurements in other species. The gravimetric measurements did not show a significantly increased FWU. However, deuterium uptake was higher when aerosols were present, indicating a lower resistance to water uptake into the leaves. Deuterium uptake was higher for freshly cut leaves than for pre-dried leaves, despite the lower leaf water potential of the pre-dried leaves. Both gmin and FWU results are consistent with bidirectional stomatal transport of liquid water along aerosol-induced pathways. The FWU could also have been generated by water vapor fluxes through 'reverse transpiration', although the functional contribution of the aerosols would remain unclear. At low leaf water potential, the pathway may dry out and become less functional for FWU, whereas it may still be noticeable as stomatal leakage, given the strong gradient of water potential from the leaf interior to the atmosphere.