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Aerosols have always been part of the atmosphere, and plant surfaces are a major aerosol sink. Given the nutrient content of aerosols and the natural stability of aerosol concentrations over evolutionary time, plants may have developed adaptations to aerosol input. Although little is known about such adaptations, leaf surface micro‐roughness appears to play a key role. This review focuses on the deposition and fate of fine aerosols that are less than 2.5 μm in diameter. Most of these aerosols are hygroscopic, and they are often deliquescent (liquid) on transpiring leaves. Such concentrated solutions may be taken up by both the cuticle and stomata, contradicting previous concepts. The establishment of a continuous liquid water connection along stomatal walls affects individual stomata and is a new concept called “hydraulic activation of stomata” (HAS). HAS enables the efficient bidirectional transport of water and solutes between the leaf interior and leaf surface and makes stomatal transpiration partly independent of stomatal aperture. The response of plants to changes in humidity can be explained by the split transpiration in an HAS pore and its interaction with neighboring stomata, i.e., as an emergent property of a stomatal patch. Normally, HAS affects only a few stomata, but if too many are activated by excessive particle accumulation or additional surfactants, hygroscopic particles may work as “desiccants,” reducing the drought tolerance of plants. This is made use of when hygroscopic salts and acids are sprayed to kill potato vine, but may cause problems in foliar fertilization. Excessive particle accumulation may also be caused by air pollution. It is hypothesized that deliquescent hygroscopic particles, due to their amorphous appearance, may have been misinterpreted as “degraded waxes.” Degraded waxes have been highly correlated to leaf loss, decreased drought tolerance, and decreased frost tolerance of trees. No sound explanation for degraded waxes has been found, and they have been interpreted as symptoms of forest decline. Because hygroscopic particles may affect the drought tolerance of trees, they could be drivers of regional tree die‐off and especially affect those trees that have adapted to capture aerosols. Several research questions are identified.

SummaryThe recent visualization of stomatal nanoparticle uptake ended a 40-yr-old paradigm. Assuming clean, hydrophobic leaf surfaces, the paradigm considered stomatal liquid water transport to be impossible as a result of water surface tension. However, real leaves are not clean, and deposited aerosols may change hydrophobicity and water surface tension.Droplets containing NaCl, NaClO 3 , (NH 4 ) 2 SO 4 , glyphosate, an organosilicone surfactant or various combinations thereof were evaporated on stomatous abaxial and astomatous adaxial surfaces of apple (Malus domestica) leaves. The effects on photosynthesis, necrosis and biomass were determined. Observed using an environmental scanning electron microscope, NaCl and NaClO 3 crystals on hydrophobic tomato (Solanum lycopersicum) cuticles underwent several humidity cycles, causing repeated deliquescence and efflorescence of the salts.All physiological parameters were more strongly affected by abaxial than adaxial treatments. Spatial expansion and dendritic crystallization of the salts occurred and cuticular hydrophobicity was decreased more rapidly by NaClO 3 than NaCl.The results confirmed the stomatal uptake of aqueous solutions. Humidity fluctuations promote the spatial expansion of salts into the stomata. The ion-specific effects point to the Hofmeister series: chaotropic ions reduce surface tension, probably contributing to the defoliant action of NaClO 3 , whereas the salt spray tolerance of coastal plants is probably linked to the kosmotropic nature of chloride ions.

SummarySince a key requirement of known life forms is available water (water activity; a w), recent searches for signatures of past life in terrestrial and extraterrestrial environments have targeted places known to have contained significant quantities of biologically available water. However, early life on Earth inhabited high-salt environments, suggesting an ability to withstand low water-activity. The lower limit of water activity that enables cell division appears to be ∼ 0.605 which, until now, was only known to be exhibited by a single eukaryote, the sugar-tolerant, fungal xerophile Xeromyces bisporus. The first forms of life on Earth were, though, prokaryotic. Recent evidence now indicates that some halophilic Archaea and Bacteria have water-activity limits more or less equal to those of X. bisporus. We discuss water activity in relation to the limits of Earth's present-day biosphere; the possibility of microbial multiplication by utilizing water from thin, aqueous films or non-liquid sources; whether prokaryotes were the first organisms able to multiply close to the 0.605-a w limit; and whether extraterrestrial aqueous milieux of ≥ 0.605 aw can resemble fertile microbial habitats found on Earth.

“Microscopic leaf wetness” means minute amounts of persistent liquid water on leaf surfaces which are invisible to the naked eye. The water is mainly maintained by transpired water vapor condensing onto the leaf surface and to attached leaf surface particles. With an estimated average thickness of less than 1 μm, microscopic leaf wetness is about two orders of magnitude thinner than morning dewfall. The most important physical processes which reduce the saturation vapor pressure and promote condensation are cuticular absorption and the deliquescence of hygroscopic leaf surface particles. Deliquescent salts form highly concentrated solutions. Depending on the type and concentration of the dissolved ions, the physicochemical properties of microscopic leaf wetness can be considerably different from those of pure water. Microscopic leaf wetness can form continuous thin layers on hydrophobic leaf surfaces and in specific cases can act similar to surfactants, enabling a strong potential influence on the foliar exchange of ions. Microscopic leaf wetness can also enhance the dissolution, the emission, and the reaction of specific atmospheric trace gases e.g., ammonia, SO2, or ozone, leading to a strong potential role for microscopic leaf wetness in plant/atmosphere interaction. Due to its difficult detection, there is little knowledge about the occurrence and the properties of microscopic leaf wetness. However, based on the existing evidence and on physicochemical reasoning it can be hypothesized that microscopic leaf wetness occurs on almost any plant worldwide and often permanently, and that it significantly influences the exchange processes of the leaf surface with its neighboring compartments, i.e., the plant interior and the atmosphere. The omission of microscopic water in general leaf wetness concepts has caused far-reaching, misleading conclusions in the past.

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Aerosols are important components of the global plant environment, with beneficial and deleterious impacts. The direct effects of aerosol deposition on plant-water relationships remain poorly characterized but potentially important. Vicia faba was grown in ambient urban air and in the same air with aerosol excluded, in a moderately polluted environment using two exposure protocols. Simultaneous measurement of gas exchange and stomatal pore aperture was combined with leaf dehydration kinetics and microscopic evaluation of leaf wetness formation and aerosol deposition patterns. The ambient aerosol was shown to be hygroscopic. Aerosol exposure increased minimum leaf conductance, shown by dehydration kinetics, and nocturnal water vapor flux, shown by dark-adapted gas exchange. Aerosol exposure decreased stomatal apertures at each level of vapor pressure deficit (VPD) and increased stomatal conductance at comparable levels of aperture. Overall, these effects were modest, and largest when stomata were wide open. The uncoupling of conductance (flux-based) from aperture (directly measured microscopically) implies that aerosol-induced water loss is not fully under stomatal control. This reduces drought tolerance and may provide a mechanism by which deposited aerosol plays a direct role in stomatal response to VPD.