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.

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Atmospheric vapour pressure deficit (VPD) is the driving force for plant transpiration. Plants have different strategies to respond to this 'atmospheric drought'. Deposited aerosols on leaf surfaces can interact with plant water relations and may influence VPD response. We studied transpiration and water use efficiency of pine, beech and sunflower by measuring sap flow, gas exchange and carbon isotopes, thereby addressing different time scales of plant/atmosphere interaction. Plants were grown (i) outdoors under rainfall exclusion (OD) and in ventilated greenhouses with (ii) ambient air (AA) or (iii) filtered air (FA), the latter containing <1% ambient aerosol concentrations. In addition, some AA plants were sprayed once with 25 mM salt solution of (NH4 )2 SO4 or NaNO3 . Carbon isotope values (δ(13) C) became more negative in the presence of more particles; more negative for AA compared to FA sunflower and more negative for OD Scots pine compared to other growth environments. FA beech had less negative δ(13) C than AA, OD and NaNO3 -treated beech. Anisohydric beech showed linearly increasing sap flow with increasing VPD. The slopes doubled for (NH4 )2 SO4 - and tripled for NaNO3 -sprayed beech compared to control seedlings, indicating decreased ability to resist atmospheric demand. In contrast, isohydric pine showed constant transpiration rates with increasing VPD, independent of growth environment and spray, likely caused by decreasing gs with increasing VPD. Generally, NaNO3 spray had stronger effects on water relations than (NH4 )2 SO4 spray. The results strongly support the role of leaf surface particles as an environmental factor affecting plant water use. Hygroscopic and chaotropic properties of leaf surface particles determine their ability to form wicks across stomata. Such wicks enhance unproductive water loss of anisohydric plant species and decrease CO2 uptake of isohydric plants. They become more relevant with increasing number of fine particles and increasing VPD and are thus related to air pollution and climate change. Wicks cause a deviation from the analogy between CO2 and water pathways through stomata, bringing some principal assumptions of gas exchange theory into question.

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Some 40 years ago, air pollution caused widespread forest decline in Central Europe and eastern North America. More recently, high levels of tree mortality worldwide are thought to be driven by rising temperatures and increasing atmospheric drought. A neglected factor, possibly contributing to both phenomena, is the foliar accumulation of hygroscopic aerosols. Recent experiments with experimentally added aerosols revealed that foliar aerosol accumulation can (i) create the microscopic impression of 'wax degradation', considered an important proxy of forest decline associated with air pollution, though the mechanism remains unexplained; and (ii) increase epidermal minimum conductance (g min ), a measure of cuticular permeability and completeness of stomatal closure-both could lead to reduced drought tolerance. Here, those studies with applied aerosol are extended by addressing plant responses to reduction of ambient aerosol.Scots pine, silver fir, and common oak seedlings were grown for 2 years in greenhouses ventilated with ambient air (AA) or air filtered to remove nearly all aerosol particles (FA). Removal of ambient aerosol prevented the development of amorphous structures viewed in the electron microscope that have typically been interpreted as degraded waxes. Lower g min values suggested that FA plants had better stomatal control and therefore greater drought tolerance than AA plants. The co-occurrence of apparent wax degradation and reduced drought tolerance in AA plants suggests a common cause. This may be mediated by the deliquescence and spreading of hygroscopic aerosols across the leaf surface. The liquid film produced may penetrate the stomata and facilitate unproductive stomatal transpiration. In this way, aerosol pollution may enhance the impacts of atmospheric drought, and may damage trees and forests on large spatial scales.