The exclusion of ambient aerosols changes the water relations of sunflower (Helianthus annuus) and bean (Vicia faba) plants

► First reported experiment on effects of ambient aerosols on plant water relations. ► The exclusion of ambient aerosols decreased the transpiration of sunflowers and beans. ► Stomatal conductance and transpiration were lowered by 20–40%. Aerosols are an ubiquitous component of the atmospheric environment of plants but their ecophysiological role is largely unknown. Here we address this role by comparing the water relations of plants grown in ventilated greenhouses with ambient air (AA), and filtered air (FA) where particle concentrations had been reduced by more than 99%. Beans and sunflowers were grown in well watered soil or hydroponics. Humidity response curves of gas exchange were recorded along with sap flow, water potentials, and osmotic potentials. Hydroponically grown FA sunflowers and FA beans showed 20–40% lower stomatal conductance and lower transpiration compared to the respective AA plants under identical conditions. In sap flow measurements, the leaf-area related transpiration of soil-grown FA sunflowers was about 20–30% lower than for AA plants, partially due to lower night time values. Midday water potentials as well as osmotic potentials of FA plants were higher compared to the respective AA plants, while pre-dawn water potentials did not differ. Reduced transpiration of FA plants with stable photosynthesis was observed for beans and can be explained by the “hydraulic activation of stomata”, where deposited hygroscopic aerosols form liquid water connections along the stomatal walls, thereby forming a second, liquid-water type of stomatal transpiration. Simultaneously decreased transpiration and photosynthesis were observed for sunflower and point to a smaller stomatal aperture of FA plants. To our knowledge, this is the first study allocating an important functional role to natural aerosol concentrations. It further supports the idea that particulate air pollution may decrease the water use efficiency and the drought tolerance of plants.