Plants perceive aerosols as an intensification of atmospheric dryness and react according to their isohydricity

Hygroscopic aerosols deposited to leaves are a local water vapor sink and can affect the water balance of plants by deliquescence and the formation of hydraulic films that penetrate into the stomata. Stomatal responses to aerosols and vapor pressure deficit(VPD) were investigated in two poplar clones grown hydroponically in ventilated greenhouses with and almost without ambient aerosols. With increasing VPD, transpiration increased in ANI, the more anisohydric clone, and decreased in ISO, the more isohydric clone, while aerosols had little effect. In ANI, stomatal conductance (gsw) and photosynthesis (A) decreased slightly with increasing VPD, but significantly with exposure to aerosols. Leaf carbon isotopes confirmed the long-term reduction in stomatal aperture by aerosols. In ISO, gsw and A decreased strongly with increasing VPD. Aerosols had no effect on stomatal conductance in ISO, but increased the minimum leaf conductance and decreased the turgor loss point. In both clones, aerosols reduced stomatal density by >20%, indicating increased water scarcity. Aerosols enhance the transmission of atmospheric dryness to the leaf, with plant responses depending on their isohydricity. Sensitive stomatal closure of isohydric plants is an effective adaptation to atmospheric dryness, but aerosol accumulation mediates a liquid pathway for water loss that undermines stomatal control.